A giant isopod is any of the almost 20 species of large Isopoda 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.][ 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
Giant isopods are of little interest to most commercial fisheries, but are infamous for attacking and destroying fish caught in Trawling.[
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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 are typically between .[
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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 can curl up into a "ball", where only the tough shell is exposed. This protects 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 eye with nearly 4,000 ommatidium, , and spaced far apart on the head.
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 ) of the central spine on the pleotelson, and the shape and structure of the and .
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 Triglyceride make up 88% of total lipids.
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.[
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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 unknown 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.][
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Ecology
Giant isopods are important in the deep-sea benthic zone. 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.[
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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 .][ Although Bathynomus have been recorded in water as warm as , they are primarily found in much colder places.]
In contrast, preliminary studies indicate that B. doederleinii stops feeding when the temperature falls below .[ They are thought to prefer a muddy or clay substrate and lead solitary lives.
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Although generalist scavengers, these isopods are mostly carnivore and feed on dead whales, fish, and squid. They may also prey on slow-moving animals such as Holothuroidea, , , , 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 its face.
As food is scarce in the deep-ocean biome, giant isopods must 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.
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[ (although some differences are seen, 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. 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 or brood plates grown from the medial border of the . The young isopods emerge from the marsupium as miniatures of the adults, known as . This is not a stage; the mancae are fully developed, lacking only the last pair of pereiopods.
Human consumption
There have been occasional attempts at utilizing giant isopods as novelty food, such as prepared in East Asian cuisine like ramen. Relative to 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.
Classification
The genus currently contains the following species:
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Bathynomus affinis
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Bathynomus brucei
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Bathynomus bruscai
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† Bathynomus civisi
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Bathynomus crosnieri
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Bathynomus decemspinosus
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Bathynomus doederleini
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Bathynomus giganteus
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Bathynomus immanis
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Bathynomus jamesi
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Bathynomus kapala
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Bathynomus keablei
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Bathynomus kensleyi
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† Bathynomus kominatoensis
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Bathynomus lowryi
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Bathynomus maxeyorum
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Bathynomus miyarei
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Bathynomus obtusus
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Bathynomus pelor
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Bathynomus propinquus ( nomen dubium)
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Bathynomus raksasa
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Bathynomus richeri
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† Bathynomus sismondai
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† Bathynomus steatopigia
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† Bathynomus undecimspinosus
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Bathynomus yucatanensis
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Bathynomus vaderi
Fossilized species
Fossilized specimens of Bathynomus are known extending back to at least the Early Oligocene (Rupelian) of Italy, with other fossils being known from Japan and Spain.
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