Crustacean - biology.

Crustacean - biology. I INTRODUCTION Crustacean, common name for any of a group of mainly aquatic arthropod invertebrates, including the crab, lobster, and shrimp. They are among the most successful animals, abundant in the sea much as insects are on land. The majority of animals in the world are marine crustaceans that belong to the copepod subclass of the crustacean subphylum. Crustaceans are also successful in fresh water, and a few, such as sow bugs, are common in moist land environments. Although most crustaceans are small, they vary widely in form and include such large members as lobsters up to 60 cm (24 in) long and a spider crab with a leg span of 3.6 m (12 ft). The subphylum contains about 26,000 known species. II STRUCTURE Like all arthropods, crustaceans have an external skeleton (exoskeleton) and a body made up of a series of segments. Each of these segments generally bears a pair of branched limbs. In the course of evolution, the segments and other parts of the body have become specialized. The limbs, used in respiration, swimming, crawling, and feeding, may be highly modified as jaws, reproductive organs, and other structures, or they may be simplified or lost. The head is usually fused with several segments behind it, forming a region called the cephalothorax, which is followed by the abdomen. Commonly an outgrowth of the head, called the carapace, covers much or even all of the body. On the head are two pairs of sensory organs (antennae) and a pair of jaws (mandibles), behind which are two other pairs of mouthparts (maxillae). The head is usually equipped with a pair of compound eyes, an unpaired eye, or both. The cephalothorax generally bears limbs used in locomotion and respiration. Often, the carapace provides a protective cover for the gills, which are part of the limbs. Some of the limbs may have pincers (chelae). Abdominal appendages may be used in locomotion but frequently have other functions, such as respiration, and they tend to be reduced in size. A tail portion (telson) that bears the anus is occasionally used in swimming. The main body cavity is an expanded circulatory system through which blood is pumped by a dorsal heart. The gut is basically a straight tube, often with a gizzard-like gastric mill used in breaking down the food, and a pair of digestive glands that not only secrete digestive fluids but also absorb food. Structures that serve as kidneys are located near the antennae. A brain exists in the form of ganglia near the sense organs, and a collection of ganglia and major nerves is found below the gut. III REPRODUCTION AND DEVELOPMENT Reproduction is primarily sexual in crustaceans. The only kind of asexual reproduction is parthenogenesis (development from unfertilized eggs), but this is rare. Usually the sexes are separate, but some parasites and most barnacles, which have difficulty obtaining mates, are simultaneous hermaphrodites (male and female at the same time). This increases the number of possible partners and may allow self-fertilization as a last resort. Some crustaceans switch sex as they get older. Many crustaceans exhibit elaborate courtship behavior, and the males may fight for the chance to mate. The young of marine crustaceans generally pass through one or more larval stages that are quite unlike the adult form. Often the larvae swim in open water to find a place to live. Freshwater and terrestrial crustaceans skip the larval stage, except for those that return to the sea to spawn. After fertilization, the developing eggs are generally cared for by the mother until they have reached the larval or postlarval condition. Otherwise, little parental care exists among crustaceans. Some live in malefemale pairs or are gregarious, but they do not form well-organized societies. The smallest crustaceans live for just a few days, but the largest ones may live for decades. IV ECOLOGY Crustaceans are modified for a wide variety of habitats and ways of life. They are important in the food chain, partly because so many of them eat small plants and animals. Many strain small particles of food from the water, but the larger crustaceans, such as shrimp and crabs, are often omnivores, scavengers, or predators. Several species are also parasites. Crustaceans are consumed by many animals, including humans, and are rich in protein. V DIVERSITY The crustacean subphylum is large and diverse. The brine shrimp, a branchiopod, lives in saline lakes and ponds. Ostracods, or ostracodes, the tiny mussel, or seed, shrimps, are protected by a two-shelled carapace that covers the body. Copepods are small animals with a simple structure that are abundant in both marine and fresh water. Many are parasites. Barnacles are immobile and highly modified as adults and capture food with their limbs. Krill are shrimplike animals that are a source of food for many whales. Crabs, lobsters, and shrimp are all decapods, each having five pairs of thoracic legs. The carapace of these animals is fused to the body to form a protected chamber for the gills. Scientific classification: The subphylum Crustacea is placed in the phylum Arthropoda. The major classes in the subphylum Crustacea are the Branchiopoda (fairy shrimps, brine shrimps), Ostracoda (mussel, or seed, shrimps), Copepoda (copepods), Cirripedia (barnacles), and Malacostraca (crabs, lobsters, shrimps, crayfish, krill, sow bugs, beach hoppers). Contributed By: Michael T. Ghiselin Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.