Text © DrSc Giuliano Russini – Biologist Zoologist


English translation by Mario Beltramini


The pretty Pterois volitans, of the group of the Osteichthyes, has the cobras’ venom in the rays of its dorsal fin © Mazza

The pretty Pterois volitans, of the group of the Osteichthyes, has the cobras’ venom in the rays of its dorsal fin © Mazza

In this general introduction on fishes, we shall begin with a look on their evolution and their natural history, treating in base on the fossil remains, through morphologic analogy, or proposed as hypothesis, the possible progenitors from where have come the various orders, infraorders, classes, genera and species in which, nowadays, such group, the most numerous in the vertebrates, is subdivided.

We shall consider, obviously, both marine forms and the freshwater or continental ones, and then we shall treat about the present taxonomy-classification.

Finally, we shall perform a general overview on the different organs and systems, in charge of the various functions (circulatory, nutritional, locomotion: swimming, sensorial, endocrinal, excretory and reproductive), in relation to the ecology (autoecology and synecology), investigating in a common way and, when necessary, highlighting their differences, the two most representative classes of such group, that of the Chondrichthyes or cartilaginous fishes (Chondrichtyes) and that of the Osteichthyes, or bony fishes (Osteichthyes).

A particular attention will be paid to the respiratory system, as, but one case only, in such animals, breathing oxygen melt in water, this system had to adapt evolutionarily, in an original way from the anatomic-physiologic point of view. And similar attention will be paid to an organ, the “swim-bladder or gas-bladder”, which, in relation to the group under study, can be independent or in communication with the oesophagus, therefore presenting either a purely dedicated to the floating and swimming of the fish function, as it works as an hydrostatic organ, or, on the contrary, in others, as having the function of an organ partly involved in breathing and in hearing, thus representing a focal point in the evolution and the natural history of these animals.

Cross section of Lancelet at the microscope. On top, at the centre, we can see the notochord. Over the spinal cord and on the sides the muscle fascicles. Under, the gills and the gullet © Giuseppe Mazza

Cross section of Lancelet at the microscope. On top, at the centre, we can see the notochord. Over the spinal cord and on the sides the muscle fascicles. Under, the gills and the gullet © Giuseppe Mazza

The kaleidoscopic world of Fishes.

Natural history and evolution.

We have to say at once that the group of the fishes represents in a generic sense, the first genuine and proper biologic expression of vertebrates, from which have later come the Amphibia, the first class of vertebrates capable to develop an amphibiotic life, that is, both in water and on the main land.

The type to which fishes belong is the Chordata that is, animals having the “notochord”, subtype Vertebrata, animals provided of spinal column. All the vertebrates are Chordates (Chordata).

Unlike the inferior or primitive chordates, such as the Ascidians (Ascidia), which have a sessile life (anchored to the bottom), whilst their larvae are mobile and the Lancelets (Amphioxus) which have a length of 5 cm and live in the sea shore sand (both still extant), where the “notochord” is permanent, in the present vertebrates, the “notochord”, a cellular cord along the back, is either reinforced or replaced by a spinal bony or cartilaginous column during the ontogenetic development, and the encephalon (brain), is protected inside a cranial capsule (braincase), which is also either cartilaginous or bony.

Presently, the classes in which the group of the fishes is subdivided are three, that of the Agnathans, or jawless fishes (Agnatha), the Chondrichthyes, or cartilaginous fishes (Chondrichthyes) and that of the bony fishes Osteichthyes (Osteichthyes).

But their natural history is very long and has begun more than 400 million of years ago, during the Palaeozoic Era.

Though the vertebrates, that is, the animals provided of a supporting endoskeleton, are only one twentieth of all the animal species known to the zoologist biologists, they include, however, most of the greatest species living in the water, on the earth and in the air: fishes, amphibians, reptilians, birds and mammals.

The oldest fossils date back to 440 million of years ago, to the Silurian Period. The vertebrates, which their habitat is strong debated, had the mouth missing of the jaws and were forming a group of small animals, rather similar to the present fishes, provided of bony bronchial arches, of brain protected by a bony skull and also of a bony cuirass, as protection of the body, in which was contained an axial organ, the notochord, prelude to the actual spine. Also during the ontogenetic development of the embryo of all vertebrates (human being included), we go, during the organogenesis, from the formation of the notochord, replaced later by the spine, as residual transitory vestigial remnant, contrarily to the most primitive forms of the vertebrates, where, instead, was and still is a permanent organ of axial support. These organisms were the ostracoderms, belonging to the class of the Agnatha, called bony-armoured animals: these are the most distant ancestors of all vertebrates, man included.

Oral disc of Lampetra zanandreai © Giuseppe Mazza

Oral disc of Lampetra zanandreai © Giuseppe Mazza

Unlike the present species of Agnatha, closest to them, the hagfishes and the lampreys, they were organisms with an alimentation based on the filtering of the debris, because they had no jaws, and this is why they were not active predators.

Examples are the extinct species like the ostracoderm Hemicyclaspis, which lived during the Devonian (345-395 million of years ago), that was filtering from the mud the materials which it was nourishing of. Another example is provided by the Pteraspis (jawless fish), extinct ostracoderm, frequent during the Devonian period in the freshwaters. The Gnatosthomata (Gnathostoma) the fishes with jaws, have maybe derived later on (after the paleontologists), from similar forms.

After about 40 millions of years, a remarkable step ahead in their evolution took place with the transformation in jaws of the anterior part of the branchial arches. The first fishes provided with jaws date back already from the Devonian.

These are the placoderms, nowadays extinct, but their descendants, the cartilaginous fishes (Chondrichthyes) are active and most fearsome predators. Between them are included the dogfishes or sharks, the chimaeras and rays. Examples of placoderms are the armoured fish (Pterichthyodes), Devonian placoderm with mobile fins, obviously extinct.

Another well known species, extinct too, was the Dunkleosteus, colossal predator which reached the 10 metres of length and could utilize both powerful jaws for breaking the cuirasses of the other marine placoderms, its preys during the Devonian. One of the most ancient sharks, the Cladoselache, extinct by now, might reach even a metre and a half of length.

It had series of sharply cusped teeth, whilst another fossile shark the Hybodon, had low and broad teeth placed in the back part of the mouth, perhaps utilized them for crushing shells. Present sharks appeared during the Jurassic (190-136 million of years ago), and since the beginning were greedy predators.

The Lampetra zanandreai is an organism with detrivorous-filtrating diet, as jawless © Giuseppe Mazza

The Lampetra zanandreai is an organism with detrivorous-filtrating diet, as jawless © Giuseppe Mazza

Between the bony fishes, were present the progenitors of sturgeons (Acipenser), pertaining to the subclass of the Chondrostei, like the Cheirolepsis, dating back from the Middle Devonian. It was a bony fish with squared scales and was already, perhaps, lived in freshwaters.

From these are coming also the progenitors of the members of the subclass of the Holostei, of which an example is the bony fish Lepidotus minor, a primitive holosteus measuring 40 cm, which abounded during the Jurassic Mesozoic Era.

These originated the Teleostei, superorder of the Teleostomi, which form the majority of present fishes. A group, characterized by the lobes sustaining the fins, was a possible progenitor of the terrestrial vertebrates; they belonged to the Crossopterygii, between which was standing the Macropoma, dating back to the Cretaceous period (136-65 million of years ago), Mesozoic Era, which might measure the 60 cm of length. It lived in the sea, like the Latimeria chalumnae, a coelacanth, living fossil recovered off the coast of Madagascar in 1938, still extant nowadays. The passage to the subaerial life was conditioned by the formation of internal naris communicating with the mouth, of lungs, of a more robust skeleton capable to support the weight out of the water and of limbs for walking. The first amphibians did coexist with the last ostracoderms as they crawled out of the water by the last part of the Devonian Period, about 350 million of years ago. Traces of such a direct origin are observed in the newts, salamanders, frogs and toads which are compelled to frequently go back to the water, from which they cannot be independent, for their reproduction and not for their dehydration.

Cyclopoma gigas : a bony fish of the Eocene © Giuseppe Mazza

Cyclopoma gigas : a bony fish of the Eocene © Giuseppe Mazza

The order of the Dipnoi (order represented by forms still extant) was triggered by the extinct Ceratodus, which lived in the Triassic (225-195 million of years ago), Mesozoic Era, living in freshwater, which was able, like present forms, to breath O2 from the air through its lungs, when the water was becoming poor of it.

To the order of the Crossopterygii, which are considered as the ancestors of the amphibians, belonged the extinct Eusthenopteron, which was an active predator in the stagnant fresh water during the last part of the Devonian.

The first proto-amphibians, after fossil findings, should be represented by to members belonging to the subclass of the Labyrinthodontia and were represented by the Primitive Amphibian (Ichthyostega), which lived during the last part of the Devonian. It had four short legs for walking on the main land (tetrapod) and a tapered shape of the body in the antero-posterior sense, similar to that of the fishes, which allowed them to swim well. It was probably spending lot of time in the water and most of its vital cycle was taking place there.

The other one was the Marsh Amphibian (Eryops), of the Lower Permian (280-225 million of years ago), Paleozoic Era; it had to go back to the water for laying its eggs and for rehydrating. About half of the 50.000 species of vertebrates nowadays living is formed by bony fishes of the class Osteichthyes, which appeared during the Devonian Period.

A biologic oddity is represented by the Mudskippers (Periopthalmus), like the Periophthalmus barbarus and the Periophthalmus argentilineatus, silvery Mudskipper, known also as Silver-lined Mudskipper, order of the Perciformes, family of the Gobiidae, subfamily of the Oxudercinae.  After some authors, the Mudskippers must be considered as members of an independent family, that of the Periophthalmidae. The debate is still ongoing.

Periophthalmus kalolo in the foreshore area in the Seychelles © Giuseppe Mazza

Periophthalmus kalolo in the foreshore area in the Seychelles © Giuseppe Mazza

The scientific name of the genus Periophtalmus comes from the presence of two great globous eyes, exophthalmic, which it utilizes in the same way as the Amphibian hippopotami (Hippopotamus amphibius), do for looking on the water, when submerged, in order to ascertain the presence of preys and predators. It nourishes of terrestrial and aquatic worms, both oligochaetes and annelids and plathelminthes, like the small crustaceans and molluscs.

It is a fish of fresh, or brackish, or marine water, living inside particular tropical vegetal associations, mainly formed by mangroves, often in the estuaries and mouths of the rivers.

