Bathynomus Jamesi

A giant isopod is any of the almost 20 species of large isopods, crustaceans distantly related to shrimp and crabs, which are decapods, in the genus ''Bathynomus''. They are abundant in the cold, deep waters of the Atlantic, Pacific, and Indian Oceans.Lowry, J. K. and Dempsey, K. (2006). ''The giant deep-sea scavenger genus Bathynomus (Crustacea, Isopoda, Cirolanidae) in the Indo-West Pacific.'' In: Richer de Forges, B. and Justone, J.-L. (eds.), Résultats des Compagnes Musortom, vol. 24. Mémoires du Muséum National d’Histoire Naturalle, Tome 193: 163–192. '' 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''). The giant isopods are noted for their resemblance to the much smaller common woodlouse (pill bug), to which they are related.

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, but are infamous for attacking and destroying fish caught in trawls. Specimens caught in the Americas and Japan are sometimes seen in public aquariums.

Description

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 . ''Bathynomus'' can be divided into "giant" species where the adults generally are between long and "supergiant" species where the adults generally are between . One of the "supergiants", ''B. giganteus'', reaches a typical length between ; an individual claimed to be long has been reported by the popular press, but the largest confirmed was .

Their morphology resembles that of their terrestrial relative, the woodlouse. Their bodies are dorsoventrally 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. 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. The isopods are a pale lilac or pinkish in colour.

The individual species generally resemble each other, but can be separated by various morphological features, notably the number (7–13) and shape (straight or upturned) of the spines on the pleotelson ("tail"), shape (simple or bifid) of the central spine on the pleotelson, and the shape and structure of the uropods and pereopods.

Range

Giant isopods have been recorded in the West Atlantic from the US state of Georgia to Brazil, including the Gulf of Mexico and the Caribbean. 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.

No species occur in both the Atlantic and Indo-Pacific. Previous records of ''B. giganteus'' from the Indo-Pacific are now considered misidentifications of other species. 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

Giant isopods are important scavengers in the deep-sea benthic environment. They are mainly found from the gloomy sublittoral zone at a depth of to the pitch darkness of the bathyal zone at , where pressures are high and temperatures are very low. A few species from this genus have been reported from shallower depths, notably ''B. miyarei'' between , the poorly known ''B. decemspinosus'' between , and ''B. doederleini'' as shallow as .

The depth record for any giant isopod is for ''B. kensleyi'', but this species also occurs as shallow as . Over 80% of ''B. giganteus'' are found at a depth between . 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 , they are primarily found in much colder places. 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 , but more abundant towards the lower temperature.

In contrast, preliminary studies indicate that ''B. doederleinii'' stops feeding when the temperature falls below . This lower temperature limit may explain their absence from temperate and frigid regions where seas at the depth preferred by ''Bathynomus'' often are colder. 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

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