From The Desk of Zelnio: Bathynomus giganteus

i-9a8f04a2f2e904bbd44032c46193ed53-when isopods attack.jpg

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.

i-579a8112244f53c459aa7e2c57322137-bathynomus front2.JPG

i-5ff4e1143ccdc5634e8fa45f6bcd193e-bathynomus dorsal.JPG
i-d4e94ae28bba9ac56693758942ded760-bathynomus ventral.JPG


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).


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).


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).


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).


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

i-025ae38ec8e42164ada0b618edc1e2cf-bathynomus mouth.JPG

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.

More like this

Kevin's wonderful post on the Giant Isopod inspired me to post on a topic I have long pondered. Frequent readers of DSN know that I am fond of Sylvia Earle and the topic of body size. Honestly, it is not just body size is all matter of size related issues. A roadside trip can be quickly…
Vulcanoctopus hydrothermalis (Mollusca: Cephalopoda: Octipodidae) Octopus are one the most fascinating and intelligent of the invertebrates. Yet, little is known about their role in the deep sea, even less is known from methane seeps of hydrothermal vents! In fact, only one species has been…
Lucernaria janetae (Cnidaria: Staurozoa: Lucernariidae) Stauromedusae are not entirely uncommon, but not entirely common either. In fact "stalked jellyfish", as they are sometimes known, are very rare in the deep sea and only about 50 species are described (5). Only one other has been…
This is a funny story about a recent expedition to Saba Bank in the Netherlands Antilles. I've been engaged in a few of these over the last couple years, and each trip was funny in its own right. For instance the time we rode four hours each way in 12 foot seas to survey a barren sandy piece…

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

This is fantastic. Do any aquariums keep Bathynomus now? I'd love a copy of the paper.

Is there some sort of taboo regarding mention of the obvious superficial resemblance of these modern critters and Trilobites (seems like I've seen a few articles here an there about marine isopods but there nearly never any mention of trilobites...)? Or is it just considered so obvious as to be not worth mentioning?
(Do they likely share a common ancestor, or is there any possibility that the modern deep-sea isopods are actually descendants of a trilobite population?)

Hi Peter,

You should have that journal in your library. I can scan it in and send it to you. I think Cocke was a student of someone at Texas A&M, maybe Wicksten.

By Kevin Zelnio (not verified) on 04 Apr 2007 #permalink

Good questions SMC. Whereas trilobites and these isopods are the same phylum (Arthropoda) they are in different subphylum. Isopods are in the Crustacea and trilobites are currently in the Chelicerates (sea spiders, true spiders, scorpions, and horseshoe crabs). There are several specific differences in anatomy between the two. For example the appendages of trilobites are relatively undifferentiated compared to isopods.

Along the lines of SMC and McClain, here is a fun website on trilobite imposters:

Several lines of evidence seem to suggest that Bathynomus is more derived than a living fossil, especially the morphology of the eyes.

By Kevin Zelnio (not verified) on 05 Apr 2007 #permalink

Thanks RPM, I'm just glad to make through the year of death. Of course not doing drugs or owning guns helps somewhat. Hopefully my 29th revolution around the sun will be successful as well (b-day was apr 2 btw, 56 min. from april fools day)

By Kevin Zelnio (not verified) on 05 Apr 2007 #permalink

Better late than never. We linked to this post for futher reading.


Lovely critters! I'm afraid the Baltic is too shallow & brackish to be much of an isopod habitat. We have to make do with Saduria entomon, which does have its own scaly charm.

I wonder if they make you eat them on Fear Factor...

that thing is awesome!

Impressive creatures. What I want to know is: How do they taste when steamed and dipped in melted whipped butter? Do we have a Homarus americanus or a Limulus polyphemus situation here?

By Dan Nurco (not verified) on 13 Jun 2009 #permalink

If you're curious and want to taste one of these isopods, you can always fly to Taiwan--specifically the seaside areas of the Dan-Shuei and Ba-Li townships (45min-1hr train ride from Taipei city). There are a lot of places there with roadside eateries cooking various seafood, andI've seen some (admittedly smaller) Bathynomus species in their "Fresh, Alive, Ready to Cook" aquariums. They're not usually what non-chinese foreigners go to for eating, so a command of Mandarin and/or Fookien Chinese would help.

I've only been to Taiwan 2 times and have never eaten them myself, but reputedly they have typical white crustacean meat (i.e. like Crabs and Lobsters). They're usually served with soy and black bean sauce, deep-steamed--while the roadside eateries do serve good food, they don't have a wide range of cooking styles.

Although I'd personally be averse to eating them. The living ones actually look kind of cute, scuttling around.

By Mike Ching (not verified) on 22 Jun 2009 #permalink

I live in Nunavut and dive in Frobisher Bay fairly regularly. In sandy area yours see fairly large 10-20cm isopods everywhere Iâve been thinking of steaming a few up and eating them. Let you know how it goes.

I wonder how they taste?

By lowell beyer (not verified) on 01 Apr 2010 #permalink