A giant isopod may be one of approximately nine species of large isopods (crustaceans related to the shrimp and crabs) in the genus Bathynomus. They are thought to be abundant in cold, deep waters of the Atlantic. Bathynomus giganteus, the species upon which the generitype is based, is the largest known isopod and is the one most often referred to by the common name "giant isopod".
Giant isopods are important scavengers in the deep-sea benthic environment; they are found from the gloomy sublittoral zone at a depth of 170 metres (560 ft) to the pitch darkness of the bathypelagic zone at 2,140 metres (7,000 ft), where pressures are high and temperatures are very low – down to about 4 °C (39 °F). Over 80 percent are found at a depth between 365 and 730 metres (1,200 and 2,400 ft). They are thought to prefer a muddy or clay substrate and lead solitary lives.
The first of these segments is fused to the head; the most posterior segments are often fused as well, forming a "caudal shield" over the shortened abdomen (pleon). The large eyes are compound with nearly 4,000 facets, sessile and spaced far apart on the head . There are two pairs of antennae.
The uniramous thoracic legs or pereiopods are arranged in seven pairs, the first of which are 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 natatory legs and rami, flat respiratory structures acting as gills. The isopods are a pale lilac in colour.
Although generalist scavengers, these isopods are mostly carnivorous and feed on dead whales, fish, and squid; they may also be active predators of slow-moving prey such as sea cucumbers, sponges, radiolarians, nematodes, and other zoobenthos, and perhaps even live fish. They are known to attack trawl catches.