Giant isopods are simply crustaceans, but they are impressive and fascinating for their large size and the difficulty to find them that keeps these animals safe from being systematically fished and killed like other species of crustaceans and fish. Their genus refers to the species Bathynomus giganteus, the largest in the world and discovered in 1879, when a young male was captured in the Gulf of Mexico.
Today, we have classified 9 species in its genus Bathynomus. Their gigantism is common among many deep sea animals like the spider crab or the giant squid, but we ignore the reasons for this, especially among invertebrates. The body structure of the giant isopod is not alien, in the reality, because very similar to that of the much smaller terrestrial woodlouse. This is its closest relative, very frequent in our gardens.
They are frequent on muddy or clay sea bottoms of the Atlantic Ocean, at depth from about -170 (-570 ft) to 2140 m (7020 ft). Here, darkness is total, pressure until 215 atm and temperature very cold, about 4°C. Their environment has changed very little during the Earth’s history and this can explain why also the giant isopod has remained nearly the same during the last 160 million years.
a) Physical description
- The giant isopod has a roughly oval and rather flattened body. Its length is very variable, between 19 and 36 cm (7.5-14.0 in), but the largest individuals are reported up to 76 cm (30 in), just in the species Bathynomus giganteus. Its body is all protected by an exoskeleton formed by a series of rigid segments and, like all isopods, it can roll itself to form a rigid ball against predators.
- The colour of its body is pale-violet or pink.
- The first frontal segment forms the head, dominated by two large and reflecting compound eyes, formed by 4000 facets. The head has also two long and sensible antennae (like the eyes, very useful in the total darkness of deep sea) and the mouth has 4 sets of jaws that perform complex movements to tear and trim food.
- The giant isopod has 7 pairs of legs and the frontal pair is modified to form the so-called “maxillipeds”; these are single-clawed arms that allow it to manipulate food and bring it to the mouth.
b) Food
Giant isopods live as scavengers, eating whatever organic residue of plants or dead animals fallen on the sea bottom, but they are also predators, when possible, and they capture fish, sponges, worms, sea cucumbers and other little creatures. They are adaptable and resistant to hunger when food is too scarce; in fact, we know they can resist up to 2 months, given that someone has performed this cruel test on some giant isopods in captivity.
c) Reproduction
The reproductive period of giant isopods is between winter and spring. After fecundation, the female forms a marsupium in her abdomen to store the eggs until their hatching. These eggs are the largest among marine invertebrates (up to 1.3 cm / 0.5 in) and this is not incredible, considering the dimensions of this invertebrate. We don’t know how much their incubation lasts, but the eggs produce little miniatures of the adults, not larvae. They are fully developed, except for the last pair of legs; bypassing the larval phase is an advantage because they are more autonomous and with more chances of reaching adulthood.
d) Scientific classification
Phylum: ................ Arthropoda
Subphylum: ......... Crustacea
Class: ................... Malacostraca
Order: .................... Isopoda
Suborder. ............. Cymothoida
Family: .................. Cirolanidae
Genus: .................. Bathynomus
