Family : Diodontidae
Text © DrSc Giuliano Russini – Biologist Zoologist
English translation by Mario Beltramini
The genus Diodon represents a group of unusual tropical fishes, called also porcupinefishes.
Taxonomically, these marine fishes are afferent to the class of Actinopterygians (Actynopterygii), order of the Tetra- odontiforms (Tetraodontiformes), family of Diodontids (Diodontidae), genus Diodon.
Three very famous species, because much commercialized in the aquariophily, whose trade is strictly controlled by the CITES, are the Long-spine porcupine fish, or Spiny balloonfish (Diodon holocanthus Linnaeus, 1758), the well known Porcupine puffer (Diodon hystrix) and the Spot-fin porcupinefish or Blotched porcupinefish (Diodon maculifer).
Biogeographically, the diffusion of the diodontids is very ample, so much that they are quite frequent close to the coasts and the coral reefs of the tropical countries of the whole world.
The porcupinefishes, in general, and therefore also the spiny balloonfish, do exclusively live in the warm seas, where they are abundant along the coasts and among the coral reefs. They live up to 20-30 meters deep and can also wander between the prairies of marine algae and phanerogams.
They are not object of any organized fishing, as their fleshes, with a sour taste, are also imbued with moderately toxic substances. But, having a much odd look, they are often seized, conserved and sold for the aquariophily as well as for curiosity; in the Chinese market, it is, once desiccated, sold also in the context of the eastern medicine. When they swell, they fill up of air which mainly dilates the belly as this has a softer skin; when they are so much inflated, they float turning the back downwards, but even if being in this unusual position, they can swim. Yet there was no evaluation by the IUCN biologists about the status of health and the density of the populations of Diodon holocanthus, as well as for the other species of the genus Diodon.
The Diodon holocanthus nourishes of sea urchins, crabs and molluscs; if kept in an aquarium, it needs temperatures of 25-26 °C; oligoelements and vitamins are to be added regularly, in order to enhance their resistance in captivity.Morpho-physiology
The most known species amongst the three aforementioned ones is the Diodon hystrix, which is also the greatest, with about 90 cm of length.
The Diodon holocanthus is smaller and reaches approximately the 50 cm.
Morphologically, these two species are much similar between them, apart the dimensions, whilst the Diodon maculifer somewhat differs from the previous ones.
The morphological lines common to the fishes of the genus Diodon are represented by a much stocky body, compressed superiorly, with short and wide head, and small and roundish mouth.
The dental system is composed by only two beak-shaped teeth, one on the jaw and the other on the mandibula; and, to this odd particularity is due the name of the genus Diodon, which rightly means “two-toothed”.
The body is thickly covered by long spiniform processes, which usually keep adherent. But when the animal is upset, the body inflates becoming almost spherical and the spines therefore rise efficiently protecting the fish against a possible predator, just like the balloonfishes afferent to the family of the Tetraodontids.
The tail is entire and with rounded edges, whilst the fins are rather reduced: the dorsal is very short and is placed in hind quarter of the body; the abdominal fins are completely absent and the anal one, similar to the dorsal as shape and size, is placed at the same level of this last one. The colour is brownish on the back, with dark brown spots encircled by white, more or less big of circular or oblong shape, present also at the base of the dorsal fin, on the cheeks and on the sides.
The belly has a whitish colouration; there is no sexual dimorphism between the two sexes. The eyes are very big if compared with the head; this species lives, as an average, 20 years.
It is a fish carrying a mainly solitary life; when in presence of the man, it does not get away, but keeps watchful. It is not aggressive; in some cases it remains motionless, while floating. The porcupinefish is an oviparous species, therefore with external fecundation (ectopic). The female lays the eggs between May and June, in some instances, even in July. Further ethological and reproductive data are not available.
The vulnerability index of this species is of 27 over 100.
Atopomycterus bocagei Steindachner, 1866; Diodon hystrix holocanthus Linnaeus, 1758; Diodon maculifer Kaup, 1855; Diodon multimaculatus Cuvier, 1818; Diodon novemmaculatus Cuvier, 1818; Diodon paraholocanthus Kotthaus, 1979; Diodon pilosus Mitchill, 1815; Diodon quadrimaculatus Cuvier, 1818; Diodon sexmaculatus Cuvier, 1818; Paradiodon quadrimaculatus Cuvier, 1818; Trichodiodon pilosus Mitchill, 1815.