It needs temperatures of 26-30° c and waters with a pH 8 or more. Their home range extends from the Red Sea to all the Indian Ocean, Sunda Islands, Australia and western Africa. It has an elongated body, very compressed on the rear; much protruding spherical eyes. Very varying colouration, depending on the species, the provenience, the age and the sex; mostly mottled integument, with white, grey, black, or brown drawings. The dimensions vary from 12 to 25 cm and even more. The sexual differences are mainly present in particular on the fins; they do not have, therefore, a marked sexual dimorphism.

During several years, they were classified as Amphibia, but even if at first sight they may be mistaken with the members of such class, they are real fish belonging to the order of the Perciformes, one of the most recent. However, they stand among the oldest ones in this order. This misunderstanding doubt came to the biologists, by the beginning of the XX century, period when they were discovered during numerous scientific expeditions. This because such animal was spending much of its time out from the water, going back there frequently, as the amphibians, and as a consequence it seemed to be characterized by an amphibiotic life.

The Periophthalmus was deemed as an amphibian for many years © Giuseppe Mazza

The Periophthalmus was deemed as an amphibian for many years © Giuseppe Mazza

Indeed, it lives always in muddy pools where there is always present some water, in the habitats it has colonized. Furthermore, as it happens for the Eel (Anguilla anguilla) it has some horizontal sack-like peri-branchial spaces where it stores volumes of water with atmospheric air.

The atmospheric oxygen will mix with the water and, at branchial level, the exchanges of oxygen, counter-current, blood-water will take place. Some species, in periods of intense drought are able, by lowering their metabolism and covering with mud, to enter a phase of ametabolic life condition, thus surviving to the extreme conditions. It has to return to the water periodically to re-stock and then to reach land again.

In both cases, it moves by means of powerful leaps obtained by using its caudal muscles. They are very lively, always in movement, chasing. They have not yet been able to reproduce them in captivity; the males, during the heath season are rather cantankerous and do often fight.

Taxonomy and present classification of Pisces

The present zoological taxonomy subdivides the group of the fishes in three main classes: the Agnathans, the jawless fishes (Agnatha), the cartilaginous fishes (Chondrichtyes) and the bony fishes (Osteichthyes).

►►►Class Agnatha: Jawless fishes.

Lampetra zanandreai lives in the rivers of North Adriatic and doesn’t parasitize fishes © Mazza

Lampetra zanandreai lives in the rivers of North Adriatic and doesn’t parasitize fishes © Mazza

The oldest vertebrates belonged to this class. The few extant belong to the only order of the Cyclostoma.  The Cyclostoma are subdivided into hagfishes and lampreys and consist of 45 species, although the lampreys condition represent a secondary adaptation, on the contrary the hagfishes seems to be mopre primitive. This pose the lampreys between the sharks and the bony fishes.

These animals are parasitic and eat debris. They do not have scales and jaws.

The hagfishes remain, during the day, buried in the sand, the mud and the gravel and come out during the night for eating dead animals or organic remains, or for acting as parasite on living animals. They do not have working eyes. The extremity of the muzzle has fleshy tentacles, sensitive to the touch, called “barbels”.

A particular condition characterizes the reproductive system of the hagfishes (called also Myxini), in all the known species. In all the observed specimens, the gonads have a “feminine” anterior tract and a “masculine” posterior one, one of them is abortive, that is not working and destined to degenerate. In the males it will be the feminine tract, in the females, the masculine one. There is therefore a condition of “Gonochorism”, but with a “Rudimentary hermaphroditism”. In the past, till the beginning of the XX century, the biologists were interpreting such situation as a “Functional protandric hermaphroditism”. It was thought that in all hagfishes the masculine territory of the gonads would work before, and then, later on along the life, the feminine territory would activate, while the masculine one was getting off.

Lastly, we have to pinpoint that in a certain percentage of all known species of hagfishes, the spontaneous degenerative regression of both sexual territories in the single gonads (both masculine and feminine), takes place, thus determining the complete sterility of the animal. The lampreys, in turn, adhere by means of mouth suckers to the side of the fishes and, scraping the flesh of their preys, suck its blood, in a few words, they are bloodsucking organisms.

Triaenodon obesus © Giuseppe Mazza

Triaenodon obesus © Giuseppe Mazza

The adult lampreys die after the deposition and the fecundation of the eggs, which are placed in the gravel on the bottom of the rivers. Some species go back to the sea once the maturity is reached; others spend their whole vital cycle in usually lotic freshwaters (rivers, streams, creeks and watercourses).

Example: the Atlantic hagfish (Myxine glutinosa), long even 45 cm, located along the coasts of northern Atlantic Ocean.

For the lampreys a typical example is the Petromyzon marinus, long up to 45-50 cm, which has the same habitat as the hagfishes. Like the other marine lampreys, it spends its adult life (from 12 to 20 months) at sea, as a parasite of other fish species.

By the end of this phase of their vital cycle, they migrate in the rivers for laying and fecundating the eggs. The female lay about 60.000 eggs on the gravelly bottom, the male fecundates them with the sperms and then buries them. Then, the adults die.

About 1% of the eggs will hatch after 10-12 days. After further 10 days, the larvae, about 5 mm long, leave the gravelly nest and go down the river or the stream, till when they find a muddy bottom, where they dig a hole for burying themselves, nourishing of the microscopic organisms carried by the current. The larva grows up, reaching the 15 cm of length during the following 2-5 years, and then transforms in adult, furnished of a raspy tongue and a round mouth, similar to a sucker. By now, the Petromyzon marinus goes towards the sea for beginning its parasitic life.

Carcharias taurus © Giuseppe Mazza

Carcharias taurus © Giuseppe Mazza

►►►Class Chondrichtyes: cartilaginous fishes.

The skeleton of these fishes is formed by cartilage which may be reinforced by calcareous salts and assume therefore a remarkable hardness; such type of skeleton is however lighter and more elastic than the bony one, allowing the typical way of swimming of the sharks, extremely agile and continuous-exploratory.

They have a cartilaginous neurocranium, containing the brain. They have an asymmetric caudal fin, or “heterocercal”, where the vertebrae extend more in one of the two lobes. In particular in the sharks (Selachii), it is “epicercal”, where the more developed lobe is the upper one. The members of this class have pair fins which have a stabilizing function while swimming.

Both jaws have a numerous series of teeth, which repeatedly fall and grow again (polyphydont dentition).

The scales are small and shaped like denticles, with the tip bound towards the tail, covered by a substance similar to the enamel. This characteristic renders the skin tough, so much that in the past it was treated and transformed in sandpaper, called Shagreen.

In these fishes there is no “gas or swim-bladder”, and the buoyancy neutrality is achieve through a lipid-rich liver. The chemo-sensing (sense of smell) is very developed, like also the system of the “lateral line”, which allows in perceiving the water vibrations.

The male fecundates the eggs inside the body of the female by means of a pair of “claspers”, sort of penises, placed on the inner edge of the ventral fins. Often the developing embryos wrap up with a horny external case, solid, rich of hooked rostra.

The Torpedo torpedo may give strong electrical shocks © Giuseppe Mazza

The Torpedo torpedo may give strong electrical shocks © Giuseppe Mazza

They are released in water close to plants or rocks and, thanks to such structures, they anchor to them and proceed in their development.

A particular sensorial structure, the “Ampullae of Lorenzini” (from the name of the Italian biologist, who, in 1890, identified it), placed in the most rostral portion of the muzzle, permits the members of the order of the Selachii, Batoidea and Chimaeriformes to perceive the electrical fields and the electric currents which a moving body, like a fish prey or predator emits, while moving in the water, inside the Earth’s magnetic field. They are of low intensity and are exploited both for the predation and for the escape from possible predators. This organ has, therefore, an enormous ecologic and etho-physiologic importance.

The class of the Chondrichtyes subdivides into two subclasses: Elasmobranchii Holocephali.

►►Subclass of the (Elasmobranchii) : to this belong the sharks and the rays. They have from five to seven pairs of branchial gill clefts for each side, numerous teeth with the maxilla not connected with the skull. This subclass is subdivided itself in 2 orders.

Order of the Selachians (Selachii), sharks or dogfishes (200 species).

All sharks are excellent swimmers with a fusiform body and branchial slits on the sides of the body turned towards the head.

Ovigerous capsule of catshark with cirri and developing embryo © G. Mazza

Ovigerous capsule of catshark with cirri and developing embryo © G. Mazza

The Carcharodon carcharias, known as white shark can reach the 8-10 m of length and the 3-4 tons of weight.

Few freshwater species do exist, some are defined “euryhaline”, which means that they can live both in marine and fresh waters, as well as in the mixed in salinity waters, which are those of the estuaries and the mouths of the tributary rivers of various seas.

This is guaranteed to them by the capacity they have to keep the Urea, by means of the mesonephric kidneys in the blood (and therefore to modify its concentration). The Urea is the osmotically active agent which allows it, not dehydrating, to live in the sea (as it functions as osmoregulator organisms).

In the blood of the marine life species, it is kept a value corresponding to the 2% of Urea, whilst in the “euryhaline” species, this value is of the 0,6%.

The Rajiforms (Raiformes) with sting, belonging to the genus Potamotrygon, stand among the few freshwater cartilaginous fishes and live in the South American rivers. They have poisonous thorns, sheathed and located in the upper part of their long pointed tail. The poison is utilized as defence. They have a flat and circular body and nourish of crustaceans.

Another example is represented by the Zambezi Shark (Carcharhinus leucas), currently improperly called by the Anglo-Saxons bull shark, which has a length of 3,5-4 m.

It lives in the Atlantic Ocean along the coasts going from Massachusetts to southern Brazil and from Morocco to Angola; in the Indian Ocean its home range extends from South Africa to Australia passing by Kenya up to Vietnam, in Indo China.

Big specimens of 4 m of length have been sighted in the Pacific Ocean, from the coasts of California to Ecuador in South America. In all these ranges it has the tendency to ascend the mouths and estuaries of the rivers that flow in these seas; it is a carnivorous and aggressive species.

Rhinoptera marginata © Giuseppe Mazza

Rhinoptera marginata © Giuseppe Mazza

The selachians have a very ample span of forms, we go from the dogfish, small shark reaching the length of 30-60 cm to the Basking shark (Cethorinus maximus) living in the northern zones of Atlantic and in the Mediterranean, 6 to 12 metres long, exceeded in size only by the Whale shark (Rhincodon typus), which reaches the length of 18 m, but which is not at all aggressive, being planktivorous and vegetarian.

