From The Desk of Zelnio: Bathynomus giganteus

Bathynomus giganteus (Arthropoda: Crustacea: Isopoda: Cirolanidae)

You know those cute little roly-poly bugs you found under rocks as a kid? You poke at them and they curl up into a little ball? Well, magnify that times 1000, take away the functional role of the eyes, head to the deep-sea and you've got the Giant Isopod, Bathynomus giganteus!

Description Bathynomus giganteus was first discovered in fishermen's nets in the Gulf of Mexico and was described as the type species of the genus by Alphonse Milne Edwards in 1879 (12). It is the largest known isopod, reaching lengths up to 50cm. That is about the length of small dog or cat! Most isopods only reach lengths between 1-5cm. They have a strong and thick exoskeleton and take on the general shape of your typical "flattened" isopod. B. giganteus has 7 pairs of pereopods, or legs, which are uniramous (meaning only one pair of legs per segment). The first pair of pereopods is modified into maxillipeds (literally "mouth feets") that help move food to its 4 sets of jaws modified for cutting and tearing.

Physiology

Interestingly, they have a compound eye with over 3,500 facets (4). The eyes are overgrown by the exoskeleton and exposure to natural daylight causes irreversible damage to their photoreceptors (4). It isn't clear whether these eyes have any function or just evolutionary remnants from a relatively young diversification (8), but they do not respond to the visible spectrum and have only low sensitivity to the ultraviolet spectrum at the 360nm wavelength (4). It is suggested that they rely mainly on chemoreception (4) and possibly mechanoreception (7) or to find prey items.

Isopods have hemocyanin for transport of oxygen, but in B. giganteus it also acts a major phenoloxidase (13). The hemocyanin protein of B. giganteus also has amino acid substitutions at the N-terminus that are more similar to primitive arthropods such as the chelicerates (sea spiders, horsesheoe crabs) and myriapods (centipedes & millpedes) (13).

Nutrition

Gut content analysis of B. giganteus show mostly fish fragments, as well cephalopod, caridean shrimp and galatheid crab remains (3). In smaller quantities, sponge fragments, other isopods, echinoderms, nematodes and tunicates have been found (1, 3). Unfortunately, large quantities of plastic were discovered in the guts of 3 specimens from north of the Yucatan Peninsula in the southern Gulf of Mexico (3). Most authors agree that B. giganteus is a scavenger (3, 5, 6), but some suggest it is also a facultative predator (3, 6). Specimens in aquaria have survived 8 weeks between feedings (5) and it speculated that this may be an adaptation for carrying its brood, which would be severely impacted by a full stomach (3). Further support for this hypothesis are the large quantities of lipid reserves in the hepatopancreas (14) and fat bodies (2) of this isopod. B. giganteus has also been collected from baited traps outside of methane seeps in the northern Gulf of Mexico (11). Stable carbon, nitrogen and sulfur isotope analyses confirm a detrital source of nutrition with some input from chemosynthetic sources, ranging from 0-45% (11).



Reproduction

Eggs of the giant isopod are also giant, up to 13mm diameter (6) and are brooded in a brood pouch above the stomach and internal organs (10). Females do not feed when brooding and seem to bury themselves in the sediment to reduce energy expenditure during brooding, which would insulate them and protect the brood and adults from predators. This unfortunately appears to make them more susceptible to trawls since brooding females have only been caught in trawls and not baited traps (1, 3). Juveniles of Bathynomus giganteus are called mancas and reach length up to 6cm. Mancas are characterized by the lack of the seventh pair of pereopods. The smallest sexual mature males and females reported are around 21cm (3) and 16.6cm (5), respectively. Individuals from the Gulf of Mexico show patterns of seasonal reproduction (3) and appear to release their brood between August and February (3), with low reproductive activity in the summer (1).



