Malacostracan, any member of the more than 29,000 species of the class Malacostraca (subphylum Crustacea, phylum Arthropoda), a widely distributed group of marine, freshwater, and terrestrial invertebrates. Lobsters, crabs, hermit crabs, shrimp, and isopods are all malacostracan crustaceans.
Malacostracans are the most numerous and most successful of the four major classes of Crustacea. Their members constitute more than two-thirds of all living crustacean species. They exhibit the greatest range of size (less than one millimetre, or 0.04 inch, to a limb spread of more than three metres, or 10 feet) and the greatest diversity of body form. Malacostracans are abundant in all permanent waters of the world: in the seas from the tropics to the poles and from the tidal zone to the abyss; in surface and subterranean fresh waters of all continents except Antarctica (where they once existed); and terrestrially on all continental landmasses and all tropical and temperate islands.
The success of malacostracans can be attributed primarily to their increased body size and to the evolution of more functional body regions and more sophisticated food-gathering appendages than possessed by their Paleozoic ancestors (542 million to 251 million years ago). This evolutionary thrust has been marked by the development of ambulatory legs and specializations for benthic life and by the brooding of eggs and suppression of free-living larval development. Especially significant has been a shift of food-gathering limbs from head to thorax and of swimming appendages and respiratory organs from head to thorax and finally to the abdomen. This rearward shift freed the antennae for the development of specialized organelles sensitive to odours, sounds, vibrations, and physical contact and added more appendages (maxillipeds) to the mouthpart field. Such changes have enabled terrestrial malacostracans to utilize efficiently the new food resources that have accompanied the evolution and proliferation of vascular plants from the late Paleozoic to the present.
Size range and diversity of structure
Some decapod crabs have leg spans of more than three metres, and others weigh more than 10 kilograms (22 pounds). Some free-living members of the orders Amphipoda, Isopoda, and Stomatopoda are lobster-sized (25–30 centimetres [0.8 to 1 inch]); most, however, are medium (one to three centimetres) in size. Paleozoic and primitive extant taxa seldom exceed 10 centimetres in body length, and the adult stages of some parasitic and subterranean groups are very small (less than one millimetre).
Malacostracans have a fixed body plan of head, thorax, and abdomen. In the adult the head consists of five segments, the thorax of eight, and the abdomen typically of six (or rarely seven) unfused segments. The head supports paired compound eyes, two pairs of antennae, and three pairs of short, chewing mouthparts, each consisting of two branches. The eyes are usually pigmented and borne on movable stalks, but they are sessile on the sides of the head in isopods, amphipods, and members of some smaller groups. The first antennae (antennules) usually have two branches (three in the subclass Hoplocarida). The outer branch of the second antennae (antennal squame), which is usually flat and bladelike for elevation and swimming balance, has two segments in stomatopods and some mysids and one segment in syncarids and eucarids; it may be small or lost entirely in amphipods, isopods, and other bottom-dwelling or subterranean taxa. The first and second maxillae are short, with variable numbers of inner biting plates (endites) and often with outer lobes (epipodites), but the palps are short or lacking.
From the hindmost (maxillary) segment projects a head shield, or carapace, which in primitive forms is large and covers the thorax, leg bases, and gill chamber. It may be fused to the dorsum of the thorax, as in the euphausiids, decapods, and other members of the superorder Eucarida, but it is variously reduced and fused only to the anterior thoracic segments in mysids or lost altogether in the isopods, amphipods, and syncarids.
The eight pairs of thoracic legs are typically biramous (two-branched). One or more pairs are modified for feeding in some groups. In free-swimming species all legs are similar in shape, and both branches are slender. In bottom-dwelling species, however, the inner branch has become a stiff walking limb, and the slender multisegmented outer branch is variously reduced (in hemicarideans) or lost altogether (in amphipods and isopods). In advanced, especially bottom-dwelling, malacostracans (such as lobsters), one or more legs are pincerlike.
The abdomen bears on each but the last segment a pair of ventral, or ventrolateral, biramous limbs called pereopods, or pleopods, which are primarily used in swimming. In the males of all eucaridans, hoplocarids, isopods, some hemicarids and syncarids, and rarely some amphipods, the anterior one or two pairs may be specially modified for sperm transfer. In males of most mysidaceans, the fourth and fifth pleopods (and the first and second uropods of some amphipods) may be modified as claspers for holding the female during mating. The last abdominal segment (of all but the leptostracans) bears a pair of biramous uropods and a median plate, or telson. The uropods are usually setose and paddle-shaped in swimming taxa and form a broad tail fan with the telson for rapid propulsion. In benthic and subterranean species the uropods are often slender, elongate, and tactile in function. The telson is bilobed in juvenile syncarids, larval eucaridans, some mysids, and most amphipods but platelike in all other malacostracans.
