The crustaceans became the major group of marine arthropods when the trilobites and giant sea scorpions (Eurypterida) became extinct. The group, called Crustacea, contains about 30,000 species and includes such animals as water fleas, barnacles, crabs, lobsters, shrimps, and wood lice (pill bugs). It also includes tiny zooplankton, which live near the surface of the sea and occupy an important position in aquatic food chains. Although Crustacea has traditionally been considered a class within the phylum Arthropoda, some zoologists believe the size of the group means it s,hould be considered a subphylum.

The microscopic Daphnia, a genus of water flea, is atypical branchiopod. It has five or six pairs of trunk appendages, which have epipodites—extensions that serve as gills. The appendages also bear setae, which collect food particles. The food is then moved into a food groove and spurts of water push it toward the mouth. Unlike other branchiopods, which use their trunk appendages for locomotion, Daphnia uses its large second antennae as paddles to move it in a jerky up-and-down motion. Also, water fleas have a pair of median compound eyes that direct them when swimming.

Characteristic features

The exoskeleton of crustaceans, which is composed of chitin and hardened with calcium salts in such species as crabs, is molted periodically to enable the animal to grow. This is an important process in the life of crustaceans and other arthropods, and occurs in a strict sequence of events. First, useful materials contained in the shell, such as calcium salts, are reabsorbed into the body; the new cuticle then begins to form under the old one. The animal swells up, causing the old cuticle to split, and the new cuticle is hardened. Before the new cuticle hardens, the animal is easy prey for predators and usually seeks shelter during this period.

In many crustaceans an outer shell, or carapace, covers the exoskeleton of the thorax or anterior trunk segments. Typically, the body consists of a head, thorax, and an abdomen, but often the head and a number of thoracic segments are fused to form a cephalothorax.

The head region has two pairs of antennae, which is characteristic of crustaceans. It also has one pair of mandibles, which in most species are heavy and have grinding and biting surfaces, and two pairs of maxillae.

The trunk region of the cephalothorax is segmented, the number of segments varying according to the species, and each segment bears a pair of appendages. These appendages are different in each species, being modified to suit particular functions. Each abdominal segment usually also has a pair of appendages structured for various functions, such as swimming, crawling, breathing, capturing food, or reproducing.

The appendages are biramous that is, each one ends in two jointed branches; they are tubular and jointed and contain muscles that contract to bend the limbs. Crustaceans swim by beating these appendages some of which have a fringe of bristly setae, to push against the water and most of the animals also crawl.

Many crustaceans breathe with gills, although some land-dwelling species, such as the robber crab, have modified them to become air chambers lined with blood vessels, which absorb oxygen. The gills also help crustaceans eliminate ammonia waste. The primary organs of excretion are special glands in the head region called the antennal glands and maxillary glands. The crustacean blood system is open the blood is pumped by the heart into a hemocoele, where it simply bathes the tissues.

The nervous system of these animals is well developed, and the sensory organs include eyes and various receptors sensitive to touch. Most crustaceans have compound eyes in the adult stage. The young, or nauplius larvae, have a median eye composed of three or four clusters of photoreceptors (ocelli), which in some species persist into the adult stage. Other sensory receptors include special touch-sensitive hairs called setae. Setae are scattered over the body surface, but are most concentrated on the appendages and statocysts, or balancing organs.

Crustaceans are usually dioecious that is, they have separate sexes but some are hermaphroditic and have both male and female sexual organs. The young hatch from eggs that are usually protected by one of the parents and take the form of free-swimming planktonic larvae that have fewer appendages than the adult. However, through successive molts, trunk segments and extra appendages develop.