Order Batoidea, rays and mantas (350 species).

They have a flat body and are provided with pectoral wing-like fins, connected to the sides of the head.

Examples are the Manta ray (Manta birostris) broad up to 6 m, living in the surface waters (pelagic life) of the temperate and tropical zone of the Atlantic Ocean.

It swims with slow beats of the pectoral fins. It feeds of plankton, collected with the fins placed on the sides of the mouth.

Another example is the Sawfish (Pristis pectinatus), which reaches the 5 m of length.

Raja undulata © Giuseppe Mazza

Raja undulata © Giuseppe Mazza

It lives in the tropical and subtropical waters of the Atlantic Ocean.

It utilizes its long serrated rostrum, which has from 24 to 32 teeth per side, for looking for food in the mud or for stunning and maiming the fishes it eats.

Or else, the Thornback ray (Raja clavata), 90 cm long, living in north-eastern Atlantic and in the Mediterranean, which nourishes of crustaceans.

►►Subclass Holocephali (complete heads), with one order only.

Order Chimaeriformes, called Chimaeras (25 species).

These fishes have the upper jaw fused with the skull, contrarily to what happens with the Elasmobranchii.  

They have only one branchial cleft per side and six pairs of dental plaques. The males have an appendix on the front, which is maybe utilized for the courting (absent in the females, sexual dimorphism) and two “claspers” for the copulation, placed on the inner edge of the ventral fins; they carry a benthonic life (they live close to the bottom of the sea).

Rhinobatus typus © Giuseppe Mazza

Rhinobatus typus © Giuseppe Mazza

In some species the dorsal fin thorn is poisonous.

A typical example which is often caught by the nets for deep fishing and trawling utilized by the fishermen, is the Rabbit fish (Chimaera monstrosa), living in the waters of eastern Atlantic and Mediterranean.

It is 90 cm long; it is piscivorous and carnivorous. The name is justified by its particular morphology, as it seems to be a sort of combination between a shark and a manta.

►►►Class of the Osteichthyes, the bony fishes.

They are by sure the most numerous in the three classes; they have a bony skeleton, teeth on both jaws and often on the palate.

The “gas or swim bladder” is often present and may be communicating with the oesophagus or can be independent.

There are two subclasses:  Actinopterygii and Sarcopterygii.

►►Subclass of the Actinopterygians (Actinopterygii), fishes with lepidotrichia. The fins are supported by bony rays.

These fishes have no internal nostrils; they live both in marine waters and in fresh or continental ones.

There are three infraclasses.

Infraclass Chondrosteans (Chondrostei): these fishes have scales covered by a layer of “ganoin”, substance similar to enamel.

They mostly live in freshwaters and are subdivided in two orders.

Order of Polypteriformes with 12 species.

They have from 5 to 18 small fins, instead of only one dorsal fin.

The Polypterus endlicheri may resist several hours out from the water © Mazza

The Polypterus endlicheri may resist several hours out from the water © Mazza

They live in tropical African rivers and nourish of small bottom animals, such as worms, larvae and small fishes; an example is the Mottled bichir (Polypterus weeksi), long up to 60 cm and living in the Congo river. It is protected by a series of scales forming a solid cuirass. It can resist, out from the water, without breathing, for several hours.

Order of Acipenseriformes, sturgeons, 22 species. They are located in the rivers and the coastal waters of the temperate and arctic regions. They have a shark-like body, with longitudinal bony plaques, developed gas bladder and tail bent upwards.

Examples are the Huso (Huso huso), long up to 8,50 m. It is common in the Caspian Sea and in the Black Sea, but it can be found also in the eastern Mediterranean and at times it goes up the Adriatic Sea entering the rivers which flow there. Its weight is of about 1 t. From the ovaries of the females they get an excellent caviar, this characteristic has placed (and still is extant) its population in risk of extinction, due to the continuous hunting. The CITES and the IUCN forbid its trade and control the density of its population.

The Common sturgeon (Acipenser sturio), long up to 3 m, lives in the Mediterranean and along the European and Atlantic coasts. With its muzzle, it continuously digs holes in the bottom, looking for small crustaceans and invertebrates which are its food.

Although if the ovaries of the females produce a quantity of caviar (red), inferior if compared to the black one of the Huso huso, also this species is in danger of extinction for the same reasons.

The Sturgeon (Acipenser sturio) reaches the 3 m but well adapts to breeding in captivity © Mazza

The Sturgeon (Acipenser sturio) reaches the 3 m but well adapts to breeding in captivity © Mazza

Luckily, unlike Huso huso it is able to adapt to live in captivity and to reproduce; as a consequence the biologists, through techniques of fish breeding, are trying to reduce the loss of the natural population taking advantage of the species bred for commercial purposes.

A fish similar to the sturgeon, belonging to the genus Polydon, like the Polydon spathula, lives in the Mississippi Basin in North America. Like the Acipenser sturio it moves continuously the mud of the bottom, by means of the paddle in which its snout ends and which is long one third of its total length, about 1,50 m. It filters small animals (crustaceans, molluscs) through its gill rakers.

Infraclass Holostei.

Similarly to the infraclass of the Chondrosteans (Chondrostei), from which they have differentiated about 300 million of years ago (Paleozoic Era, Pennsylvanian or Superior Carboniferous period), the Holostei have scales covered by a layer of “ganoin”.

They differ from the Chondrostei in the characters of the skull and the jaw.
They live in freshwaters and subdivide in two orders.

Order of Semionotiformes, 7 species. These fishes, which reach at times the remarkable length of 3 m, are easily recognizable for their elongated snout and for the dorsal and anal fins, placed very behind, close to the caudal.

The Lepisosteus osseus is swept by the currents and has many similarities with the fossil fishes © Mazza

The Lepisosteus osseus is swept by the currents and has many similarities with the fossil fishes © Mazza

They swim slowly and are capable to grasp the preys with rapid movements of their strong jaws.

An example is the Longnose gar (Lepisosteus osseus), 1,50 m long, which lives in the freshwaters of North America, in the Mississippi Basin.

It is swept by the current, like a floating log, for approaching stealthily the preys, usually fishes.

It has many affinities with fossil fishes species.

Order Amiformes (one species only): Bowfin (Amia calva) which lives in the rivers and the marshes of North America.

Long up to 90 cm, it has a long dorsal fin, without thorns. The males, during the reproductive season, prepare the nests utilizing aquatic plants, small branches and mud, where the fecundated eggs will be incubated, after the females has laid them.

Furthermore, the males guard the nests, often fluttering the tail by its entrance, in order to keep constant the oxygenation which is necessary for the physiological development of the eggs; finally, the males will breed the young fry born after the hatching.

Many teleosts, like the Lutjanus kasmira, are gregarious © Giuseppe Mazza

Many teleosts, like the Lutjanus kasmira, are gregarious © Giuseppe Mazza

They eat invertebrates, crustaceans and small fishes.

Infraclass Teleostei. The teleosts, superorder of the Teleostomes (Teleostomi).

The clade of the teleostomes includes ganthostomata (but the condrichthyes and the ostracoderms), tetrapods and bony fishes.

They are not to be mistaken with the teleosts (group of bony fishes, which are one of its superorders, but also an infraclass of the class of the Osteichthyes), they form the most numerous group of extant vertebrates.

They evolved from the Holostei (Holostei),), about 190 million of years ago (Mesozoic Era, Jurassic Period).

Their scales are not covered by “ganoin”, the caudal fin is symmetric or “homocercal” (that is, the two lobes of the fin have the same dimensions, where the vertebrae do not extend in the lobes, and the fin is almost completely symmetric) and has the rays inserted on bones of the terminal extremity of the spinal column.

The upper jaw articulates on the skull in several ways. The “swim or gas bladder” is absent in several species. Many teleosts are gregarious; presently there are about 30 orders of them.

Order Elopiformes, Tarpon and similar (12 species). Primitive group, very close to the Holostei. Their larvae are colorless and have a flattened form. An example is the Atlantic tarpon (Tarpon atlanticus), long up to 1,80 m, living in the tropical and subtropical Atlantic. It can jump over the surface of the water up to 1,80-2 m of height.

Order Anguilliformes, eels, morays and like (300 species).

The eels (Anguilla anguilla) perform long migrations for reproducing © Mazza

The eels (Anguilla anguilla) perform long migrations for reproducing © Mazza

These fishes are mostly marine. The larvae, called “leptocephali”, are metamorphosing, transparent and ribbon shaped. They migrate, from the Sargasso Sea, to the various rivers spread in almost all the world, where they will metamorphose in a morphologically and physiologically different organism, which, once the sexual maturity will be reached, will go down the currents for reproducing in the same area of sea where it was born.

Only the eels (as member of this order) have this particular vital cycle, which is the reverse of that of the salmons and for this reason is called by the biologists “catadromous”.

They are therefore called also “catadromous” species (which go down the currents), whilst the salmons, which spend their adult life in the seas for going to the rivers for reproducing, are defined “anadromous”, or “anadromous” species (which ascend the currents).

As a matter of fact, the adults of the eels live in the rivers and once reached the full sexual development, by the time of the reproduction, migrate back to the areas of sea of origin where they were born, for reproducing, both males and females; a homing process, the nature of which for the biologists is even less known than that of the sea turtles and the birds (see Chelonia).

After some authors, the homing, both for salmons and eels, might be effected through a combination of geomagnetic phenomena, of identification of physical geographic signs during the displacement (identification of coasts, rocks, bottoms, etc.) and the disposition of the stars in the sky, as well as also the perception in the variation of the salinity of the waters where they are swimming, during the migration in the two directions (sea-river for the salmons, river-sea for the eels). Some zoologist biologists speculate that the variation of the photoperiod (the light in the sea as well as in the river, at the depth where salmons and eels live, is still intense enough for differentiating a period day-night), is the stimulus which triggers the heat season, whilst the variation of the salinity perceived in the water during their voyage (for both species of fishes), might act in some way at a neuro-endocrine level, stimulating the release of various hormones involved in the processes of ovulation and in the spermatogenesis.

The Gymnothorax funebris is a 30 kg tropical moray © Giuseppe Mazza

The Gymnothorax funebris is a 30 kg tropical moray © Giuseppe Mazza

The zoologists seem to be convinced that all species of eels come to reproduce in the Sargasso Sea, even those living in the European rivers, facing very long trips.

Last, after the fecundation of the eggs (like for salmon in the rivers), the eels die, probably furnishing a nourishing substratum to the “leptocephalic” larvae (embryos) living free, under development.