Distribution and abundance
The class Malacostraca contains more than 29,000 living species and represents about half of all known crustacean species. It is the single largest group not only of marine arthropods but also of all fully aquatic arthropod taxa. Within the Malacostraca, Decapoda is the largest order, with more than 10,000 described species, followed by the orders Isopoda (10,000 species) and Amphipoda (6,200 species). The other major orders have fewer than 1,000 species each.
Most malacostracans are marine. Among the decapods, the ancient palinurans, their modern brachyuran (10-legged crab) derivatives, and the dendrobrachiate and stenopodid shrimps dominate in tropical and temperate marine shallows. The decapod caridean shrimps, astacidean lobsters and crayfish, and anomurans (hermits and eight-legged crabs), however, dominate in cold-water and polar regions, in the deep sea, and in continental fresh waters. The amphipods and isopods are also abundant along cold-water marine shores and in the abyss and have widely penetrated fresh waters. They are also widespread in underground waters and terrestrial environments. Stomatopods are largely confined to tropical marine shallows; tanaids and cumaceans are found mainly in the colder deeps; and mysids, though mainly marine, are also abundant in relicts of northern glacial lakes.
Importance to ecology and to humans
Malacostracans are often predators and scavengers. They are important ecologically in ridding the sea bottom and seashores of decaying animal and plant matter and in serving as middle-level converters of organic food energy to animal protein in a form suitable for fish, sea birds, marine mammals, and ultimately humans. The decapods and euphausiids (krill, order Euphausiacea) are the only malacostracan groups that have a major direct economic value to humans.
Reproduction and life cycles
The malacostracan life cycle typically involves an egg stage; a series of free-swimming, plankton-feeding larval stages; a series of immature (subadult) growth stages; and finally a sexually mature (reproductive) adult stage. Hermaphroditic adults are present in a few isopods. In the primitive swarming type of reproduction the male seeks out the female in the open water, usually in synchrony with lunar periodicity, cycles of temperature, or food availability. Mating (copulation) is very brief, often completed in a few seconds and usually following the reproductive molt of the female, when her exoskeleton is still soft. The eggs are fertilized as they are extruded from the oviductal opening on the sternum of the sixth thoracic segment. In many species males do not feed, do not reproduce again, and do not live long after mating. Fertilized eggs may be shed freely in the sea, where they hatch, usually into nauplius larvae. In marine groups that brood the eggs by attaching them to the pleopods, the eggs hatch as late-stage larvae, which are often carnivorous (e.g., zoeae and phyllosoma larvae of decapods, antizoeae and pseudozoeae of stomatopods). These larvae eventually sink or swim to the bottom and pass through one or more stages prior to attaining the juvenile stage. Where embryos develop within a thoracic brood pouch, the larval stages are suppressed. The embryos typically hatch as immature forms of the adult (e.g., Isopoda, juveniles of the orders Mysidacea and Amphipoda), but parental brooding may be continued for a further few molts. In the deep sea and in fresh waters, whether embryos are laid freely (superorder Syncarida) or brooded on pleopods (decapods) or in a thoracic pouch (isopods and amphipods), they hatch as juveniles or immature adult forms.
In the more advanced, especially bottom-dwelling, malacostracans or in those with specialized habits, mating usually takes place on or in the bottom. Males may attend, guard, or carry the female for some time (preamplexus) prior to copulation (amplexus), and mating may be prolonged for several hours; the male usually continues to feed, molt, and mate further (in isopods, creeping decapods, and benthic amphipods). Where the female exoskeleton variously hardens prior to mating, the oviductal opening is often complex, and sperm transfer is assisted by correspondingly modified first and second pleopods of the male (“internal” fertilization of stomatopods, isopods, and the superorder Eucarida). Newly hatched late larvae or juveniles may be initially guarded or carried by the female (in stomatopods and some amphipods and isopods).
Malacostracans are primarily swimmers and secondarily walkers, clingers, and burrowers. Swimming is accomplished primitively by coordinated, synchronous beating of the biramous head appendages in early larval stages and thoracic appendages in later larval stages and the adult stages of leptostracan shrimp, mysids, and syncardids and in krill, decapods, and other eucarid malacostracans. The swimming action characteristic of adult malacostracans is provided by abdominal pleopods. Typically, each of the first five abdominal segments bears, on the ventral (lower) surface, a pair of pedunculate, biramous pleopods. In order to beat in unison, each pair is usually hooked together by spines on the inner margin of the peduncle (retinacula) or the inner ramus (“clothespin spines”). The amphipods are unique in having only three pairs of pleopods, the last two pairs being modified as stiff, thrusting uropods. In primitive forms the pleopod rami are slender and segmented (annulate), as in amphipods and procarididean decapods, all of which are primarily swimmers as adults; however, in all the other malacostracan groups, most of which are crawlers and burrowers, the rami are broad, flaplike, and unsegmented. The pleopods are typically reduced, or even lost, in many burrowers. The swimming crabs use paddlelike fifth thoracic legs for propulsion. Abrupt swimming propulsion is provided by the tail fan. In amphipods the tail fan (consisting of three pairs of uropods and telson) provides a sudden forward thrust. In eucaridans (especially decapods) the tail fan (paired uropods and telson) provides a characteristic “tail-flip” or sudden backward escape reaction.