This subclass of mostly freshwater animals have earned their name (meaning “gill feet”) from their thoracic appendages, which are modified for respiration as well as filter-feeding and locomotion. The group consists of four orders: Anacostraca, or fairy shrimps, which have no carapace; Notostraca, or tadpole shrimps, whose head and front thoracic region is covered by a carapace; Conchostraca, or clamp shrimps; and Cladocera, or water fleas. The animals in this last group, which include the common genus Daphnia, are covered by a carapace that encloses the trunk but not the head. The carapace of these animals is usually transparent, but can appear red or pink, depending on the level of oxygen in the water. Daphnia, for example, is transparent in oxygenated water, but turns pink in stagnant water, when it produces hemoglobin to extract more oxygen.
Branchiopods are filter-feeders that collect food particles on the bristles of the trunk appendages. The food is carried to the mouth by streams of water that pass between the trunk appendages as they move back and forth. The first maxillae finally push the food into the mouth.

The sexes are generally separate, and copulation is the means of fertilization, but under favorable conditions the females reproduce parthenogenetically—that is, the eggs develop without fertilization. The eggs of water fleas hatch into females for several generations until adverse conditions, such as low water temperature or short food supply, induce the production of males, after which eggs are again fertilized by copulation.

Cyclops are freshwater copepods. These tiny animals can stagger the development of eggs that are produced from a single mating. After fertilization, one or two egg-containing sacs form on the genital segment of the female-each sac holds up to 50 eggs. A number of eggs are hatched from half a day to five days after fertilization, and a new batch is then brooded. The eggs hatch as nauplius larvae, just as the one shown above, swimming away from the parent.

Ostracods and copepods

These two subclasses of tiny crustaceans contain freshwater and marine species, and plank

tonic, or shallow-water, as well as benthic, or deepwater, types. The ostracods are similar to the conchostracan branchiopods in that they have a bivalve hinged carapace. The head is more developed than the remainder of the body, and its appendages (especially the an-tennules and antennae) are modified for crawling, swimming, and feeding. Most ostracods are filter-feeders, but some species are scavengers, predators, or parasites.

Most copepods live in salt water and occur in huge numbers, making them of great economic and ecological importance because they form a large part of the diet of many fish. The body is short and tubular, with a trunk that is usually composed of six segments, an abdomen, and a reduced head region. The first antennae are long, whereas the second pair is short and often branched. They can move rapidly by beating their thoracic appendages, which causes jerky movements. They are also able to glide slowly by moving the second antennae.

Planktonic copepods usually live in the upper 650 to 975 feet (200 to 300 meters) of the ocean and commonly migrate to and from the surface daily. Light appears to be the trigger for this behavior, but its value to the animal is far from certain.

Many copepods are filter-feeders, some are predatory, others are scavengers, and still others are parasitic. The parasitic species feed on freshwater and marine fish, some living on the gill filaments, and others in the intestines.

Neither ostracods nor copepods have gills, and gaseous exchange occurs in the valves (in ostracods), or across the body surface (in copepods). Both groups have an open circulatory system. Median eyes are found in both, and only one group of ostracods has compound eyes. One outstanding feature of these animals is their luminescence. In both groups, fertilization is generally by copulation, although parthenogenesis is known in some ostracod species.

The anatomy of crayfish is similar to that of most decapods, but the male (left) and female differ in some respects. For instance, the female has large swim-merets, on which she carries newly hatched larvae, whereas those of the male are small. The male appendages are modified for the transmission of sperm.


Cirripedia. The subclass derives its name from the six pairs of featherlike, two-branched legs (cirri) that protrude from the shell and that are used for filter-feeding.

All barnacles are marine, and most are found attached to rocks, shells, or driftwood. Some species, however, live on the bodies of other crustaceans. Barnacles are surrounded by a carapace of calcareous plates, which develops from a clamlike carapace in the young, or cypris, larva.

Most barnacle species are hermaphrodites, but cross-fertilization is common. The young hatch as nauplius larvae, as in other crustaceans, but then develop into cypris larvae that settle on a suitable surface. Finally, metamorphosis takes place, and through a series of molts the animal rapidly adds the carapace and reaches adulthood.