Ecology

Isopods apparently make good taxis as well! Several epibionts have been documented on the carapaces of B. giganteus including the barnacle Octolasmis aymonini, the gastropod Mitrella amphisella, serpulid polychaetes and hydrozoans (1, 3). Interestingly, Mitrella amphisella, from a family of carnivorous snails (Columbellidae), is found mainly on females and in the brood pouch (3). Not much is known about what types of organisms eat Bathynomus giganteus, but one paper reports a specimen in the gut contents of a tiger shark (3).

Biogeography

B. giganteus occurs at depths between 310-2140m (6), although in one rare case a single specimen was reported from 80m off the coast of Brazil (9). The giant isopod has a more or less global distribution. No one has studied to my knowledge the genetic connectivity between these isolated populations, but there is no doubt that further sampling should turn up more specimens from other areas of the world

Further Reading:

1.    Barradas-Ortiz, C., P. Briones-Fourzan, and E. Lozano-Alvarez. 2003. Seasonal reproduction and feeding ecology of giant isopods Bathynomus giganteus from the continental slope of the Yucatan peninsula. Deep Sea Research Part I: Oceanographic Research Papers 50:495-513.

2.    Biesiot, P. M., S. Y. Wang, H. M. Perry, and C. Trigg. 1999. Organic reserves in the midgut gland and fat body of the giant deep-sea isopod Bathynomus giganteus. Journal of Crustacean Biology 19:450-458.

3.    Briones-Fourzán, P., and E. Lozano-Alvarez. 1991. Aspects of the biology of the giant isopod Bathynomus giganteus A. Milne Edwards, 1879 )Flabellifera: Cirolanidae), off the Yucatan Peninsula. Journal of Crustacean Biology 11:375-385.

4.    Chamberlain, S. C., V. B. Meyer-Rochow, and W. P. Dossert. 1986. Morphology of the compound eye of the giant deep-sea isopod <I>Bathynomus giganteus</I>. Journal of Morphology 189:145-156.

5.    Cocke, B. T. 1986. Deep-sea isopods in aquaria. Tropical Fish Hobbyist 35:48-52.

6.    Holthuis, L. B., and W. R. Mikulka. 1972. Notes on the deep-sea isopods of the genus Bathynomus A. Milne Edwards, 1879. Bulletin of Marine Science 22:575-591.

7.    Klages, M., S. Muyakshin, T. Soltwedel, and W. E. Arntz. 2002. Mechanoreception, a possible mechanism for food fall detection in deep-sea scavengers. Deep Sea Research Part I: Oceanographic Research Papers 49:143-155.

8.    Kussakin, O. G. 1973. Peculiarities of the geographical and vertical distribution of marine isopods and the problem of deep-sea fauna origin. Marine Biology 23:19-23.

9.    Lemos de Castro, A. 1978. Descricao de uma especie nova gigante do genero Bathynomus Milne-Edwards

do litoral brasileiro (Isopoda, Cirolanidae). Revista Brasileira de Biologia 38:37-44.

10.    Lloyd, R. E. 1908. The internal anatomy of bathynomus giganteus, with a description of the sexually mature forms. Memoirs of the Indian Museum 1:81-102.

11.    MacAvoy, S. E., R. S. Carney, C. R. Fisher, and S. A. Macko. 2002. Use of chemosynthetic biomass by large, mobile, benthic predators in the Gulf of Mexico. Marine Ecology Progress Series 225:65-78.

12.    Milne-Edwards, A. 1879. On a gigantic isopod from the great depths of the sea. Annals and Magazine of Natural History 5:241-243.

13.    Pless, D. D., M. B. Aguilar, A. Falcon, E. Lozano-Alvarez, and E. P. Heimer de la Cotera. 2003. Latent phenoloxidase activity and N-terminal amino acid sequence of hemocyanin from Bathynomus giganteus, a primitive crustacean. Archives of Biochemistry and Biophysics 409:402-410.

14.    Steeves, H. R. I. 1969. Lipid contents of the hepatopancreas of the isopod Bathynomus giganteus A. Milne Edwards, 1879. Crustaceana 16:135-138.