Examples are the Eel (Anguilla anguilla), long up to 1,20 m.

It is practically present in all the rivers of the planet. Unlike other fishes, it can survive for long time out of the water, because the gill cavity holds a substantial quantity of the same, ensuring the oxygenation in external location and for its typical capacity of cutaneous respiration.

As described, the homing of the Eels, has major obscure points for the ichthyologist biologists as for the moment they have not yet been able to understand how the European species (Anguilla anguilla), as well as the American one (Anguilla rostrata), may begin their vital cycle in the Sargasso Sea; sea delimited in a western portion of the Atlantic Ocean, included between the archipelagos of the West Indies (on the west) and the Azores (east), name which comes from a genus of alga growing there, the Sargassum.

The metamorphosing larvae, of both species called Leptocephalus, which represent a morphologically different form from the adult one, live in the case of the American species for one year, for the European three, closer to the marine coasts, after such period they transform into a an intermediate form between the larval and the adult ones, called “blind”.

At this stage, the migration begins towards the lotic and fresh continental waters (rivers, streams, etc.). They will live in these waters for about 7-20 years, being this time valid for both species.

The migration begins close to, or when reached, the sexual maturity for both sexes; during this stage the digestive organs atrophy, the reproductive ones develop completely and the eels leave towards the Sargasso Sea for reproducing and then dying.

After a theory of about 15 years ago, advanced by some English and Russian zoologist biologists (which was not accepted unanimously and is still discussed by the zoological scientific community), it is not sure the European eel stock go really back to the Sargasso Sea.

The Muraena helena reaches the length of 1,5 m and 15 kg of weight © Giuseppe Mazza

The Muraena helena reaches the length of 1,5 m and 15 kg of weight © Giuseppe Mazza

After these authors, it might be possible that also the European eels belong to the American species, and so the generating population should come from this last and that the differences between the eels of the two continents should be due to environmental-ecological factors which influence the development of the European leptocephalic larvae during the first stages of their vital cycle. Finally, another migrating fish, besides salmons and eels, has been identified in 1955 by Japanese biologists; it is the Plecoglossus altivelis, born in the rivers of Japan, Korea and Taiwan Island, but which spends most of its life in coastal waters. It lays the eggs (anadromous species) on the gravel of the upper part of the rivers (towards the source), where the fast waters are well aerated, and the temperature of the water is of 12-15° C. When the fry are born, they are carried by the current of the river to the sea. After about fifteen months, the go back up in the rivers, where they spawn and then die.

Other Anguilliformes, specifically with marine life and not migrating are the members of the family of the Muraneidae.

Between these stands our Muraena helena living in the Mediterranean Sea, the eastern Atlantic Ocean (from the south of England up to Senegal).

It is also found along the coast of the Azores, Madeira, Canary Islands and Cape Verde. It is a reserved animal which prefers to live in the anfractuosities of the rocks.

It nourishes mostly by night time, eating molluscs, fishes and crustaceans. This indicates that it lives in such depths where the light is still able to get, permitting them to discriminate between the night and the day, that is, between the 200 m of depth.

Even if its saliva is toxic, the bites of Muraena helena are not venomous © Mazza

Even if its saliva is toxic, the bites of Muraena helena are not venomous © Mazza

It can reach a length of 1,5 m and a weight up to 15 kg.

It is commonly thought, among the people, that the moray is a poisonous fish, but this is not true, at least in the strict sense of a venomous organism, as a jellyfish, a viper or an imperial scorpion can be.

Yes, it emits, with its saliva, a thermolabile toxin, which has, anyways, a mildly toxic effect, which is secreted by glands present in the palatal mucosa.

But, actually, the pain felt when bitten by one of these specimens, is caused by the strong teeth which originate deep lacerations in the tissues.

The infections which may follow are caused by bacteria which grow, abundant, in the oral cavity thanks to the organic debris present in the mouth.

Order Notacanthiformes, notacanths and like (20 species). They are deep sea species, with elongated body and with larvae similar to those of the Anguilliformes. This is to give an example the case of the Lipogenys (Lipogenys gillii), long up to 40 cm, living in the deep waters of the northern zones of the Atlantic Ocean.

The Gymnothorax unicolor is a rare Mediterranean moray © Giuseppe Mazza

The Gymnothorax unicolor is a rare Mediterranean moray © Giuseppe Mazza

Order Clupeiformes, herring, sardines and anchovies (about 350 species).

They mostly live in groups close to the surface of the sea, but many species are found also in freshwaters.

The aggregation in crowds of enormous dimensions (millions of specimens !) is a defence strategy against the predators.

The dorsal fin is by the centre of the body and the tail is forked.

The gas bladder is connected with the labyrinth, a particular adaptive situation from the evolutionary point of view, being explained later on.
A well-known example is the Atlantic herring (Clupea harengus), long up to 35 cm, which lives in the northern Atlantic.

The crowds effect seasonal displacements and migrations in relation with the reproduction.

The herrings, in fact, meet in reproductive areas, where each female lays on the bottom from 12.000 to 160.000 eggs. These ones open after 1-3 weeks from the fecundation (external). The larvae (embryos), 6 mm long, after a period of planktonic life, swim towards the coast, where they remain till the age of 1-2 years, reaching a length of 8 cm.

Osteoglossum bicirrhosum of South American rivers has fossils also in the north and in England © Mazza

Osteoglossum bicirrhosum of South American rivers has fossils also in the north and in England © Mazza

At this stage, they are called “whitebait”.

The young fishes move then to the open sea where they reach the sexual maturity and a length of about 35 cm, before beginning their voyage to the reproductive areas.

The death rate is elevated in each stage of growth. Pinnipeds, dolphins and other various cetaceans, between which the sperm whales, stand among their worst foes, besides the human being, who is the most dangerous due to the fishing.

They are piscivorous and planktophagous, they may be also omnivorous.

Order Osteoglossiformes, Arapaima and similar (16 species).

They are autochthonous in the rivers of the Tropics.

They are provided with thick and thorny scales and with a tongue sustained by bony pieces equipped with small teeth.

Examples are the Arapaima (Arapaima gigas), 4-5 m long, which lives in the rivers of South America and is one of the biggest freshwater fishes. It can weigh even about 400 kg.

Gnathonemus petersi, a Mormyriformes which locates its preys generating an electrical field © Mazza

Gnathonemus petersi, a Mormyriformes which locates its preys generating an electrical field © Mazza

The Silver arowana (Osteoglossum bicirrhosum), long up to 75 cm, lives in the ribers of South America, even if its fossils have been discovered in North America and in England. After some zoologist biologists, the female carries the eggs, once fecundated, inside its mouth till their opening.

Order Mormyriformes, mormyrus and gymnarchus (about 150 species). Also these fishes, similar to the Osteoglossiforms (Osteoglossiformes), have the tongue furnished with small teeth and are found in Africa only. They are usually insectivorous and live in fresh water. Many of them have a tubular snout, bent downward, and have electrical organs with which they localize any object included in their electrical field. A typical example is the Elephant-snout fish (Mormyrus kannume) living in the Nile and the Victoria Lake, close to the reed beds. It is about 75 cm long, and utilizes it flexible snout for looking for food on the bottom: organic debris, small crustaceans and all what can be eatable.

Order Characiformes, piranhas, with several hundred species. The most representative family is that of the Characidae, to which belongs the subfamily of the Serrasalminae, whose members are the piranhas (in Portuguese), pirañas in Spanish.

Usually, the piranhas (Pygocentrus nattereri) live in crowd © Giuseppe Mazza

Usually, the piranhas (Pygocentrus nattereri) live in crowd © Giuseppe Mazza

Usually, they are 15 to 25 cm long, even if some biologists during the forties of the last century have discovered some specimens which were reaching the length of 40 cm. They are carnivorous-piscivorous, they attack in group and, like the sharks, when they perceive the smell of the blood and they assume a frenetic obsessive activity which, in these fishes, stimulates them to unite in a crowd capable to eat a horse in a few minutes. They are dangerous also for the man.

Zoo-geographically, they live in the Amazon River and in the Guyana ones.

They are also present in the lake zones of the Amazon Basin, from the Orinoco to the Rio de la Plata.

They are well known to the autochthonous populations, the “Indio” of the Amazonia who fear them. Some join them to the indigenous cannibal populations living in the Caribe, reason for which, they are called also with this term, Caribe, besides “Pira” or “Pirana”. The piranha is a gregarious fish, lives in groups also very numerous which, at times, can even populate entire parts of a river.

Salmo trutta fario © Giuseppe Mazza

Salmo trutta fario © Giuseppe Mazza

Some biologists aver that there are species mainly carnivorous and others only vegetarian; after other authors, some species, during their vital cycle, influenced also by the temperature and the pH of the water, may nourish of weeds and algae, for then assuming an evidently carnivorous alimentation, when adult, possibly necessary for the reproduction.

Some well known species are the Serrasalmus spilopleura, the Pristobrycon calmoni, the Ossubtus xinguensi, the Metynnis fasciatus and the Colossoma bidens.

Order Salmoniformes, salmons, pikes, trout and stomiatoidei (500 species).

Group of fishes much diversified for style of life and habitat.

The salmonids have a small adipose fin behind the dorsal fin, possibly a reserve of fat.

The stomiatoidei live in the marine depths and have luminous organs. The pikes live usually in the lotic freshwaters (rivers, water streams, creeks), are extremely aggressive and excellent predators; the same habitat is colonized by the trout which is present in all Eurasia and in the New World. A typical example is given to us by the Pike (Esox lucius) which has two jaws with teeth having apexes bent backward.

The Exos lucius may be 1,5 m long and has a very tasty flesh © Giuseppe Mazza

The Exos lucius may be 1,5 m long and has a very tasty flesh © Giuseppe Mazza

This allows trapping the prey, once bitten, with no escape. Besides fishes, it nourishes of aquatic birds and even of mice. It lives in freshwaters from Europe to North America; it can reach even the 1,5 m of length, is carnivorous-piscivorous and has some very tasty flesh.

Other members are the Black-belly dragonfish (Stomias atriventer), long up to 30 cm, living in the depths of the Gulf of California and off Chile, and the Salmon (Salmo salar), anadromous species (see explanation in the text of the Anguilla anguilla), living in the zones of the northern Atlantic, in cold and temperate waters and reaching the remarkable length of 1,20 m.

After its birth, it spends 1-3 years in freshwater, then reaches the sea; after the sexual maturation, by ascending the currents, will go to reproduce in the same area of the river, where it was born.

