Giant isopod

Taxobox
name = Giant isopods


image_width = 230px
image_caption = "Bathynomus giganteus"
regnum = Animalia
phylum = Arthropoda
subphylum = Crustacea
classis = Malacostraca
ordo = Isopoda
subordo = Flabellifera
familia = Cirolanidae
genus = "Bathynomus"
genus_authority = A. Milne-Edwards, 1879
subdivision_ranks = Species
subdivision = "Bathynomus affinis"
"Bathynomus decemspinosus"
"Bathynomus doederleinii"
"Bathynomus giganteus"
"Bathynomus immanis"
"Bathynomus kapala"
"Bathynomus miyarei"
"Bathynomus pelor"
"Bathynomus propinquus"A giant isopod may be one of approximately nine species of large isopods (crustaceans related to the shrimp and crabs) in the genus "Bathynomus". They are thought to be abundant in cold, deep waters of the Atlantic. "Bathynomus giganteus", the species upon which the generitype is based, is the largest known isopod and is the one most often referred to by the common name "giant isopod".

French zoologist Alphonse Milne-Edwards was the firstcite journal |quotes=no |author=P. Briones-Fourzhn & E. Lozano-Alvarez |year=1991 |title=Aspects of the biology of the giant isopod "Bathynomus giganteus" A. Milne Edwards, 1879 (Flabellifera: Cirolanidae), off the Yucatan Peninsula |journal=Journal of Crustacean Biology |volume=11 |pages=375–385 |doi=10.2307/1548464] to describe the genus in 1879 [cite journal |quotes=no |author=A. Milne Edwards |year=1879 |title=Sur un isopode gigantesque des grandes profondeurs de la mer |journal=Les Comptes rendus de l'Académie des sciences |volume=88 |pages=21–23] after fishing 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. Females were not recovered until 1891.

Giant isopods are of little interest to most commercial fisheries owing to the typical paucity of catches and because ensnared isopods are usually scavenged beyond marketability before they are recovered. However, in Northern Taiwan and other areas, they are not uncommon at seaside restaurants, served boiled and bisected with a clean lateral slice. The white meat, similar to crab or lobster in texture, is then easily removed. The few specimens caught in the Americas with baited traps are sometimes seen in public aquaria.

Physical description

Maturing to a length between 19 and 37 cm (7.5 to 14.5 in), and maximally reaching a weight of approximately 1.7 kg (3 lb) in "B. giganteus",Fact|date=June 2007 giant isopods are a good example of deep-sea gigantism (cf. giant squid); most other isopods range in size from 1–5 cm. Their morphology is nonetheless familiar to most people as giant isopods closely resemble their terrestrial cousins, the woodlice: their bodies are dorso-ventrally compressed, protected by a rigid, calcareous exoskeleton composed of imbricate segments. The first of these segments 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 [cite journal |quotes=no |author=Steven C. Chamberlain, V. Benno Meyer-Rochow & William P. Dossert |year=1986 |title=Morphology of the compound eye of the giant deep-sea isopod "Bathynomus giganteus" |journal=Journal of Morphology |volume=189 |issue=2 |pages=145–156 |doi=10.1002/jmor.1051890205 |url=http://www3.interscience.wiley.com/cgi-bin/fulltext/109919813/PDFSTART] . There are two pairs of antennae.

The uniramous thoracic legs or "pereiopods" are arranged in seven pairs, the first of which are 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 natatory legs and "rami", flat respiratory structures acting as gills. The isopods are a pale lilac in colour.