Barnacles can be stalked; that is, cemented to a surface by a peduncle, or nonstalked and attached directly to a surface. The peduncle of the stalked goose barnacle carries the capitulum, or body, which is surrounded by hard plates. The upper plate is called the carina.


The order Decapoda contains most of the larger crustaceans in the subclass Malacos-traca, including lobsters, crabs, crayfish, and shrimps. About 10,000 species have been described, and most are marine, although some species live in fresh water. All decapods have eight pairs of thoracic appendages; the rear five are modified for walking and are the origin of the name decapod (five feet). The front three pairs serve as feeding appendages (maxillipeds). In some decapods, the first pair of walking legs is heavier and stronger than the others and has pincers called chelae.
The abdomen is composed of six segments and a tail, or telson. The abdominal appendages are called swimmerets, and in some species they are very small. The sixth abdominal segment usually has a pair of appendages called uropods, which together with the telson form a tail fan.

Decapods have compound eyes on jointed movable stalks, and the central nervous system is well developed. Their wide variation in color usually depends on the habitat, and is caused by the pigment-producing cells called chromatophores in the exoskeleton. Aquatic decapods breathe with gills, usually five pairs, which run vertically in the cephalothorax between the endoskeleton and the other organs. Water flows in at the base of the front five appendages, bathes the gills, and flows out through vents under the second antennae.

An elaborate courtship ritual is typical of many decapods prior to copulation, and scented chemicals called pheromones play an important role in their sexual behavior. In many species, the eggs are laid soon after copulation and are cemented onto the swim-merets of the female.
Lobsters, like some crabs, have a single huge claw that is used for crushing; the other claw of the pair is much smaller but has sharp edges and is used for seizing and tearing prey. They are scavengers, but also catch fish and break open shelled animals. Crayfish are similar in appearance to lobsters, but most live in fresh water. Also, like lobsters, crayfish are nocturnal and feed on almost any organic matter, living or dead.

Crabs are probably the most successful decapods, in that they can live on land as well as in water. They are found at all depths of water, in all parts of the world. They have a wide carapace and, unlike lobsters, a small abdomen, which is tucked tightly under the cephalothorax. The female uses its swimmerets only for brooding eggs. Crabs can walk forward, but more usually move sideways. The crab’s large front claws, called chelipeds, are not used for walking.

Crabs range in size from the pea crabs (Pinnotheridae)—which live in the tubes constructed by marine annelids, on sea urchins, or in the mantle cavity of gastropods—to the Japanese spider crab, whose body measures about one inch (2.5 centimeters) across, and whose leg span is more than 3 feet (1 meter). They exist in a wide diversity of forms, including the hermit crab, which has no shell of its own and takes over empty gastropod shells for protection.
Crabs are filter-feeders, predators, and scavengers, and their method of obtaining food is usually reflected by the shape of their chelipeds.

With their narrow bodies and well-developed abdomens, shrimps and prawns are much better designed for swimming than the lobster, but even so, most of them are bottomdwellers. Their thoracic legs are usually long and slender, and the first three pairs may have claws. The abdomen has long, fringed swimmerets used for swimming. In females, eggs are attached to them.

Krill are not decapods, even though they are shrimplike in appearance, but belong to the order Euphausi-acea. Like some shrimps, some species of krill are luminescent. They are well designed for swimming, but do not have a carapace completely enclosing the gills, as do most decapods. Most krill are filter-feeders, feeding off zooplankton and phytoplankton.


This order contains the only group of truly terrestrial crustaceans the pill bugs although most species are marine. The shield-shaped body is flat, has no carapace, and has seven pairs of legs. Isopods have a pair of compound eyes and two pairs of antennae.

Water loss can be a problem to wood lice because they do not have a waxy cuticle like some other land-dwelling arthropods, such as insects. They survive by living in fairly damp habitats and by leading a nocturnal existence. Other behavioral adaptations, such as rolling up into a ball, help to reduce water loss.