Crustacea

(invertebrate zoology) A class of arthropod animals in the subphylum Mandibulata having jointed feet and mandibles, two pairs of antennae, and segmented, chitin-encased bodies.

A highly variable, species-rich group of arthropods that have inhabited marine environments since the beginning of the Cambrian Period. Within the marine realm the crustaceans occupy as diverse a spectrum of habitats as the insects inhabit on land.

The hierarchical rank of the Crustacea is a matter of continuing debate. The Crustacea are regarded as a phylum, distinct from other arthropods, by proponents of the concept of polyphyly in the Arthropoda. Alternatively, they are given the rank of subphylum or superclass by those who view the Arthropoda as a monophyletic taxon.

Species of Crustacea such as the shrimp, prawn, crab, or lobster are familiar. However, there are many more with less common vernacular names such as the water fleas, beach fleas, sand hoppers, fish lice, wood lice, sow bugs, pill bugs, barnacles, scuds, slaters, and krill or whale food. The Crustacea are one of the most difficult animal groups to define because of their great diversity of structure, habit, habitat, and development. No one character or generalization will apply equally well to all.

Crustaceans have segmented, chitin-encased bodies; articulated appendages; mouthparts known as mandibles during some stage of their life, however modified they may be for cutting, chewing, piercing, sucking, or licking; and two pairs of accessory feeding organs, the maxillules and maxillae. One or the other pair is sometimes vestigial or may be lacking. The Crustacea are unique in having two pairs of antennae: the first pair, or antennules, and the second pair, the antennae proper. The latter, are almost always functional at some stage of every crustacean's life.

Taxonomy

Not only is there a lack of agreement on the rank of the Crustacea per se, but there is no consensus on hierarchial levels of subordinate taxa. The classification presented here is restricted to extant taxa and is, at best, a compromise among opposing opinions.

Superclass Crustacea

     Class Cephalocarida

     Class Branchiopoda

               Order: Anostraca

                                   Spinicaudata

                                   Laevicaudata

                                   Ctenopoda

                                   Anomopoda

                                   Onychopoda

               Order: Haplopoda

                                   Notostraca

     Class Remipedia

     Class Ostracoda

          Subclass Myodocopa

               Order: Myodocopida

                                   Halocyprida

          Subclass Podocopa

               Order: Platyocopida

                                   Podocopida

     Class Maxillopoda

          Subclass Mystacocarida

          Subclass Cirripedia

               Order: Ascothoracica

                                   Thoracica

                                   Acrothoracica

                                   Rhizocephala

          Subclass Copepoda

               Order: Calanoida

                                   Harpacticoida

                                   Cyclopoida

                                   Poecilostomatoida

                                   Siphonostomatoida

                                   Monstrilloida

                                   Misophrioida

                                   Mormonilloida

          Subclass Branchiura

          Subclass Tantulocarida

     Class Malacostraca

          Subclass Phyllocarida

               Order: Leptostraca

          Subclass Hoplocarida

               Order: Stomatopoda

          Subclass Eumalacostraca

               Superorder Syncarida

               Order: Bathynellacea

                                   Anaspidacea

          Superorder Peracarida

               Order: Spelaeogriphacea

                                   Mysidacea

                                   Mictacea

                                   Amphipoda

                                   Isopoda

                                   Tanaidacea

                                   Cumacea

General morphology

The true body segments, the somites or metameres, are usually somewhat compressed or depressed. Each typically includes one pair of biramous appendages. The linear series of somites making up the body of a crustacean are more or less distinctly organized into three regions or tagmata: the head, thorax, and abdomen. Where regional organization of the postcephalic somites is not clearly marked, they collectively form the trunk. The somites are variously fused with one another in diagnostic combinations in different groups of the Crustacea.

Body

A dorsal shield or carapace of variable length arises from the dorsum of the third cephalic somite and covers the cephalon and cephalothorax to varying extent. The carapace reaches its greatest development in the malacostracan Decapoda (shrimps, lobsters, and crabs). See also Decapoda (Crustacea).

The chitinous cuticle covering the crustacean body is its external skeleton (exoskeleton). The chitin is flexible at the joints, in foliaceous appendages, and throughout the exoskeletons of many small and soft-bodied species, but it is often thickened and stiff in others. It becomes calcified in many species as a result of the deposition of lime salts.