Ecology

Giant isopods are important scavengers in the deep-sea benthic environment; they are found from the gloomy sublittoral zone at a depth of 170 m (550 ft) to the pitch darkness of the bathypelagic zone at 2,140 m (7,020 ft), where pressures are high and temperatures are very low (down to about 4 °C) [Cite book |author=B. T. Cocke |year=1987 |title=Morphological variation in the giant isopod "Bathynomus giganteus" (suborder Flabellifera: family Cirolanidae) with notes on the genus |publisher=Master's thesis, Texas A&M University |pages=129 pp] . Over 80% are found at a depth between 365 m and 730 m [Cite journal |quotes=no |author=L. B. Holthuis & W. R. Mikulka |year=1972 |title=Notes on the deep-sea isopods of the genus "Bathynomus" A. Milne-Edwards, 1879 |journal=Bulletin of Marine Science |volume=22 |pages=575–591] . 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 be active predators of slow-moving prey such as sea cucumbers, sponges, radiolarians, nematodes and other zoobenthos, and perhaps even live fish. They are known to attack trawl catches. As food is scarce in the deep ocean biome, giant isopods must make do with what fortune brings; they are adapted to long periods of famine and have been known to survive over eight weeks without food in the aquarium. When a significant source of food is encountered, giant isopods gorge themselves to the point of compromising their locomotive ability. One study examining the contents of 1651 "giganteus"' guts found that fish were most common there, followed by cephalopods and decapods, particularly carideans and galatheids.

In 1990, the Scavengers of East Australian Seas expedition (SEAS) started to document the scavenging crustaceans along the east coast of Australia by setting traps. The deeper the water, the fewer number of species they 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 it is known that "Bathynomus" existed more than 160 million years ago, before the break-up of the supercontinent Pangaea, so it has not evolved independently in all three locations. Over this length of time it would be expected that "Bathynomus" would evolve independently in the various locations. However, the SEAS study found that the giant isopods in all three locations were almost identical. Andrew Parker in his book "In the Blink of an Eye" (from where this description of the SEAS expedition is taken) links this lack of evolution to the extremely low light levels of their habitat [cite book |author=A. Parker |title=In the Blink of an Eye: How Vision Kick-started the Big Bang of Evolution |pages=121-32 |publisher=The Free Press |year=2003 |id=ISBN 0-7432-5733-2] .

Reproduction

Study of the seasonal abundance of "B. giganteus" juveniles and adults suggests a peak in reproductive capacity in the spring and winter months. This is apparently 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 "oostegites" or brood plates grown from the medial border of the pereopods. The fertilized eggs — thought to be the largest of all marine invertebrates — are retained safely within the marsupium for an unknown period.

The young isopods emerge from the marsupium as miniatures of the adults, known as "manca". This is not a larval stage: the manca are fully developed, lacking only the last pair of pereopods. A brooding female is at risk of losing her eggs if she overindulges in food to the point of bloating.

In popular culture

*Fossilised giant isopods play a key role in the thriller "Deception Point" by Dan Brown.
*The Godzilla comics featured gigantic giant isopods which attacked boats before the arrival of Godzilla.
*In the Blizzard Entertainment game "StarCraft", the Zerg Overlord Unit was based on the giant isopod.
*Jeph Jacques used the giant isopod in merchandise, a shirt and a totebag, derived from his webcomic Questionable Content.

References

External links

* NOAA, Ocean Explorer [http://oceanexplorer.noaa.gov/explorations/02mexico/logs/oct13/oct13.html Gulf of Mexico 2002 Exploration: October 13 Log]
* [http://oceanexplorer.noaa.gov Ocean Explorer (www.oceanexplorer.noaa.gov)] - Public outreach site for explorations sponsored by the Office of Ocean Exploration.
* [http://oceanexplorer.noaa.gov/history/history.html NOAA, Ocean Explorer History]
* [http://oceanexplorer.noaa.gov/gallery/gallery.html NOAA, Ocean Explorer Gallery] - A rich collection of images, video, audio and [http://oceanexplorer.noaa.gov/explorations/podcast/oceanexplorer_podcast.xml podcast] .
* [http://www.explore.noaa.gov NOAA's Office of Ocean Exploration]
* [http://www.youtube.com/oceanexplorergov NOAA, Ocean Explorer YouTube Channel]
* [http://tolweb.org/notes/?note_id=3004 Guide to California Isopods] at Tree of Life Web Project. Accessed May 2008
* [http://www.whozoo.org/Anlife2001/chelsy/clh_Bathynomus.htm Giant Isopods at WhoZoo]
* [http://jamiesrunoutgroove.blogspot.com/2007/11/songs-about-giant-isopods-join-fight.html News Story] about compilation album 'Songs About Giant Isopods' to be released in the UK