A 10 kg female may lay about 14.000 eggs and then will die. Its migrations are fairly known to the zoologist biologists.

The period of 1-3 years spent in the lotic (rivers, streams, creeks) freshwaters, is called phase “parr”, in correspondence with this phase of its vital cycle the salmon meets deep morpho-chromatic transformations, assuming on the back a steel-blue colouration, whilst on the belly and on the sides it becomes yellowish, phase “smolt”.

The Electrophorus electricus, 1,5 m long, is South American and can emit strong electric shocks © Mazza

The Electrophorus electricus, 1,5 m long, is South American and can emit strong electric shocks © Mazza

After the previously discussed migratory phases as “anadromous” species and then, after the fecundation of the eggs laid, both males and females begin to pass away.

Some ichthyologists have remarked that a very small percentage of Salmo salar, does survive, going back to the sea for restoring. After this, they ascend again the water streams for laying and fecundating the eggs. This phase has been called “kelt” by the zoologist biologists.

Order Myctophiformes, lanternfishes (300 species). These fishes live in deep sea and have some photophores (organs producing light), on the sides

They are diffused up to 1.000 m of depth, but they look for food close to the surface. Some species have an adipose dorsal fin.

An example is the Lanternfish (Myctophum punctatum) living in the Atlantic Ocean and in Mediterranean.

They have big eyes, in order to see at the low depths where the darkness is predominating; this is an adaptation similar to that assumed by the plants living in the thick pluvial-tropical forests, where the tall layers of the forest, due to the tops of the trees, allow scarce light to penetrate, therefore the plants living in the lower layers have leaves with an enormous surface for catching the maximum possible quantity of the penetrating light. The photophores can be also useful to the individuals of the same species for reciprocal recognition. It is 5-7 cm long.

Order Ctenothrissiformes: one species only, the Macristium (Macristium chavesi), sole survivor of this order. It lives in depth in the southern Atlantic and measures 10 cm. Many fossil species are known, similar to the lanternfish (Myctophum punctatum).

Fantail Carassius auratus © Giuseppe Mazza

Fantail Carassius auratus © Giuseppe Mazza

Order Gonorhynchiformes, gonorhynchs and like, (15 species).

They are intermediate between the Ctenothrissiformes and the Cypriniformes. They have a forked caudal fin and are lacking of teeth. Examples are the Chanos chanos, living in the surface waters of the Indian Ocean. They are up to 90 cm long and the female can lay even 9.000.000 eggs!

Order Cypriniformes, cyprinidae, carps, characins and gymnotus (3.000 species). The members of this great order which includes almost all the freshwater fishes, have a series of small bones which connect the “swim and gas bladder” with the skull, adaptation which probably increases the hearing capacity. Usually, their body is covered by scales.

Examples are the Gymnotus (Electrophorus electricus), long up to 1,20 m; it lives in fresh water, in South America and has electric organs, and the Carp (Cyprinus carpio), native to Asia, found from the Black Sea to the Turkestan. It reaches the length of 90 cm and may have an omnivorous or vegetarian alimentary diet.

Another example of cyprinid is the common goldfish, either of breeding or present in our aquariums (Carassius auratus), which, through various mutations and hybridizations, may appear with the most different morphologies (for instance, the enormous head of the oranda lion headed, or, always in the orandas, the enormous exophthalmic eyes), as well as in the chromatism of its colouration.

Order Siluriformes, catfishes (2.000 species).

The catfishes are mainly located in the freshwaters in Africa, Asia and South America; some of them live also in northern regions and others are marine. Usually, they have “barbels” close to the mouth and some species have an adipose dorsal fin. The labyrinth and the gas bladder are connected, thus improving the hearing; the skin is naked or covered only by bony plaques. Examples are the Armoured Catfish (Corydoras aeneus), which lives in the freshwaters of South America and which is long up to 8 cm.

Malapterurus electricus : an electrical catfish © Giuseppe Mazza

Malapterurus electricus : an electrical catfish © Giuseppe Mazza

Some species, like the Catfish of the Mekong, belonging to the family of the Ictaluridae, (scientific name Ictalurus melas), may reach the 2 m of length and 300 to 400 kg of weight.

All species are carnivorous-piscivorous and extremely aggressive and greedy, when of such dimensions, also against the man.

Another species is the Synodontis batensoda which lives in Africa and backstrokes, just under the water in the various rivers, streams and lakes. This particular adaptation to the swim allows to catch the food (insects, insects larvae and amphibians), floating on the surface film.

In some catfishes, like the Ictalurus nebulosos, which live on the bottom of the fresh water streams are present sensorial organs placed in the “barbels”, with which they explore the presence of preys. Other species have pectoral venomous thorns.

Order Percopsiformes, 10 species. They remind the cyprinids, but they have thorns in the dorsal fin. An example is the Pirate perch (Aphredoderus sayanus), which is located in the fresh waters of the USA and is about 10-12 cm long.

Order Batrachoidiformes, toadfish and like (10 species). They are marine and predators. They have a big head, fusiform body, robust teeth. The first rays of the dorsal fin are spine-shaped. An example is given by the Oyster toadfish (Opsanus tau), a littoral fish living in the deep waters of northern Atlantic.

Order Gobiesociformes, Barred clingfish (100 species). Small marine fishes, which adhere to the littoral rocks with a sucker formed by the ventral fins. An example is the Barred clingfish (Tomicodon fasciatus), 4 cm long. It lives in the stony shoals of the Brazilian coasts.

The unusual fishing rod moved by the Antennarius pictus attracts small preys, swallowed without even noticing © Giuseppe Mazza

The unusual fishing rod moved by the Antennarius pictus attracts small preys, swallowed without even noticing © Giuseppe Mazza

Order Lophiiformes.

The anglerfishes and like (150 species). They have a depressed body, with a big head and an ample mouth.

The first ray of the anterior dorsal fin has a fleshy terminal lap which is moved as it were a fishing rod equipped with bait for attracting small preys.

The males of some species are smaller than the females, to which they stick and from which they do wholly depend for the food. An example about this is given us by the well known Antennarius scaber, 13 cm long, living in the Atlantic Ocean which has, over the mouth, its own small “fishing rod”.

The cod (Gadus morhua) may be 120 cm long © Giuseppe Mazza

The cod (Gadus morhua) may be 120 cm long © Giuseppe Mazza

Order Gadiformes, cods and allies (450 species).

The body is very long and tapered.

The dorsal and anal fins are usually without thorns and may be subdivided in more parts.

A typical specimen is the Cod (Gadus morhua), long up to 120 cm, living in the North Atlantic Europeand and North American waters.

The cods form huge crowds especially during the time of the reproduction and as a defence strategy.

400 million of cods are fished each year, as this fish enters the diet of many populations of the Boreal hemisphere, in particular of Europe and North America.

Order Beryciformes, squirrel fishes and allies (150 species).

The dorsal and anal fins have anterior rays spine-shaped; the caudal is formed by 18-19 rays. Examples are the Squirrelfish (Holocentrus xantherythrus), 15 cm long, living close to the Pacific Ocean coral reefs.

Order Atheriniformes, flying fishes and gambusias (600 species).

Due to the climatic changes, this Squirrelfish (Sargocentron rubrum) has invaded the Mediterranean © Mazza

Due to the climatic changes, this Squirrelfish (Sargocentron rubrum) has invaded the Mediterranean © Mazza

Small fishes, with primitive characters, living in the surface sea water and in the fresh waters of the Tropics.

Examples are the well known Flying fish (Oxyporhymphus micropterus), 25 cm long, living in the tropical zones of the Atlantic and Pacific Oceans and the Exocoetus volitans, also a circum-tropical flying fish, very common in the tropical belt of all seas and oceans.

During the summer, it can penetrate into the Mediterranean Sea, including the Italian Seas (Tyrrhenian, Ionian and Adriatic), but it is not very frequent.

The wing-like pectoral fins allow this fish to perform planed flights out of the water, reaching a distance of even 150 metres.

The flying fish hits the surface of the water in order to begin planning with the caudal fin, which it starts when escaping from a predator; the dolphins, very agile swimmers, like the Bottlenose dolphin (Tursiops truncatus) or the Short-beaked dolphin (Delphinus delphis), which stands between its most ferocious foes.

Zeus faber. A big mouth for a good prey… but then ends up in a restaurant menu © Mazza

Zeus faber. A big mouth for a good prey… but then ends up in a restaurant menu © Mazza

Order Zeiformes, John dory and like (60 species).

Littoral marine fishes forming a group similar to the squirrelfishes.

They have 11 to 14 rays on the caudal fin.

An example is the John dory (Zeus faber), 75 cm long, living in the Atlantic Ocean and in the Mediterranean.

It has a much compressed body and can approach, unseen, the prey (fishes) which it quickly grasps with its protractile mouth.

Order Lampridiformes king fish, regalecus, ribbonfish (50 species).

They live in deep waters. They have a protractile mouth, anterior ventral fins and skin without scales. It is matter, for instance, of the King Fish (Lampris regius), 1,80 m long. Very diffused in the oceans, is oval and with a very compressed body on the sides; it weighs almost 300 kg! It eats squid and octopus.

Order Gasterosteiformes, fishes like the three-spined stickleback, the seahorse and the pipefish (150 species).

The fishes of this order have an elongated body, more or less protected by a bony cuirass.

The Hippocampus guttulatus is a Mediterranean seahorse © Giuseppe Mazza

The Hippocampus guttulatus is a Mediterranean seahorse © Giuseppe Mazza

The mouth is small, often placed at the extremity of an elongated snout.

An example can be the Three-spined stickleback (Gasterosteus aculeatus), present both in Arctic and temperate regions and in fresh and brackish waters.

It has a very ample diffusion, spreading from the Pyrenees up to the oceanic coasts of Siberia.

It lives in several rivers, between which the Danube (for such reason it was one of the biologic subjects preferred by Nikolaas Tinbergen and, partially, also by Konrad Lorenz, two of the three biologists who, together with Karl von Frisch, founded the Ethology and won the Nobel Prize for the physiology and Medicine in 1973), and in rivers tributary to the North Sea, Baltic Sea and Black Sea, as well as to the Atlantic Ocean and the western Mediterranean Sea.

It is capable to live well and reproduce even in the seas with modest salinity (Black Sea and Baltic).

For reproducing, it is in need of very clean and limpid waters.

The mating between male and female, before the laying and following fecundation of the eggs, is composed by very complex rituals.