The paired appendages are typically biramous and consist of two branches: the endopod and exopod. The endopod is definitely segmented in the higher Crustacea. The endopods are variously modified to serve a variety of functions and needs such as sensory perception, respiration, locomotion, prehension and comminution of food, cleansing, defense, offense, reproduction, and sex recognition and attraction. If retained in the adult, the exopod may remain leaf- or paddlelike, or become flagellated structures, facilitating swimming or aiding respiration.

The digestive system is a relatively straight tube that curves dorsally from the mouth, which is situated on the underside of the head between the mandibles. In the alimentary tract three regions are recognizable: the foregut, midgut, and hindgut. The anterior part of the foregut often is esophageal in nature, whereas the posterior and greater part usually consists of two parts. In the anterior portion in the Eucarida, a complex and elaborate grinding mechanism, the gastric mill, is developed. The posterior chamber is divided into dorsal and ventral filtering compartments for the straining of food. The midgut in most cases is provided with several ceca or diverticula which produce digestive secretions or serve as organs for the absorption of food. When these ceca are present in considerable number, they become organized into a sizable gland, the midgut gland of higher Crustacea. The hindgut is typically short and terminates in a muscular anus on the underside of the telson.

Most crustaceans have a heart perforated by openings, or ostia, which admit venous blood from the pericardial sinus in which it is located. The heart may be elongated and tubular and extend through the greater part of the body, but generally it is a more compact organ. It pumps blood through an arterial system or through connecting sinuses or lacunae within the body tissues. Some lower Crustacea do not have a heart, and the muscular movements of the animal or its alimentary tract circulate the blood through the body cavities or sinuses. The blood of the great majority of the crustaceans is bluish because it contains the respiratory pigment hemocyanin. A few crustaceans have red blood as a result of the presence of erythrocrurion. See also Respiratory pigments (invertebrate).

Crustacea take up oxygen by means of gills, the general body surface, or special areas of it. Some of the few species that have become more or less terrestrial in their habits have developed modifications of their branchial mechanism such as water-retaining recesses which when sufficiently moist enable them to breathe air. Some sow or pill bugs have special tracheal developments in their abdominal appendages for the same purpose.

In the crustacean nervous system a supraesophageal ganglion somewhat larger than the other ganglia is considered to be the brain. It is connected by circumesophageal commissures to a double ventral nerve cord with segmentally arranged ganglia. These become reduced in number and, in the brachyuran crabs, form a large ganglionic mass centered in the thorax.

The organs of special sense are the eyes, the antennules, and the antennae. Crustacean eyes (photoreceptors) are of two types: the median (nauplius eye) and simple eyes (frontal organs); and compound eyes. Simple and median eyes and lateral eyes consist only of light sensory cells. In contrast, compound eyes are composed of many subunits (ommatidia), each having separate optical elements. Compound eyes, therefore, provide varying amounts of actual vision, and in at least some species color differentiation. The antennules and antennae are provided with a variety of sensory structures for the reception of chemical and mechanical stimuli. Taste chemoreceptors are usually found on the mouthparts and the pereiopods, or walking legs. Many of the hairs (setae) and bristles found on the crustacean body and appendages act as mechanoreceptors. Organs of balance (statocysts) are also present on basal segments of the antennules in many crustaceans. See also Chemoreception; Eye (invertebrate); Photoreception.

Those glands recognized as definite excretory organs are the maxillary and antennal glands in the adult crustaceans. They are located in the cephalon and rarely are both present at the same time. Many crustaceans, especially deep-sea forms, have phosphorescent or luminous organs. The barnacles and some other crustaceans have cement glands. Species that produce encysted or drought-resistant eggs have other glands of special secretion. The most studied crustacean gland is the sinus gland of the eyestalks. This gland is part of a neurosecretory system producing hormones that control color change and pattern, molting cycle, oogenesis, and egg development within the ovary. See also Endocrine system (invertebrate); Neurosecretion.

Reproductive system

The sexes are separate in most Crustacea and usually can be differentiated from each other by secondary sex characters. Chief among these characters are the size and shape of the body, appendages, or both, and placement of the genital apertures. Hermaphroditism is the rule in the Cephalocarida, Remipedia, some ostracods, sessile Cirripedia (barnacles), in isolated cases in other crustaceans, and in certain parasitic forms. Parthenogenesis (eggs developing and hatching without prior fertilization) occurs frequently in some of the lower crustaceans that have what might be called an alternation of generations. The parthenogenetic generations alternate with a generation produced by fertilized eggs. See also Cirripedia.

