Class Malacostraca

1. Bilaterally symmetrical.
2. Segmented calcareous skeleton.
3. 14 segments, plus telson, -8 thoracic, -6 abdominal.
4. Jointed appendages.
5. Molting nust occur in order to allow growth.
6. Blood system contains haemocyanin.
7. Eggs are brooded by the female.
8. Gonochoristic, rarely hermaphroditic, with internal fertilization.
9. Contains 22,651 species, of the 38,000 crustaceans.

The class Malacostraca contains the largest number of crustaceans species. Malacostracans are made upof 16 orders and 6 suboreders. Most malacostracans live in the marine environments,but a few are found in the freshwater and terrestrial environments. They occupy all three dimensions in the water column, such as crawling on top of the sediment, burrowing in the substrate, or swimming with the oceans currents. Most malacostracans are predatory by scavenging, but few are detritus feeders, filter feeders, parasitic, and herbivoures. In the aquatic members, the general trend is to use the gills for the site of ammonia excretion. Fertilization is internal and sexual. The female carries the eggs under the carapace in a brood pouch until they are ready to be released as larvae. The largest order is the Decapoda and contains about one quarter of all the species of crustaceans. These are the familiar lobsters, shrimps, and crabs. Three pairs of the first thoracic appendages are modified as maxillipeds, which are used for feeding. The remaining five pairs of legs are used for locomotion, and the first pair is usually modified as a large claw or cheliped. The second largest group is the infraorder Brachyura, sometimes called "true" crabs. The folding of the abdomen beneath the thorax allows for the center of gravity to shift forward underneath the legs, thus providing better mobility. The remaining orders represent the smaller pelagic and sedimentary species.

• Members of the class Malacostraca have successfully occupied
• the marine habitat, as opposed to the terrestrial and freshwater
• ecosystems.
• They occupy every zone, from the tidal pools, to the coral reefs, and depths of several kilometers. Some have been found crawling on top of 300 degree Celsius surfaces at the Galapagos Rift hot springs. Others have been found living in near freezing temperatures below the artic icesheets.
• Some shrimp exhibit veritcal migration, from depths of about 800m up to about 100m, while others live their entire lives with a tiny vicinity of a larger organism, such as the anenome crab.
• The size of the malacostracans differs as much their distribution. An american lobster may grow as large as 60cm. and weight up to 22kg, while a commensal pea crab is under 1cm.
• Some predatory malacostracans have tremendous powers of hunting. The mantis shrimp can produce an impact of a 0.22 caliber bullet upon striking with its claws.
• Another interesting malacostracan is the cleaner shrimp. As the name implies, the cleaner shrimp pick parasites off of larger fish such as moray eels, trigger fish and groupers. These fish consider any other malacostracans as part of their regular diet. However, the cleaner shrimp are allowed to venture into the mouths and gills of these large predators, without a scratch.

1. Barnes,R.S.K, P. Calow and P.J.W. Olive. 1993. The
2. Invertebrates, a new synthesis. Cambridge: University Press.
3. Cone, Joseph. 1991. Fire under the Sea. New York.
4. Ruppert, Edward and Robert Barnes. 1994. Invertebrate Zoology. New York: Saunders.