Why Is The “Deep Sea Monster”, Bathynomus giganteus, So Big?

Bathynomus giganteus (Richardson, 1905)
Bathynomus giganteus (Richardson, 1905)

Bathynomus giganteus, first described by French zoologist Alphonse Milne Edwards (1879) from collections made by an expedition by Alexander Agassiz in 1877, is the largest known isopod crustacean, reaching sizes of 46 centimeters (about the size of a large lobster). I mentioned it in this post on cymothoid isopods. It’s a deep sea predator and scavenger that lives on muddy seafloors and near cold seeps 300 to 2000 meters in depth, in the Caribbean, the northern Indian Ocean, and the southern Atlantic.

Other Bathynomus species are also large, between 10 to 30 cm on average, and all are also deep sea creatures. The evolution of these sizes, especially the giant size of B. giganteus, is somewhat similar to the evolution of giant sizes of small animals on islands. A well-supported idea in biogeography states that the deep sea and shallow sea are analogous to islands and the mainland, respectively. Part of the support for this idea comes from the sizes of deep sea animals, such as B. giganteus. Other examples of deep sea gigantism include the ostracod Gigantocypris agassizii, or even just the giant legs of most sea spiders. The cause is decreased temperature and predation, allowing these usually tiny animals to reach giant sizes (Timofeev, 2001). The converse, by the way, is also true: usually large animals get smaller in the deep sea, e.g. ascidians; this is due to lack of food and nutrients (Thiel, 1979).

There is another interesting characteristic of B. giganteus: vision. Bathynomus species are some of the few deep sea organisms to have enlarged eyes, as opposed to the more common trend of reduction due to lack of light. Typical isopods have ~40 facets in each compound eye. B. giganteus has ~3500 (Chamberlain et al., 1986). In addition, their compound eyes have moved from the usual isopod position at the top of the head: they’re located at the base of the antennae. That the eyes have undergone so much change is a hint that vision has been selected for and maintained; however, physiological studies on their vision are currently lacking because they are very sensitive to light. Get them exposed to light while collecting them, and their vision gets irreparably damaged (Chamberlain et al., 1986).

By the way, B. giganteus isn’t an uncommon species to catch in the regions where it’s found, and many fishermen who use deep nets will catch them – much to the fishermen’s dismay, since the crustaceans are voracious predators and will happily feed on the helpless fish trapped alongside them. Marine biologist lore says they can be used as an effective substitute for crab meat, although it’s a bit sweeter.


Chamberlain SC, Meyer-Rochow VB, Dossert WP. 1986. Morphology of the compound eye of the giant deep-sea isopod Bathynomus giganteus. Journal of Morphology 189, 145-156.

Milne Edwards A. 1879. Sur un isopode gigantesque des grandes profondeurs de la mer. Comptes Rendus Hebdomadiares des Séances de l’Académies des Sciences, Paris 88, 21-23.

Richardson H. 1905. Monograph on the isopods of North America. Bulletin of the United States National Museum 54.

Thiel H. 1979. Structural Aspects of the Deep-Sea Benthos. Ambio Special Report 6, 25-31.

Timofeev SF. 2001. Bergmann’s Principle and Deep-Water Gigantism in Marine Crustaceans. Biology Bulletin 28, 646-650.

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