The eggs of most crustaceans are carried attached to the female until hatched. Some females develop brood pouches in which the young are retained for a time. A nutrient secretion which sustains the young until they are released is produced in some species having a brood chamber. Penaeid shrimp and a few of the lower Crustacea deposit their eggs in the medium in which they live, in some cases attaching them to aquatic vegetation.

Development

The nauplius larva is characteristic of Crustacea. This first larval stage is common in the lower forms, but in many of the higher forms it occurs during development in the egg, and the young are hatched as a different and more advanced larva or, as in many Malacostraca, in a form similar to the adult. Life histories vary from the simple to the complex within the different groups of Crustacea.

Molting (ecdysis)

This process involves several steps: (1) preparation, which includes some degree of resorption of the old cuticle; (2) the formation of a new, temporarily soft and thin one within it; (3) the accumulation and storing of calcium in the midgut gland or as lenticular deposits (gastroliths). The preparatory period is less complicated in the thinly chitinous forms. The actual molt follows. The old shell or cuticle splits at predetermined places, permitting the crustacean within, already enclosed in new but still soft exoskeleton, to withdraw. A temporary absorption of water enables the animal to split or crack its housing. Upon withdrawal of the entire animal, absorption of water again rapidly takes place with a pronounced increase in body size. The chitinous lining of the foregut and hindgut as well as the endophragmal skeleton are shed along with the exoskeleton. In the immediate postmolt period the crustacean is quite helpless and, if unsheltered, is at the mercy of its enemies. The tender new cuticle is reinforced rapidly by the resorbed chitin, and hardened by whatever reserves of calcium the animal may have stored, supplemented and extended by the far more plentiful supplies in solution in the sea which may be absorbed or ingested by the growing crustacean. Molting takes place quite frequently in the larval stages when growth is rapid, but becomes less frequent as the animal ages. In many species there is a terminal molt at maturity. Hormones from the sinus gland play an important role in both initiating and inhibiting molting. See also Gastrolith.

Autotomy and regeneration

The mechanisms of autotomy and regeneration are developed in the crustaceans to minimize injury or loss to an enemy. When an appendage is broken, it is cast off or broken at the fracture or breaking plane. This sacrifice often enables the victim to escape. Even more remarkable is the fact that crustaceans, by voluntary muscular contraction, can part with a limb which may be injured. Crustacea also have the ability to regenerate lost parts. Although the regenerated parts are not always the same size as the original in the first molt after injury, increase in size in successive molts soon restores a lost limb to virtually its former appearance.

Geologic history

Exceptionally well-preserved arthropods have been discovered in Late Cambrian rocks. The oldest true crustaceans are ostracods, dating from 570 million years ago (Ma). Although the ostracods are not the most primitive crustaceans, their remains constitute the oldest authentic record of the Crustacea. The appearance of more primitive crustaceans later in geologic history, the reverse of what would be expected, is more related to their preservation potential than to their evolutionary history. The more advanced classes, the Maxillopoda and the Malacostraca, appeared in the fossil record, respectively, in the Middle Cambrian, about 530 Ma, and the Late Devonian, about 370 Ma. See also Cephalocarida.

Bionomics and economics

Crustacea are ubiquitous. They live at almost all depths and levels of the sea, in fresh waters at elevations up to 12,000 ft (3658 m), in melted snow water, in the deepest of the sea's abysses more than 6 mi (9 km) down, and in waters of 0°C (32°F) temperature. Some species live on land, although most must descend to salt water areas again to spawn their young. Some live in strongly alkaline waters and others in salt water which is at the saturation point, still others in hot springs and hydrothermal vents with temperatures in excess of 55°C (131°F).

Crustacea are of all sizes, ranging from copepods 0.01 in. (0.25 mm) long to huge spider crabs of Japan, which span 12 ft (3.7 m) from tip to tip of the laterally extended legs. The American lobster, the heaviest so far known, tops all crustaceans at 44½ lb (20 kg).

Most crustaceans are omnivorous and essentially scavengers. Many are filter feeders and screen particulate life, plankton, and organic detritus from the waters in which they live; others are largely carnivorous, still others vegetarian. Among the vegetarians are the grazers of the ocean meadows which convert the microscopic plant life (diatoms) into flesh and food for larger animals which in turn are harvested as food for humans.

Zoological class of hard-shelled marine arthropods (shellfish) including crabs, crayfish, lobster, prawns, scampi, shrimps.

A class of arthropods including the lobsters, crabs, shrimps, wood lice, water fleas and barnacles.