In most benthic malacostracans the hind five to seven pairs of thoracic legs have become essentially uniramous (single-branched)—the inner branch is thickened and stiffened and adapted for walking or crawling. In amphipods the first four pairs are pointed forward and the last three backward, an adaptation for perching, clinging, climbing in “inchworm” fashion, or jumping.
In burrowing malacostracans, especially decapods and stomatopods, the distal segments of some legs attain a pincerlike form that facilitates both digging and removal of the soft substratum. In many species of burrowing amphipods, the claws are reduced, but the adjacent segments are much broadened, strongly spined, and powerfully muscled. Rapid leg movements, often aided by the fanning action of setose antennae and the hydraulic tunneling motion of powerful pleopods, enable these torpedo-shaped crustaceans to swim through loose sandy substrata, feeding as they go.
Food and feeding
Malacostracans consume virtually every available kind of organic matter, plant or animal, living or dead. The small- to medium-sized animals primarily consume detritus and plankton, and some parasitize other aquatic organisms. The larger-sized malacostracans are mainly carnivores and scavengers, preying on a wide range of small invertebrates and fishes or devouring the carcasses of whales, seals, fishes, and large invertebrates. Burrowing and small groundwater malacostracans are filter feeders, consuming microorganisms and bacteria from the sediments. Terrestrial isopods and amphipods consume forest leaf litter and algae at the tide lines.
Malacostracans capture or obtain their food primarily by using their thoracic legs. In early free-swimming larvae and the adults of some filter-feeding or deposit-feeding amphipods, isopods, and hemicarideans and in large carnivorous palinuran decapods, food may be gathered (occasionally killed) by means of the antennae and other head appendages. In carnivorous, or raptorial, species one or more of the thoracic legs are enlarged, and the tips are pincerlike, allowing the animal to capture, kill, and initially shred its prey. In lobsters and crayfish the first walking leg (fourth thoracic) is fully cheliform (pincerlike), and either the left or right claw is massive, with pavementlike teeth for crushing hard-shelled prey such as snails and clams. In “spearer”-type stomatopods the raptorial claw is toothed and spiny for stabbing soft-bodied prey. “Smashers” have a swollen, hammerlike claw for crushing hard-bodied prey.
Malacostracans (except for leptostracans) typically have one to three pairs of thoracic limbs modified as accessory mouthparts. These maxillipeds (or “jaw legs”) pass food to the masticatory, or chewing, mouthparts of the head proper. The thoracic segment of the first pair of maxillipeds is usually fused to the head, forming a cephalon. In stomatopods the first five pairs are called maxillipeds, but only the first pair is functionally so and its body segment is not fused to the head. In amphipods the first two pairs of thoracic legs may also function as food-pushing limbs, but their segments are typically free. In decapods the first two or three pairs serve as maxillipeds, and their segments are fused within the cephalothorax.
The mouthparts generally reflect feeding habits. In flesh eaters and scavengers the mandibular incisors are typically large and the plates and palps of the maxillae and maxillipeds are armed with strong spines and cutting edges, whereas the molar is small or lacking. In those species that consume all organic material and in those that consume only plants, the molar is usually strong, with an inner grinding surface. In filter feeders the plates of the maxillules, the maxillae, or both may be enlarged and equipped with a large number of fine-filtering (plumose) setae. Accessory (baler) plates, for directing feeding currents, are often well developed (e.g., in cumaceans and haustoriid amphipods).
Although malacostracans are typically free-living animals, members of several taxa, especially among the amphipods, decapods, and isopods, have formed symbiotic, commensal, and even fully parasitic relationships with other invertebrates, fishes, marine mammals, and reptiles. Many decapods, especially porcellanid and xanthid crabs, live permanently in cavities among sponges, corals, and bryozoans. Some amphipods live within the respiratory and feeding cavities of sponges, tunicates, and anemones. Lafystiid and some lysianassid amphipods, as well as aegid, cymathoid, and immature gnathiid isopods, are external parasites of fish. Cyamid amphipods occur on whales and some hyalid amphipods in the buccal cavities of marine turtles. Epicaridean isopods are fully parasitic on other crustaceans, especially decapods. The body of the host may be much deformed and the body of the parasitic female very much transformed, quite unlike the small, symmetrically segmented, and otherwise normal male.
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