These ones were brilliantly studied by the biologists N. Tinbergen and K. Lorenz; the male during the heat season, once identified the female, will be subjected to a modification of the colour in the ventral part of the body, which will assume a more intense bright red colour, while in the mean time the eyes will become bright blue.

In the complicated world of the seahorse, it’s the male who delivers © Giuseppe Mazza

In the complicated world of the seahorse, it’s the male who delivers © Giuseppe Mazza

These changes determine the “nuptial livery” which will be later on subject to “eclipsing”, once the mating will be completed.

By then, the male gets again its natural colours, brownish on the back and silvery with dark spots on the sides.

The ventral red dot, in particular, will serve as sexual attractor for the female.

In this phase the males become dangerously aggressive, fighting, utilizing their three powerful dorsal spines and biting, till to cause injuries or even to kill, in the extreme cases.

The male builds up a nest in the corner of a rock or in a rocky ravine, formed by air bubbles, debris and small branches, where the water, flowing, provides the necessary oxygen, and will survey it carefully.

The female, once laid the eggs will be sent away by the male who will take care of their hatching after the fecundation.

New borns of seahorse © Giuseppe Mazza

New borns of seahorse © Giuseppe Mazza

Also in this species, the male, periodically, will agitate its tail by the opening of the nest, thus guaranteeing a constant input of oxygen.

The male will also assume the parental cares towards the fry, which, during the day, are left free to swim and nourish, with the father remaining close to them for constantly verifying the presence of possible predators.

If, by chance, a situation of danger takes place, the male opens the mouth wide and all fry will shelter there.

They will leave the nest once reached the sexual maturity for going to look for a female with which mate.

Another interesting example is the Long-snouted seahorse (Hippocampus guttulatus), long up to 15 cm which is a littoral fish of the Mediterranean and recently found also in the Channel waters.

In spring, the female lays the eggs in a ventral pouch of the male, where they will be fecundated and then incubated.

Channa micropeltes has an accessory breathing organ for resisting long time out from the water © Mazza

Channa micropeltes has an accessory breathing organ for resisting long time out from the water © Mazza

The new born will remain inside this for some days, then will get and will swim freely.

Order Channiformes, ophiocephalus (five species).

Freshwater fishes, provided with an additional respiratory organ which allows them to remain out from the water for long time. An example stands in the Snakehead murrel (Channa striata), long up to 75 cm.

This fish lives in rivers, ponds and marshes of Asia.

It can survive to drought burrowing into the mud and remaining in cryptobiosis, a sort of torpor.

Order Synbranchiformes. Rice-eels and like (seven species).

It’s a small group of freshwater fishes of southern Asia and of Africa. They do not have any close affinity to the eels, even if their outer morphology can recall them.

Pterois radiate, a luxury clearfin lionfish © Giuseppe Mazza

Pterois radiate, a luxury clearfin lionfish © Giuseppe Mazza

They do not have pair fins and do not have scales.

An example is the Rice-eel (Amphipnous cuchia), which is about 75 cm long.

It lives in the fresh and brackish waters of India and Burma.

It breathes air staying for long time out from the water in the grass, close to the water streams.

This is possible because of the presence of two auriferous sacs, similar to the lungs, connected to the gill cavities.

These sacs operate like the aeriferous sacs of the birds and are present also in some reptilians; they allow getting a supply of atmospheric air which can be utilized during the phases of external life.

Order Scorpaeniforems, red scorpionfish (whiskery scorpionfish), tub gurnards and bullyheads (700 species).

They have bony crests and thorns on the head, typical of these fishes, which, sometimes, are commonly defined as “with cuirassed cheeks”.

A well known specimen is the Red scorpionfish, or Whiskery scorpionfish (Scorpaena porcus), about 20 cm long, common in the rocky bottoms of the Mediterranean, and main ingredient, in spite of its will, in the Italian cooking, for the preparation of the delicious chowder.

Trigla lucerna © Giuseppe Mazza

Trigla lucerna © Giuseppe Mazza

The Tub gurnard (Chelidonichthys lucerna), about 75 cm long. It is common along the coasts of the Mediterranean and of the eastern Atlantic. The dactylopters (6 species) belong to the order of the Dactylopterifomrs (Dactylopteriformes). They are typical due to their ample pectoral fins.

The head is cuirassed by bony plaques. Example of this group is the Dactylopterus orientalis 38 cm long.

It lives close to the coasts of the Indian and Pacific Oceans.

It has pectoral fins bigger than those of the flying fishes, but these do not allow it to effect jumps out of the water.

Order Pegasiforems, dragonfishes (4 species).

Small fishes, cuirassed, typical of the Indo-Pacific region, with great and aliform pectoral fins and small mouth placed under the snout.

Dactylopterus volitans © Giuseppe Mazza

Dactylopterus volitans © Giuseppe Mazza

An example is the Dragonfish (Pegasus draconis), 12 cm long, living between the corals in shallow waters.

Order Perciformes, perches, mackerels, tuna, groupers, basses, sharksuckers, swordfishes, marlins (6.500 species).

Belong to this order the famous Barracuda, suborder (Scombroidei), which, as well as the Sphyranea barracuda, live in all the tropical seas, particularly in the Caribbean.

But they are present also in the western Atlantic and in the Mediterranean Sea, especially along the coasts of Sardinia and Corsica.

They are carnivorous-piscivorous, very ferocious and greedy.

Murderous attacks against human beings have been registered during the years ’30, ’50, ’60 and ’70 of the last century.

The Epinephelus marginatus is a protected Mediterranean species © Giuseppe Mazza

The Epinephelus marginatus is a protected Mediterranean species © Giuseppe Mazza

During the mating period they unite in rather populated groups.

Probably a female may be fecundated by more than one male, polygamy.

It is a very heterogeneous group, where we find the Tuna (Thunnus thynnus) or the European seabasses (called “branzino” in northern Italy, as well as the Dicentrarchus labrax or “ragno” and “lupo di mare” along the Tuscany coasts), the groupers and several other species.

Many of these species have spiniform rays in the fins.

The caudal fin has 17 rays; the scales have often a jagged edge.

Good examples are the Heckel discus (Symphisodon discus), 22 cm long, located in the fresh waters of South America, or the Blue-spotted grouper (Cephalopholis argus), long up to 1 m and living close the coral reefs in the Indian and Pacific Oceans. Some species of groupers may reach the 3 m of length per 100 kg of weight, like the Epinephelus lanceolatus.

Cephalopholis miniata © Giuseppe Mazza

Cephalopholis miniata © Giuseppe Mazza

They are protogyny hermaphrodite organisms, which means that the juveniles are mostly females, and transform into males when growing up.


The growth if of about 1 kg per year, and usually, the groupers remain adolescent till when they reach the three kilos of weight; they nourish of fishes.

The Perch (Perca fluviatilis), 60 cm long, lives in lakes, rivers and ponds of Eurasia.

The bio-geographic analysis of the biologists have concluded that the perches, characids and carps live only in fresh waters, as they cannot go into the marine and usually more salty ones.

The cichlids, on the contrary, can perform short passages in salty water.

The Atlantic Mackerel (Scomber scomber), long up to 60 cm, lives in surface waters close to the coasts of the Atlantic Ocean.

The well-known Live sharksucker (Echeneis naucrates), about 75-80 cm long, lives in the tropical oceanic zones. It is always associated with sharks, mantas and sea turtles.

The famous Discus fish (Symphysodon aequifasciata) much loved by the aquarists © Giuseppe Mazza

The famous Discus fish (Symphysodon aequifasciata) much loved by the aquarists © Giuseppe Mazza

It keeps stuck, by means of a sucker generated by a modified dorsal fin, to the host which transports it: the belly of a shark or of a manta, or the carapace of a turtle.

On one side, it cleans its host by the pests and on the other it nourishes of the leftovers from the banquets of these animals.

It’s a real symbiosis.

To this order belong also the members of the family of the Istiophoridae, like the Marlin (Istiophorus albicans) and (Istiophorus platypterus), they have a length of 2,5 m, a lance-shaped snout and a long rigid dorsal fin extending back to form a crest.

The marlins are fast swimmers; they can be found in all seas and hunt big and small fish.
Or, the so-called Swordfish (Xiphias gladius), family Xiphiidae.

Specimens of Xiphias gladius are spread in the tropical, subtropical and temperate zones of all the oceans, and also in the Mediterranean Sea, Black Sea, Azov and Marmara Seas, where they tend to concentrate when the marine currents are so strong to attract huge crowds of more or less big fishes which they will eat.

The well-known Perch (Perca fluviatilis), much loved in Italy by the gourmets © Mazza

The well-known Perch (Perca fluviatilis), much loved in Italy by the gourmets © Mazza

These fishes may reach huge dimensions, with a maximum length of more than 4,6 m and a weight well exceeding the 400 kg.

Order Pleuronectiformes, flatfish (500 species).

Having no “swim or gas bladder”, they live almost all the time on the sea bottom, laying on a side.

After the larval phase, one eye moves on the opposite side where is located, in such a way that both eyes end for being on the same side.

This process of ocular migration is still to be clarified in its physiology by the biologists.

The fish, therefore, lies on the bottom leaning on the “blind” side.

The soles and the plaices have both eyes on the right side, the turbots on the left one.

The reason of this difference of lateralization should have its explanation at embryonal level.

The flatfishes have derived evolutionally from the Perciforms (Perciformes), but do not have thorns. A well known example is the European plaice (Pleuronectes platessa).

The White-eyed flounder (Bothus podas) has no swim bladder, and one eye shifts to the upper side © Mazza

The White-eyed flounder (Bothus podas) has no swim bladder, and one eye shifts to the upper side © Mazza

75 cm long, it lives along the north-western coasts of Europe. It blends with the bottom and buries into the sand.

Order Tetraodontiformes, the colourful triggerfish, the unusual boxfish and the pufferfish (250 species).

They are fishes with a small mouth, robust teeth and tall body, short and often covered by thorns.

The ventral fins are either small or absent; the anterior one may have some thorns.

Examples are the Bandtail puffer (Sphoeroides spengleri), which may be even 48 cm long.

It lives close to the coral reefs of the tropical and subtropical Atlantic.

It is provided with spinulas instead of scales and when it inflates because in danger, these ones, stiffening, form a valid defensive barrier.

The Lactoria cornuta is a typical Boxfish © Giuseppe Mazza

The Lactoria cornuta is a typical Boxfish © Giuseppe Mazza

Subclass Sarcopterygii, fishes with lobed fins. The second subclass of fishes with bony skeleton, that of the fishes with lobed fins, did originate the amphibians (please see natural history, above). The fins are supported by big fleshy lobes which contain skeletal pieces.

