Crustacea

Crustacea is usually a subphylum-level classification of arthropods. This group includes barnacles, copepods, crabs, prawns, lobsters, and wood lice. More than 30, 000 species have been identified, the majority of which are marine dwelling. Terrestrial species such as woodlice and pill bugs are believed to have evolved from marine species. Most crustaceans are free-living but some species are parasitic—some even on other crustaceans. Some species swim in open water, while others are specialized at crawling or burrowing in soft sediments.

Despite having radiated into an extraordinary diversity of species, most crustaceans share distinctive characteristics. The head usually bears five pairs of appendages: two pairs of antennae that play a sensory role in detecting food as well as changes in humidity and temperature; a pair of mandibles that are used for grasping and tearing food; and two pairs of maxillae that are used for feeding purposes. The main part of the body is taken up with the thorax and abdomen, both of which are often covered with a toughened outer skeleton, or exoskeleton. Attached to the trunk region are a number of other appendages, which vary both in number and purpose in different species. In shrimp, for example, one pair of appendages may be modified for swimming, another for feeding, another for brooding eggs and yet another for catching prey.

Crustacea exhibit a wide range of feeding techniques. The simplest of these are those species that practice filter feeding such as the copepods and tiny shrimps. Feeding largely on plankton and suspended materials, the animal creates a mini water current towards the mouth by the rhythmic beating of countless number of fine setae that cover the specialized feeding appendages. Food particles are collected in special filters and then transferred to the mouth. Larger species such as crabs and lobsters may be active hunters of small fish and other organisms, while some species adopt a scavenging role, feeding on dead animals or plants and other detritus.

Apart from the smaller species, which rely on gas exchange through the entire body surface, most crustaceans have special gills that serve as a means of obtaining oxygen. Simple excretory organs ensure the removal of body wastes such as ammonia and urea. Most crustaceans have a series of well-developed sensory organs that include not only eyes, but also a range of chemical and tactile receptors. Nearly all crustaceans are probably capable of detecting a light source but in some of the more developed species, definite shapes and movements may also be detected.

Breeding strategies vary considerably amongst the crustacea. Most species are dioecious (being either male or female), but some, such as the barnacles, are hermaphrodites. Fertilization is usually internal through direct copulation. The fertilized eggs then mature either in a specialized brood chamber in some part of the female’s body, or attached directly to some external appendage such as a claw. Most aquatic species hatch into a free-swimming larvae that progresses through a series of body molts until finally arriving at the adult size.

See also Zooplankton.

Crustacea

The crustacea (subphylum Mandibulata, class Crustacea) are a diverse group of animals. This class includes some of the more familiar arthropods , including barnacles , copepods , crabs , prawns, lobsters , and wood lice . More than 30,000 species have been identified, the majority of which are marine-dwelling. Terrestrial species such as woodlice and pill bugs are believed to have evolved from marine species. Most crustaceans are free-living but some species are parasitic—some even on other crustaceans. Some species are free-swimming, while others are specialized at crawling or burrowing in soft sediments.

Despite such an extraordinary diversity of species, many crustaceans have a similar structure and way of life. The distinctive head usually bears five pairs of appendages: two pairs of antennae that play a sensory role in detecting food as well as changes in humidity and temperature ; a pair of mandibles that are used for grasping and tearing food; and two pairs of maxillae that are used for feeding purposes. The main part of the body is taken up with the thorax and abdomen, both of which are often covered with a toughened outer skeleton, or exoskeleton. Attached to the trunk region are a number of other appendages which vary both in number and purpose in different species. In crabs, for example, one pair of appendages may be modified for swimming, another for feeding, another for brooding eggs and yet another for catching prey .

Crustacea exhibit a wide range of feeding techniques. The simplest of these are those species that practice filter feeding such as the copepods and tiny shrimps. Feeding largely on plankton and suspended materials, the animal creates a mini water current towards the mouth by the rhythmic beating of countless number of fine setae that cover the specialized feeding limbs of these species. Food particles are collected in special filters and then transferred to the mouth. Larger species such as crabs and lobsters are active hunters of small fish and other organisms, while some species adopt a scavenging role, feeding on dead animals or plants and other waste materials.

Apart from the smaller species, which rely on gas exchange through the entire body surface, most crustaceans have special gills that serve as a means of obtaining oxygen . Simple excretory organs ensure the removal of body wastes such as ammonia and urea . Most crustaceans have a series of well-developed sensory organs that include not only eyes, but also a range of chemical and tactile receptors. All crustaceans are probably capable of detecting a light source but in some of the more developed species, definite shapes and movements may also be detected.

Breeding strategies vary considerably amongst the crustacea. Most species are dioecious (being either male or female), but some, such as the barnacles, are hermaphrodite . Fertilization is usually internal through direct copulation. The fertilized eggs then mature either in a specialized brood chamber in some part of the female's body, or attached directly to some external appendage such as a claw. Most aquatic species hatch into a free-swimming larvae that progresses through a series of body molts until finally arriving at the adult size.

See also Zooplankton.

Crustacean

A crustacean is an invertebrate (an animal without a backbone) with several pairs of jointed legs and two pairs of antennae. It is covered by a tough exoskeleton (a hard outer support structure) with overlapping plates that thin out at the joints to allow maximum movement. Its body is divided into two main regions that are fused together. Most crustaceans live in water.

