Giant Isopod

Giant isopods are important scavengers in the deep-sea benthic environment; they are mainly found from 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.[11] A few species from this genus have been reported from shallower depths, notably B. miyarei between 22 and 280 m (72 and 919 ft),[12] the poorly known B. decemspinosus between 70 and 80 m (230 and 260 ft), and B. doederleini as shallow as 100 m (330 ft).[1] The depth record for any giant isopod is 2,500 m (8,200 ft) for B. kensleyi, but this species also occurs as shallow as 300 m (980 ft).[1] Over 80% of B. giganteus are found at a depth between 365 and 730 m (1,198 and 2,395 ft).[13] 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.[1] Although Bathynomus have been recorded in water as warm as 20 °C (68 °F), they are primarily found in much colder places.[14] For example, during a survey of the deep-sea fauna of Exuma Sound in the Bahamas, B. giganteus was found to be common in water between 3.25 and 13 °C (37.8 and 55.4 °F), but more abundant towards the lower temperature.[15] In contrast, preliminary studies indicate that B. doederleinii stops feeding when the temperature falls below 3 °C (37 °F).[16][17] This lower temperature limit may explain their absence from temperate and frigid regions where seas at the depth preferred by Bathynomus often are colder.[16][17] They are thought to prefer a muddy or clay substrate and lead solitary lives.

Although generalist scavengers, these isopods 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 trawl catches. One giant isopod was filmed attacking a larger dogfish shark in a deepwater trap by latching onto and eating the animal's face; this footage was aired during the 2015 episode of Shark Week called "Alien Sharks: Close Encounters".[18] As food is scarce in the deep-ocean biome, giant isopods must make take advantage of whatever food they have available; they are adapted to long periods of famine and have been known to survive over 5 years without food in captivity.[19][20] When a significant source of food is encountered, giant isopods gorge themselves to the point that they could barely move. A study examining the digestive system contents of 1651 specimens of B. giganteus found that fish were most commonly eaten, followed by cephalopods and decapods, particularly carideans and galatheids.[4]

Giant isopods collected along the east coast of Australia by setting traps exhibit a variation in diversity with water depth. The deeper the water, the fewer number of species found, and the larger the species tended to be. The giant isopods found in very deep waters off Australia were compared to those found off Mexico and India. From the fossil record, Bathynomus is thought to have existed more than 160 million years ago, so it did not evolve independently in all three locations, but since then Bathynomus likely would show divergent evolution in the various locations. However, the giant isopods in all three locations are almost identical in appearance[21] (although some differences are seen, and they are separate species).[1] This reduced phenotypic divergence is linked to the extremely low light levels of their habitat.[21]