Presently, there are two orders.

Order Crossopterigii, with only one extant species: the Coelacanth (Latimeria chalumnae), up to 1,5 m long and weighing 90-100 kg.

The first living specimen was caught in 1938, by chance, by African fishermen, off the coasts of Madagascar.

Till then the group of the coelacanths was known only, as a fossil, to the biologists and it was thought to have extinguished since 60 million of years!

It has robust fins which are utilized for removing the bottom; it is a greedy and pitiless predator of fishes.

Order Dipnoi, lungfishes (6 species). These fishes belong to two families: the Ceratodids (Ceratodidae), with only one species, Australian, having a stocky body, great scales and pair fins, equipped with rays and a single lung, and the family of the Lepidosirenids, or Lepidosirens (Lepidosirenidae) endemic to Africa and South America.

They have a thin body, small scales, pair fins without rays and get the 95% of the oxygen they necessitate for living, utilizing a double lung. The members of both families live in aquatic environments poor of oxygen and in very warm biotopes.

An example is the Queensland lungfish (Neoceratodus forsteri), long up to 90 cm. It does not build dens in the mud for overcoming the drought periods, so it dies if the river where it lives dries up.

Comparative physiology of organs and systems, the dilemma of the “swim or gas bladder”

Balistapus undulates. As the shape and name suggest, it’s a Triggerfish © Mazza

Balistapus undulates. As the shape and name suggest, it’s a Triggerfish © Mazza

The Circulatory system: the chondrichthyes and the osteichthyes, in principle, have a similar anatomy for this system.

In the fishes the heart is simple with one “atrium” and one “ventricle”, there is no duplication (through a division with interatrial and interventricular septa) of these structures, which begins to emerge in the reptilians, for then, later one, to finally overt in the class of the birds and the mammals.

The heart consists of an atrium which is, upstream, connected with a “venous sinus”, from which the not oxygenated blood, the downstream atrium is in communication with a ventricle, which is followed in a “conus arteriosus”.

The system comes from that of the lamprey and of the amphioxus, talking in an evolutionary way.

In the elasmobranchii, however, this organ is more robust than in the cyclostomes and maintains a pressure of 30-40 mmHg (millimetres of mercury) in the ventral aorta. In the osteichthyes as well as in the elasmobranchs, the blood pressure is of about 40 mmHg in the ventral aorta, during inactivity, and then reduces of 10-20 mmHg when the blood passes through the gills. As a consequence, the circulation is very slow and the venous blood pressure in the recirculation is very low.

Balistoides conspicillum is the king of tropical marine aquaria © Mazza

Balistoides conspicillum is the king of tropical marine aquaria © Mazza

The highest ever measured blood pressure has occurred in the Pacific Salmon (Onchorhynchus iscawytscha), well known to the zoologist biologists for its vigour.

In its ventral aorta they have recorded about 120 mmHg. In both classes members, on the relevant outgoing branchial arteries, there are “pressure receptors” which send impulses to the encephalon.

The brain replies to the information it has received and regulates, consequently, the blood pressure slowing down or accelerating the heartbeats. This mechanism permits to protect the branchial capillaries against the pressure changes.

Both in the bony fishes and the cartilaginous ones, the blood flows from the heart, anteriorly, in a massive “ventral aorta” which divides to form a couple of “innominate arteries”.

The gill branches generate from these last ones.

From the outgoing branchial arteries taking away the oxygenated blood from the gills the same is carried to the head by means of the “carotidal arteries” and, for the entire trunk, by means of a big arterial median impair vessel, the dorsal aorta.

The pectoral fins are receiving by means of a couple of “subclavian arteries”, whilst an impair arterial vessel, the “celiac artery”, irrigates the stomach, liver and part of the intestine.

The pelvic fins, when present in the osteichthyes and in those of the cartilaginous are irrigated by a couple of “iliac arteries”. The caudal artery forms from the continuation of the “dorsal aorta”.

The venous blood, effluent from the head, reaches the “ducts of Cuvier” and the sinus venosus through the “anterior cardinal veins”.

The venous blood of the trunk splits in that coming back from the intestinal walls, drained by the “hepatic portal vein”, by means of two hepatic veins, to the sinus venosus.

That from the tail flows in two renal veins (right and left), for recovering in the “posterior cardinal veins”, which, collected the blood also from the gonads, as well as from the mesonephric kidneys, end into the “ducts of Cuvier” and from here, to the heart.

Striking picture of the gills of a trout © Giuseppe Mazza

Striking picture of the gills of a trout © Giuseppe Mazza

The only difference found in the osteichthyes is that the “conus arteriosus” is smaller than in the cartilaginous, and the series of valves present reduces to one, whilst the initial tract thickness of the ventral aorta increases, transforming in an “arterial bulb”.

The “lymphatic conduits” in the Actinopterygians (Actinopterygii) are much more separated from the venous system than in the Agnaths (Agnatha) and in the elasmobranchs (Elasmobranchii).
They do not have valves and lymph glands, but in some species of Teleostomi, there are lymphatic hearts which facilitate the draining of the lymph in the venous system.

The erythrocytes are less numerous than in the tetrapoda, but are bigger. The quantity of haemoglobin (Hgb) varies a lot depending on the species, being greater in the very active and better swimming ones and living in poorly oxygenated waters. Some Antarctic species, fairly static, do not even have this respiratory pigment. The Endocrine system is, in both classes, phyilogenetically old and as much developed as that of the terrestrial tetrapods.

The Hypophysis (pituitary gland, appendix of the diencephalon), produces hormones controlling the reproductive activity, both in males and in females. It does no excrete, however, neither “vasopressin”, nor “oxytocin”.

The gonads are controlled by the pituitary “fullicolins”, secreting steroid hormones (estrogens, androgens) involved in the ovulation and the spermatogenesis.

Scales called Mustelus shagreen, once used as sandpaper © Giuseppe Mazza

Scales called Mustelus shagreen, once used as sandpaper © Giuseppe Mazza

There is a thyroid gland, originating from the pharynx, as an eversion, during the embryogenesis; the thyroid hormones, like for all vertebrates, are important regulators of the metabolism and, like amphibians, in the species with metamorphosing larva (embryo), such as Anguilla anguilla, are indispensable for a correct development. There is a “chromaffin” tissue, biologic equivalent of the suprarenal (adrenal) glands in the mammals.

It produces hormones such as the “adrenaline”, called also “epinephrine” and “noradrenaline”, or “norepinephrine”.

There are also the “inter-renal organs”, similar to suprarenal cortex of mammals, producing “corticosteroid glucoactive” hormones, and “mineralocorticoid corticosteroid” hormones, under the control of the adrenocorticotrophic hormone “ACTH”. The Pancreas has the “Islets of Langerhans”, as endocrine component, which regulate the glycemia.

Digestive System: the derivation from a detritivorous-filtrating alimentary regimen, like that of the Cyclostoma, to a solid one (formed by preys), like for the cartilaginous and bony fishes, has required an anatomic-physiologic rehash, of the digestive system in evolutionary terms.

Therefore, there was the passage from a simple absorbing “alimentary duct” of the early Agnatha, to the formation of a gastric pouch (stomach), possibly coming from an evagination of the esophagus, gradually more and more specialized, where was started the production of hydrochloric acid (HCl), as means of digestion, but also for avoiding the putrefaction of the food, caused by an increase of the bacterial flora fought by this acid which acts as bacteriostatic.

Ctenoid scale of Trachyrhynchus scabrous - Ganoid scale of Calamoichthys - Spiny scale of Coelorhynchus coelorhynchus © Giuseppe Mazza

Ctenoid scale of Trachyrhynchus scabrous – Ganoid scale of Calamoichthys – Spiny scale of Coelorhynchus coelorhynchus © Giuseppe Mazza

Ctenoid scale of Scorpaena - Cycloid scale of Barbus - Cycloid-proximal scale of Rhombus © Giuseppe Mazza

Ctenoid scale of Scorpaena – Cycloid scale of Barbus – Cycloid-proximal scale of Rhombus © Giuseppe Mazza

In the chondrichthyes as well as in the osteichthyes, there are glands annexed to the digestive system, liver, pancreas and gallbladder. In the Selachii, as well as in the Batoidea and the Chimaeriformes, the short intestine amplifies its absorbing power by means of a “spiral-shaped valve” which lengthens and slows down the route of the bolus. In the osteichthyes the absorbing duct (intestinal) is longer. The Nervous system has, in both classes, a central component, the brain (encephalon), well developed and contained in a bony capsule in the osteichthyes and in a “cartilaginous neurocranium” in the cartilaginous, acerebellum and a brainstem.

Swim bladder of Bleak (Alburnus alburnus), a “Physoclistic” Cypriniformes. Left on top we note a white component rich in fat surrounding the pharynx, in red. For this reason, in the past, it was called “fat swim bladder” © Giuseppe Mazza

Swim bladder of Bleak (Alburnus alburnus), a “Physoclistic” Cypriniformes. Left on top we note a white component rich in fat surrounding the pharynx, in red. For this reason, in the past, it was called “fat swim bladder” © Giuseppe Mazza

The neurocranium of the cartilaginous, together with the “splanchnocranium”, or “viscerocranium”, from which generate the gill slits, forms the “chondocranium”.

There is also an Autonomic Nervous System which controls the non voluntary activities and all visceral organs. There are also cranial and spinal nerves. The sense organs, “Ampullae of Lorenzini” included, integrate by means of these structures. The Urogenital system: it consists of two kidneys at the stage of “mesonephros”, preceded by that of pronephros at embryonic level.

In the females, both in the osteichthyes and the chondrichthyes, the urinary tract is clearly separated from the reproductive one; in the males the ureters are utilized also for the emission of the sperm besides that of the urine. At the level of the cloaca, there is, in the chondrichthyes, a sac-like structure which operates as “urinary bladder” even if not similar to that of the superior vertebrates.

The mesonephric kidneys of the Elasmobranchii have an additional segment located in the renal tubules, a “segment capable to reabsorb the Urea”, the active osmolyte, present in the blood of these fishes, which allows them, depending on the haematic concentration, to live in the sea, fresh or estuary water, as we have seen for the “euryhaline” sharks, for instance the South American “Rajiformes”.