A crustacean is a member of the phylum Arthropoda, the largest and most successful phylum in the kingdom Animalia. It is also a member of the class Crustacea which is one of the three major groupings of arthropods (the other two are Arachnida and Insecta). The name crustacean is derived from the Latin word cursta meaning "crust," and refers to the hard outer shell that this class of invertebrate wears. There are about 40,000 species of crustaceans, including the better-known animals like shrimps, lobsters, crayfishes, and crabs as well as barnacles, water fleas, and isopods like the wood louse. Some are predators and eat other invertebrates, while others are herbivores and eat only plant material. While there are simple crustaceans, most usually have a large diversity (and a large number) of paired appendages (like legs, arms, or pincers). To be classified as a crustacean, an animal must have two joined body parts—a cephalothorax (a head and middle region) and an abdomen (the lower part of the body). The head has two compound eyes that are located on the ends of retractable and flexible stalks. A compound eye is made up of many separate compartments, each having its own lens. A crustacean must also have two pairs of antennae with which it feels and receives chemical stimuli. It must also have at least four pairs of walking legs, and often has more. Shrimp, lobster, crabs, and crayfish are called decapods because they have ten legs. All have a broad, paddle-like tail used for swimming.

As with a representative crustacean species like the crayfish, its first set of legs are adapted as claws. It uses these claws or pincers to obtain food and to defend itself. The other four pairs of smaller legs are used for walking. Behind these walking legs and attached to the lower half of its body called the abdomen are tiny appendages called swimmerets used for swimming and during reproduction. The respiratory system of a crayfish consists of gills over which water passes as the animal moves. The gills are really feathery outgrowths located on both sides of its body. The crayfish has a heart that moves its blood through arteries. Other crustaceans have variations of these systems. For example, a crab has an especially strong claw used for tearing up seaweed and attacking another animal. The barnacle, which attaches itself to a rock by a long stalk, has appendages almost like feathers that are used to comb or sift the water for microscopic food.

The smallest crustacean might be the 7,500 species of copepods, some of which are no more than a few millimeters long. These free-swimming herbivores play an important role in the diet of many fish. All crustaceans reproduce sexually and develop through a series of larval stages.

Crustacea (crustaceans; phylum Arthropoda) Diverse subphylum of mandibulate arthropods, the body usually divided into three parts: head, thorax, and abdomen. In some crustaceans (e.g. crayfish) the head and thorax may be joined to form the cephalothorax. The head bears two pairs of antennae, one pair of mandibles, and two pairs of maxillae. The limbs are biramous, and are adapted for a wide range of functions. Closely placed setae on the limbs function as filters in filter-feeding species. Respiratory gills are situated on the appendages, but vary greatly in location and number; they are absent only in very small species. In addition to the antennae, sense organs include a pair of compound eyes, and a small, dorsal, median, nauplius eye, comprising three or four closely applied ocelli (clusters of photoreceptors). The nauplius eye, characteristic of crustacean larvae, is absent in many adults; and some groups lack the compound eyes. Mainly marine, but there are many fresh-water species, and a relatively small number have invaded the land. Four classes of crustaceans have an important fossil record. The Malacostraca (crabs, lobsters, woodlice, etc., Cambrian to Recent) includes the earliest crustaceans of the subclass Phyllocarida. The Branchiopoda (similar to modern water fleas, Lower Devonian to Recent) are valuable index fossils in non-marine strata. The Cirripedia (barnacles) occur from Upper Silurian to Recent, and the Ostracoda from Lower Cambrian to Recent. The living class Cephalocarida (e.g. Hutchinsonella) is thought to be closest to the ancestral crustacean stock, but the group is without any unequivocal fossil representative.

Crustacean

The Crustacea are a subphylum of the animal phylum Arthropoda. This is a large and diverse group with more than forty thousand species, including crabs, shrimp, lobsters, crayfish, barnacles, and many near-microscopic members of the zooplankton community. The subphylum is characterized especially by having mandibles and compound eyes and living in mostly aquatic habitats, although the "pillbugs" found under rocks and boards are also crustaceans, and many crabs spend much of their time on land.

The Crustacea are named for their hard, crusty exoskeletons , well known to anyone who has dined on lobster or crab. The hardness of the exoskeleton comes partly from chitin , but moreover from a heavy deposit ofcalcium carbonate. The edible blue crab, for example, has as much calcium carbonate in its exoskeleton as four sticks of chalk. The rigid exoskeleton requires crustaceans to molt, or shed it periodically, in order to grow. Some crustaceans can mate only during the brief time just after they have molted and the new exoskeleton is still soft. This is also a time of great vulnerability to predators, so crustaceans often seek a place to hide before molting.

Some crustaceans resemble miniature adults from the moment they hatch, but many species have larval forms with little or no resemblance to the adult. These larvae, and some adult crustaceans, such as krill and cope-pods, are very important members of the freshwater and oceanic plankton community and are a major source of food for corals, fish, baleen whales, and other animals. A few crustaceans turn the tables on these predators by parasitizing the skin of fishes. These parasitic crustaceans are often wormlike and scarcely recognizable as relatives of shrimp and crabs.

see also Animalia; Arthropod; Lakes and Ponds; Ocean Ecosystems; Plankton

Kenneth S. Saladin

Bibliography

Pechenik, Jan A. Biology of the Invertebrates, 4th ed. Boston: McGraw-Hill, 2000.

Ruppert, Edward E., and Robert D. Barnes. Invertebrate Zoology, 6th ed. Fort Worth, TX: Saunders College Publishing, 1994.