Penaeid shrimp belong to the largest phylum in the Animal Kingdom, the Arthropoda, characterized by jointed appendages and an exoskeleton or cuticle that is periodically molted. There are thousands of terrestrial species in this phylum, and a large, predominately aquatic subphylum, the Crustacea. The more highly evolved crustaceans (Class Malacostraca) include the penaeid shrimp (Order Decapoda). The subphylum Crustacea contains about 42,000 species belonging to 10 classes. The class Malacostraca contains about three-fourths of the known species and includes crayfish, lobsters, shrimps and crabs (Bailey-Brock & Moss, 1992). Superorders Peracarida and Eucarida are the most speciose Malacostraca, and are distinguished from one another by a number of morphological and life-history traits. Peracarida are characterized by direct development (i.e. no independent larval stage), a ventral brood pouch or "marsupium" for retaining developing young, and the presence or absence of a carapace uniting the head and thoracic segments. Eucarida have indirect development (i.e. one or more independent larval stages), no brood pouch (developing eggs are usually attached to appendages), and a distinct carapace. Shrimp-like species are found in both superorders, and some peracarids are important to world fisheries (e.g. the Mysidacea); although the most desirable fishery species are decapod eucarids (e.g. lobsters, shrimps, and crabs) (Bailey-Brock & Moss, 1992). Decapods can be distinguished from other higher crustaceans by examining differences in the thoracic appendages. The first three pairs of thoracic appendages, the maxillipeds, are modified for feeding and the remaining five pairs are the walking legs, hence the name Decapoda or "ten-legs". Penaeid appendages typically consist of two branches (biramous), the exopodite and endopodite. These structures are variously developed for feeding, locomotion or burrowing; or they bear feathery gills (modified epipodites) contained beneath the carapace, or sensory structures on the antennae and antennules (Bailey-Brock & Moss, 1992). Appendages associated with the head region include the antennules, antennae, mandibles and two pairs of maxillae. The thorax bears three pairs of maxillipeds and five pairs of walking legs (pereiopods). The endopodites of the walking legs are attached to the cephalothorax by a short joint, the coxa. Legs consist of a number of articulating sections that terminate at the dactylus which may be chelate (first, second and third walking legs) or simple (fourth and fifth walking legs). Sections between the coxa and dactylus are called the basis, ischium, merus, carpus and propus. The presence of a spine or row of spines on the ischium is of taxonomic importance for some species. The six-segmented abdomen has five pairs of swimmerets (pleopods) and a pair of uropods that form the tail fan with the telson (Bailey-Brock & Moss, 1992). |