Malacostracans are distributed worldwide in marine, freshwater, and terrestrial environments. There are roughly 25,000 species in as many as fifteen orders. Decapoda (crabs, lobsters, and shrimps) is the most speciose group within the Malacostraca.
Malacostracans exhibit the hard, calcified exoskeleton typical of crustaceans. The body is divided into three tagmata, cephalon, thorax, and abdomen. The head and thorax are fused into a cephalothorax and may be difficult to distinguish. All malacostracans possess five segments in the head, eight in the thorax, and six in the abdomen, excepting the 20-odd species in the Phyllocarida, which have seven abdominal segments. As a general rule, each segment bears a pair of appendages, but in some organisms appendages are lacking on several abdominal appendages. The anterior one to three thoracic appendages are modified into maxillipeds, which are used in feeding. The anteriormost five abdominal appendages are, almost without exception, biramous. In most malacostracans, the posteriormost abdominal appendages, if present, are flattened and form a tail fin with the telson.
Orders are often categorized by the specialization of specific limbs and body segments. There is a great deal of morphological diversity within the class, which is the most familiar of all the crustacean taxa.
Malacostracans have the typical crustacean body plan. Internal gills are protected by the carapace. The circulatory system in large organisms may be highly developed and extensively venous, though it is still considered open (as opposed to closed). The nervous system is highly centralized. A large brain near the eye is connected to a number of ganglia via a paired ventral nerve cord, which runs the length of the body. Green glands in the second antennae serve an osmoregulatory and excretory function. Disposal of nitrogenous wastes probably also occurs across the gills or body wall itself. The mouth of malacostracans leads to a two-chambered stomach, which possesses a grinding structure called the gastric mill. Digestion occurs throughout the gut, and waste matter is expelled through a posterior anus on the telson.
Malacostracans are dioecious, and sex is genetically determined. The gonads are located in the sixth thoracic segment in females, and the eighth in males. Copulation is the rule as the unflagellated sperm are nonmotile. The anterior one or two abdominal appendages in males are modified into reproductive structures designed to aid in sperm delivery. Development ranges from direct to metamorphic among members of class Malacostraca. In peracardians, eggs are brooded behind the thorax. In other malacostracans, eggs are laid. Most metamorphosing malacostracans have a nauplius larva, but in many species eggs hatch into zoea larvae.
Virtually every imaginable feeding strategy is demonstrated by at least one member of the class. Many malacostracans are strictly carnivorous, and are active hunters. Organisms representing many orders possess thoracic appendages modified for spearing or catching and crushing prey. Several malacostracan taxa are parasitic. Still others are scavengers. Herbivorous malacostracans, as well as filter-feeders, also exist.
Malacostracans are generally active. Among benthic taxa, however, some burrowing species remain fairly inactive. Many pelagic forms are active hunters. Decapods are known for elaborate courtship displays, such as those demonstrated by the fiddler crab.
Malacostracans have a powerful role in the economy. Humans consume large amounts of decapods, and huge industries have developed around the capture or farming and sale of shrimp, lobster, and crabs. There is also a large aquarium trade, supplying animals both as pets and as food for fish and amphibians. Most malacostracan parasites invade fishes and crustaceans. For this reason, parasitic malacostracans have a negative impact on fish, shrimp, lobster, and crab industries. Malacostracans play such an important role in aquatic ecosystems that their conservation is an important issue. Commercial over-fishing may eventually put populations in danger. Ironically, it is the important role that malacostracans play in the human economy that is endangering them. Both the fishing and farming of malacostracans can be environmentally damaging.
The taxonomy of malacostracans centers around the specialization and arrangement of appendages and body segments. Unfortunately, many researchers suspect that a high degree of convergence is obscuring the phylogeny of malacostracan orders. For this reason, the taxonomic divisions between many groups of malacostracans should be viewed as a good general guidelines rather than strict phylogenetic relationships.
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Dan Atwater (author), Daphne G. Fautin (author).
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature