Giant Sea Isopods

A true monster of the deep, with a length of up to 50cm being the largest scientifically confirmed measurement. Although some anecdotal reports from fisherman have claimed they have witnessed these super giant isopods measuring more than 76 cm long!

Giant sea isopods are one of 20 species of large isopod within the genus Bathynomus. Giant sea isopods are a good example of deep-sea gigantism. They are abundant in the cold, deep waters of the Atlantic, Pacific, and Indian Oceans. The four known Atlantic species are B. obtusus, B. miyarei, B. maxeyorum, and B. giganteus, and the last of these is the only species recorded off the United States. The remaining Bathynomus species are all restricted to the Indo-Pacific ocean.

Interestingly, there are no species that occur both in the Atlantic and Indo-Pacific. The most species rich oceans are found off eastern Australia with five recorded species inhabiting this part of the worlds ocean. In general however, the distributions of giant isopods are imperfectly known, and undescribed species may exist.

A giant isopods morphology is very reminiscent of their terrestrial relatives. Their bodies are dorsoventrally compressed, protected by a rigid, calcareous exoskeleton composed of overlapping segments. Like some isopods, they can curl up into a "ball", leaving their tough shell exposed to protect their soft underside from predators. The first shell segment is fused to the head; the most posterior segments are often fused, as well, forming a "caudal shield" over the shortened abdomen (pleon).

They have large eyes which are compounded with nearly 4,000 facets, sessile, and spaced far apart on the head. They have two pairs of antennae. The uniramous thoracic legs or pereiopods are arranged in seven pairs, the first of which is modified into maxillipeds to manipulate and bring food to the four sets of jaws. The abdomen has five segments called pleonites, each with a pair of biramous pleopods. These are modified into swimming legs and rami, flat respiratory structures acting as gills. Giant isopods are a pale lilac or pinkish in colouration.

Giant isopods like Bathynomus giganteus store substantial organic reserves in their midgut gland and fat body, with lipids forming a significant component, particularly in the fat body where triacylglycerols make up 88% of total lipids.

Giant isopods are important scavengers in the deep-sea environment. They are mostly carnivorous and feed on dead whales, fish, and squid. They may also prey on slow-moving animals such as sea cucumbers, sponges, radiolarians, nematodes, and other zoobenthos, and perhaps even live fish. They are known to attack fisherman’s trawl catches. There have been reports of giant isopods attacking a larger dogfish shark in a deepwater trap by latching onto and eating its face!

They are adapted to long periods of famine which means their metabolism is very low. They have incredibly been known to survive over 5 years without food in captivity. When a significant source of food is encountered, giant isopods gorge themselves to the point that they could barely move. eaten, followed by cephalopods and decapods, particularly carideans and galatheids.

Giant isopods are thought to prefer a muddy or clay substrate and lead solitary lives and are mainly found in the gloomy sublittoral zone at a depth of 170 m (560 ft) to the pitch darkness of the bathyal zone at 2,140 m (7,020 ft), where pressures are high and temperatures are very low. A few species from this genus have been reported in shallower depths, notably B. miyarei between 22 and 280 m (72 and 919 ft), and the poorly known B. decemspinosus between 70 and 80 m (230 and 260 ft), and B. doederleini as shallow as 100 m (330 ft). Over 80% of B. giganteus are found at a depth between 365 and 730 m (1,198 and 2,395 ft). The giant isopods reduced phenotypic divergence is believed to be linked to the extremely low light levels of their habitat due to the depths they prefer to inhabit.

In regions with both "giant" and "supergiant" species, the former mainly live on the continental slope, while the latter mainly live on the bathyal plain. Although Bathynomus have been recorded in water as warm as 20 °C (68 °F), they are primarily found in much colder places. There are some studies which indicate some species will stop feeding or reproducing if temperatures fall below 3 °C (37 °F) suggesting water temperatures are of key importance.

There have been occasional attempts to cook giant isopods to be used in East Asian cuisines line ramen. Relative to a giant isopods total size, there is not very much "meat" to be harvested. The meat is sometimes described as resembling lobster and/or crab in taste, with a somewhat firmer, chewier texture.

If you were interested in keeping one of these giant isopods as a pet, you would need to be prepared to invest in a large, pressurised tank capable of holding gallons of sea water and maintain cool temperatures. There are reported cases of giant isopods in captive aquariums across the world that are thriving. However, it is often a complex and life-threatening process to have giant isopods brought from the deep ocean up to the surface for captive transportation or study. These deep-sea monsters are truly a fascinating species of isopod.