In the Elasmobranchii, which have an internal fertilization (through the “claspers”) of the males, the “ovaries” of the females, they have a development more similar to the amphibians than to the other tetrapods, Gnatosthoma and fishes; they are, usually, paired organs, single in some species, and are, or is, placed behind the liver. They emit the ovules in the “coelomic cavity” which sends them by means of the cilia covering it, to the mouth of the oviducts.

In the oviparous species, inside the oviduct are present “nidamental glands” which express the albumen forming the shell of the eggs. Other species, such as the Bluntnose sixgill shark (Hexanchus griseus), are ovoviviparous (with omphaloplacenta or alloplacenta), whilst others are viviparous. In the oviparous species, the eggs are “telolecithal”, very rich in yolk, at partial discoidal segmentation, enclosed inside a rectangular horny shell, provided with cirri.

In the viviparous species, inside the uterus, during the advanced phases of the foetal development, it is secreted from special glands a particular liquid, the “uterine milk or histotrophic”, which has the charge of nourishing the embryo-foetus under development.

The lateral line is well visible in this electrical Catfish (Malapterurus electricus) © Giuseppe Mazza

The lateral line is well visible in this electrical Catfish (Malapterurus electricus) © Giuseppe Mazza

In the Teleosts (bony fishes par excellence), like the Salmo trutta, the reproductive system is separated from the urinary one both in males and females. In the male, the testicles pour out the sperm in “vas deferens” which forms in the caudal part of the gonad; the two ducts unite in a single duct which leads outside, behind the anus, through a “genital pore”, detached from the excretory pore.

The bony fishes, which are also provided with a couple of mesonephric kidneys, utilize the urinary excretion for the osmoregulation (like the cartilaginous), whether of freshwater or of sea.

The elimination of the catabolites, such as the ammonia (NH3) and in this instance the urea happens rather at the level of the gills.

For what the system of the Lateral line sense is concerned, in both classes, apart some differences, it has a similar anatomic-histological structure, and physiological equivalent; it consists in detecting the surrounding waters vibrations. It is present on both sides of the body, from the tail to the head. It is formed by a thin subcutaneous canal, which, in the head, parts, with a different drawing, depending on the species.

It is thought that in the gregarious fishes the lateral line helps in synchronized swimming © G. Mazza

It is thought that in the gregarious fishes the lateral line helps in synchronized swimming © G. Mazza

It communicates with the environment through small ducts which open via pores on the skin.

In each one of these canal pores are present sensorial hair cells, united in groups of small reliefs, overlooked by a jelly-like cupule, protruding in the lumen of each canal.

The primary function of this organ was, most probably, that of orienting the animal in respect to the currents in the water streams.

Later on, with an improved sensitivity of the fishes, the system of the lateral line became capable to perceive the far away acoustic vibrations and the water perturbations caused by other animals or inanimate objects.

Essentially, it is a remote tactile sensory organ.

The inner ear in both classes does not differ much from that of the other vertebrates, even if the endolymphatic duct in the bony fishes does not open to the outside.

The functions in the cartilaginous as well as in the bony ones, are basically those of maintaining of the muscular tone, the reception of the angular acceleration, of the gravity and the sounds.

The eyes are similar in the two classes and, in general lines, also to those of the other vertebrates. The “cornea” has an irregular surface, not necessarily smooth, as the tissue forming it has more or less the same optical properties of the water, thus not causing any diffraction phenomenon.

The eyes of the fishes have a still iris and a spherical lens © Giuseppe Mazza

The eyes of the fishes have a still iris and a spherical lens © Giuseppe Mazza

The “lens” is spherical, and in the bony fishes, the “suspensory ligaments” and the “lens retracting muscle” to which it is connected, when contracting, push it backwards, focusing it for the far sight.

This is exactly the contrary to what happens in the Elasmobranchii, where the contraction pushes the lens ahead, adapting it to a close vision.

The iris does not move; depending on the species, there are, in different proportions, both cones and rod cells and the quantity of light absorbed is regulated by means of the migration of granules of photosensitive pigment in the retina.

We can now discuss about the real dilemma of the swim or gas bladder. For centuries, this has vexed the zoologist biologists for explaining its functions and the evolution origin.

Sac-like diverticulum, mostly simple, at times double, derived from the pharynx. It is present, even if with different functions, in almost all the Teleostomi and in all the Dipnoi.

Thanks to the swim bladder the Calamoichthys calabaricus has some autonomy out from water © Mazza

Thanks to the swim bladder the Calamoichthys calabaricus has some autonomy out from water © Mazza

It is absent in the Chondrichtyes and in the Agnatha.

Some biologist zoologist think that it might have already existed in some Placodermi, but this hypothesis is not accepted by all.

In the most archaic species between the present Actinopterygians (Actinopterygii), genera Polypterus and Chalamoicthys, order of Polypteriforms (Polypteriformes), living in the African freshwaters, subject to warming and consequently losing oxygen, the swim or gas bladder originates from the ventral side of the pharynx and parts in two lateral lobes which tend, during the development, to shift laterally.

Its wall contains arteries coming from the last arch of the aorta and which operate as pulmonary arteries and, during the periods of low oxygenation in the water, it fills of atmospheric air through the pharynx, helps the breathing activity of the gills, where, when going counter current, do happen the gassy exchanges between water and blood.

This particular function, however, it is not so important to allow the fish to survive for very long periods outside from the water.

In the Protopterus dolloi the swim bladder has even greater breathing functions © Mazza

In the Protopterus dolloi the swim bladder has even greater breathing functions © Mazza

A similar function is found in other archaic actinopterygians of the North American freshwaters, like the genus Lepisosteus of the order Semionotiformes and the genus Amia, order Amiformes, where this organ generates, however, from the dorsal side of the pharynx, and is distinctly placed on the back in respect to the alimentary canal, separating, only partially, in lateral lobes.

Although the wall of the gas bladder of these last fishes is folded and alveolated, thus increasing the breathing surface, its functionality is not however sufficient for allowing these animals to carry a subaerial life.

A similar alveolate look, associated to a type of pulmonary breathing, is even more evident in the bladder of the Dipnoi, which, however, comes, like for the Polypteriformes, from the ventral side of the pharynx, and part in two lateral-dorsal lobes in Lepidosiren and Protopterus, whilst in Neoceratodus, it remains single.

In the sturgeons (Acipenseriformes) and in all the Teleostei (the most evolved Teleostomes), the bladder, evolutionary talking, assumes quite a different role. Always single, it is placed on the median line of the body, dorsally to the intestine and in retroperitoneal position; it generates from the dorsal side of the pharynx, or from a side of the pharynx, to which often remains connected through a retroflexed duct.

In snappy fishes, like the Seriola dumerili, the swim bladder is an important hydrostatic organ © Mazza

In snappy fishes, like the Seriola dumerili, the swim bladder is an important hydrostatic organ © Mazza

Only in an extremely limited number of cases, for instance in the genus Erythrinus of the family of the Characins (Characidae), order Cypriniforms (Cypriniformes), living in South American freshwaters, often poorly oxygenated, it maintains respiratory functions, being at least in part provided of blood vessels, pulmonary type arteries, coming from the last arch of aorta.

Normally, however, the blood which irrigates its wall is already well oxygenated as it comes from arteries coming from the dorsal aorta, caudal to the gill regions.

Therefore, the gasses filling it do not come from the atmosphere, but from the blood.

As a matter of fact, they are mostly oxygen with a certain percentage of nitrogen and are provided by particular structures called “rete mirabile”, placed in the front region of the wall of the swim or gas bladder, thickly covered by capillaries

In many groups of teleosts, between which the most ancient (Clupeiformes and the Cypriniformes) the gas bladder keeps its connexion with the pharynx (and the gullet), by means of a “pneumatic duct” which is utilized for emptying into the pharynx the exceeding gas.

As would otherwise do this darting Labrus mixtus in love, in nuptial livery © Mazza

As would otherwise do this darting Labrus mixtus in love, in nuptial livery © Mazza

The teleosts, which are equipped with this, are called “physostomous”.

In other groups, among which the most evolved Perciforms (Perciformes), the pneumatic duct, on the contrary, occludes and regresses, and the excess of gas is then reabsorbed by richly vascularised zones, from the wall of the back part of the gas bladder, at times concentrated in a structure called “oval body”, whose shape is regulated by a muscular sphincter which surrounds it. The teleosts not having this “pneumatic duct” are called by the zoologist biologist “physoclisti”.

The function of the swim or gas bladder which, in these more evolved species, coincides better with its name, it is not any more respiratory, but of an “hydrostatic organ”, with hydrostatic sensitivity, as it operates as water floating and positioning for the fish. In these fishes the swim bladder is full of gasses coming from the blood. Its walls are riche of nervous sensitive terminations which register the pressure exerted by the gasses contained in it and which also regulate their exit or reabsorption.

In this way, a status of dynamic balance is maintained between the hydrostatic pressure done by the environmental water on the body of the fish at the level of the “lateral line” and the fish itself. The bony fishes, good swimmers, get in this way precise information about the depth of the water where they are swimming, both during the ascent or the descent in it (inflating or deflating the bladder) and also in the horizontal swim.

But it is of no use for Uranoscopus scaber. It lives in the sand and attracts preys with a vermiform line © Mazza

But it is of no use for Uranoscopus scaber. It lives in the sand and attracts preys with a vermiform line © Mazza

They regulate the modalities of swimming, after the information received from the lateral line and from the walls of the bladder, and integrate them.

The floating of the fish is caused by the swim bladder, which, filling of gasses coming from the blood, causes a variation in the specific weight in water of the fish.

In the cypriniforms the swim bladder is divided in an anterior and a posterior lobe, the first presses against the “membranous labyrinth” of the inner ear through a chain of ossicles, coming from the cephalic portion of the spine, forming the “Weberian apparatus”.

This correlates the hydrostatic function, furnished by the swim bladder and the regulation of the equilibrium and of the position of the fish, by means of the inner ear.

Some forms of teleosts do not have swim or gas bladder and carry on a benthic life (on the marine or freshwater bottom); then we have the genus Trachinus and Uranoscopus, belonging to the order of the Perciformes, which live interred into the sand of the coastal and meso-littoral sea-beds. Or in the Pleuronectiforms order (Pleuronectiformes), such as the soles and turbots, where the adult is benthic and has no gas bladder, whilst the sub-adult and young forms are pelagic because provided of swim bladder which later on degenerates.





→ To appreciate the biodiversity within the Osteichthyes, the BONY FISH, please click here.

→ To appreciate the biodiversity within the Chondrichthyes, the CARTILAGINOUS FISHES please click here.