Giant isopod
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Bathynomus affinis |
A giant isopod may be one of approximately nine species of large isopods (crustaceans related to the shrimps and crabs) in the genus Bathynomus. They are thought to be abundant in cold, deep waters of the Atlantic and Pacific Ocean. 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 first to describe the genus in 1879 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 no interest to commercial fisheries owing to the typical paucity of catches and because ensnared isopods are usually scavenged beyond marketability before they are recovered. The few specimens caught with baited traps are sometimes seen in public aquaria.
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[edit] Physical description
Reaching a maximum length of approximately 45 cm (18 inches) and a weight of 1.7 kg (3 lb) in B. giganteus, 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. 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.
[edit] 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). 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 echinoderms 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.
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 millions 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.[1]
[edit] 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.[1] Mature females develop a brood pouch or marsupium when sexually active, the pouch 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.
[edit] References
- ^ Parker,A: In the Blink of an Eye: How Vision Kick-started the Big Bang of Evolution, pages 121-32, The Free Press, 2003
[edit] External links
- Oceanexplorer Gulf of Mexico 2002 Exploration: October 13 Log Trapping for Predators, Diving for Prey
- What is an Isopod? at Tree of Life Web Project. Accessed December 2006
