Giant isopod

Giant isopod
A giant isopod and shrimp
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Crustacea
Class:	Malacostraca
Order:	Isopoda
Suborder:	Cymothoida
Family:	Cirolanidae
Genus:	Bathynomus A. Milne-Edwards, 1879
Species
Bathynomus affinis Bathynomus brucei Bathynomus bruscai Bathynomus crosnieri Bathynomus decemspinosus Bathynomus doederleinii Bathynomus giganteus Bathynomus immanis Bathynomus kapala Bathynomus keablei Bathynomus kensleyi Bathynomus lowryi Bathynomus miyarei Bathynomus obtusus Bathynomus pelor Bathynomus propinquus (nomen dubium) Bathynomus richeri

A giant isopod is any of the almost 20 species of large isopods (crustaceans distantly related to the shrimp and crabs, which are decapods) in the genus Bathynomus. They are thought to be abundant in cold, deep waters of the Atlantic, Pacific and Indian Oceans. Bathynomus giganteus, the species upon which the generitype is based, is often considered the largest isopod in the world, though other comparably poorly known species of Bathynomus may reach a similar size (e.g., B. kensleyi).

French zoologist Alphonse Milne-Edwards was the first to describe the genus in 1879 after his colleague Alexander Agassiz collected a juvenile male B. giganteus from the Gulf of Mexico; this was an exciting discovery for both scientists and the public, as at the time the idea of a lifeless or "azoic" deep ocean had only recently been refuted by the work of Sir Charles Wyville Thomson and others. No females were recovered until 1891.

Giant isopods are of little interest to most commercial fisheries owing to the typical scarcity of catches and because ensnared isopods are usually scavenged beyond marketability before they are recovered. The species are noted for resemblance to the common woodlouse or pill bug, to which they are related. The few specimens caught in the Americas and Japan with baited traps are sometimes seen in public aquariums.

Description

A frontal view of Bathynomus giganteus, showing its large, highly reflective compound eyes

Giant isopods are a good example of deep-sea gigantism (cf. giant squid), as they are far larger than the "typical" isopods that are up to 5 centimetres (2.0 in). Bathynomus can be divided into "giant" species where the adults generally are between 8 and 15 centimetres (3.1 and 5.9 in) long and "supergiant" species where the adults generally are between 17 and 50 centimetres (6.7 and 19.7 in). One of the "supergiants", B. giganteus, reaches an average length between 19 and 36 centimetres (7.5 and 14.2 in), with a maximum weight and length of approximately 1.7 kilograms (3.7 lb) and 76 centimetres (30 in) respectively.

Their morphology resembles that of their terrestrial cousin, the woodlouse: their bodies are dorso-ventrally compressed, protected by a rigid, calcareous exoskeleton composed of overlapping segments. Like some woodlice, they also possess the ability to curl up into a "ball", where only the tough shell is exposed. This provides protection from predators trying to strike at the more vulnerable underside. 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). 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 swimming legs and rami, flat respiratory structures acting as gills. The isopods are a pale lilac in colour.

Range

Giant isopods have been recorded in the West Atlantic from off Georgia (USA) to Brazil, including the Gulf of Mexico and the Caribbean. The three known Atlantic species are B. obtusus, B. miyarei and B. giganteus, and the last of these is the only recorded off the United States. The remaining Bathynomus species are all restricted to the Indo-Pacific. Giant isopods are not known from the East Atlantic or East Pacific. The greatest species richness (five species) is found off eastern Australia, but it is possible other regions that are not as well-sampled match this figure. In general, the distributions of giant isopods are imperfectly known, and undescribed species may exist.

Ecology

The underside of Bathynomus giganteus

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 metres (560 ft) to the pitch darkness of the bathypelagic zone at 2,140 metres (7,020 ft), where pressures are high and temperatures are very low – down to about 4 °C (39 °F). A few species from this genus have been reported from shallower depths, notably B. miyarei between 22 and 280 metres (72 and 919 ft), the poorly known B. decemspinosus between 70 and 80 metres (230 and 260 ft) and B. doederleini as shallow as 100 metres (330 ft). The depth record for any giant isopod is 2,500 metres (8,200 ft) for B. kensleyi, but this species also occurs as shallow as 300 metres (980 ft). Over 80 percent of B. giganteus are found at a depth between 365 and 730 metres (1,198 and 2,395 ft). In regions with both "giant" and "supergiant" species, the former mainly live on the continental slope, while the latter mainly live on the bathyal plane. 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 and killing 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". As food is scarce in the deep-ocean biome, giant isopods must make do with whatever comes along; they are adapted to long periods of famine and have been known to survive over five years without food in captivity. When a significant source of food is encountered, giant isopods gorge themselves to the point of compromising their locomotive ability. 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.

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 it is thought that Bathynomus existed more than 160 million years ago, before the break-up of the supercontinent Pangaea, so it did not evolve independently in all three locations, but since then it would be expected that Bathynomus would show divergent evolution in the various locations. However the giant isopods in all three locations were almost identical in appearance (although there are some differences, and they are separate species). This reduced phenotypic divergence is linked to the extremely low light levels of their habitat.

Reproduction

A study of the seasonal abundance of B. giganteus juveniles and adults suggests a peak in reproductive capacity in the spring and winter months. This is observed to be due to a shortage of food during the summer. Mature females develop a brood pouch or marsupium when sexually active, the pouch being formed by overlapping oostegites or brood plates grown from the medial border of the pereiopods. The young isopods emerge from the marsupium as miniatures of the adults, known as mancae. This is not a larval stage: the mancae are fully developed, lacking only the last pair of pereiopods.