Decapoda

(invertebrate zoology) A diverse order of the class Crustacea including the shrimps, lobsters, hermit crabs, and true crabs; all members have a carapace, well-developed gills, and the first three pairs of thoracic appendages specialized as maxillipeds. An order of dibranchiate cephalopod mollusks containing the squids and cuttle fishes, characterized by eight arms and two long tentacles.

(Crabs, shrimps, and lobsters)

Phylum: Arthropoda

Subphylum: Crustacea

Class: Malacostraca

Order: Decapoda

Number of families: 151

Thumbnail description
Crustaceans with a large carapace covering the head and thorax and enveloping the gill chambers; also possess five pairs of legs

Evolution and systematics

The order Decapoda falls within the class Malacostraca, a group of crustaceans that comprises twice as many species as all other crustacean classes combined. Malacostracans are distinguished by bodies divided into 19 segments (five head, eight thoracic, and six abdominal segments) and the location of their genital openings. Many of the divisions within the Malacostraca are based in part on the number of thoracic appendages that have been integrated into the head region to function as mouthparts; these appendages are termed maxillipeds. In the decapods, the first three thoracic appendages are maxillipeds, leaving five pairs of legs or ten feet for walking. The order name Decapoda literally means "ten-footed."

The two suborders within the Decapoda are based in part on differences in the gill structure. The gills of the Dendrobranchiata consist of bundles of branching filaments, while those of the Pleocyemata are unbranched—either as filaments (trichobranchs) or more commonly as unbranched leaflike plates (phyllobranchs). The two groups also differ in their method of reproduction: female Pleocyemata brood their eggs on their abdominal appendages until they hatch, while female Dendrobranchiata spawn their eggs into the sea.

Many of the subsequent divisions within the Pleocyemata can be recognized by the number and position of appendages with claws. The classification of crustaceans remains in flux, however, reflecting the great degree of uncertainty as of 2003 about the relationships among the groups. The following list represents one of the more common classifications of the major groups within the Decapoda:

• Suborder Dendrobranchiata
• Superfamily Penaeoidea, the penaeid shrimps
• Superfamily Sergestoidea, the sergestids
• Suborder Pleocyemata
• Infraorder Caridea, the true shrimps
• Infraorder Stenopodidea, the boxer shrimps
• Infraorder Astacidea, the clawed lobsters and crayfish
• Infraorder Thalassinidea, the ghost and mud shrimps
• Infraorder Palinura, the spiny and slipper lobsters
• Infraorder Anomura, the hermit crabs, king crabs, and porcelain crabs
• Infraorder Brachyura, the true crabs

The earliest decapod to appear in the fossil record is Palaeopalaemon newberryi, which lived during the late Devonian period, about 360 million years ago (mya). It shares characters of both lobsters and shrimps, leaving unanswered questions about its taxonomic affiliations. The Dendrobranchiata are considered the most primitive extant decapods, but early fossils of dendrobranchs are lacking. Fossil representatives of all of the infraorders except the Stenopodidea are known from the Triassic (213 mya) and Jurassic (145 mya) periods onward, and the Brachyura have shown extensive radiation (diversification) since the Eocene epoch, 33.7 mya.

Physical characteristics

In addition to having five pairs of legs, decapods are distinguished by the carapace that covers the dorsal portion of the head and thorax, forming a single functional unit termed the cephalothorax. The sides of the carapace extend downward to envelop the gills, forming lateral branchial chambers. Although the vast majority of decapods are aquatic and breathe with gills, some terrestrial forms have developed blood vessels in the inner surface of the branchial chambers so that they function as lungs.

Decapods have two pairs of antennae, as do all crustaceans. The first pair typically bears special chemosensory structures that govern the sense of smell, while the second pair is often elongate and tactile. The foremost legs often have claws that perform several functions related to feeding, mating, and defense.

The structure and function of the abdomen varies among the different decapod taxa. Lobsters and shrimplike forms have large muscular abdomens that terminate in a flattened tail fan. To evade predators, these animals snap their tail fans rapidly beneath their abdomens, which propel them backwards. Shrimps and other decapods that don't have heavy or thick exoskeletons use their abdominal appendages to swim forward. At the other extreme, the true crabs or brachyurans have lost most of their abdominal appendages; their abdomen plays no role in locomotion. The few remaining appendages in brachyurans are used only for egg attachment in females or as copulatory structures (gonopods) in males.

Decapods exhibit tremendous diversity in shape, size, and color. They range in size from minute parasitic pea crabs to the giant Japanese spider crab Macrocheira kaempferi, which has legs spanning up to 12 ft (3.7 m). Many species have distinctive color patterns, while others are able to change color by expanding or contracting specialized groups of pigment cells in the epidermis underlying their exoskeleton.

The many variations of the basic decapod body plan reflect the great success of this group and their adaptation to a wide range of habitat types and ecological roles. Although the familiar terms "shrimp," "lobster," and "crab" have no formal taxonomic meaning, they do represent three distinct and recognizable body plans. Of the shrimplike forms, the Dendrobranchiata and the Caridea are the most familiar; they include all commercially harvested shrimps and prawns.

The Dendrobranchiata have perhaps the least-modified body plan of all the Decapoda and also show little diversity in form. The first three pairs of legs on penaeid shrimps have small claws that are equal in size. This is a relatively small group of approximately 350 species, and is primarily tropical or subtropical in distribution. Some Caridea superficially resemble the penaeids, but the third pair of legs never has claws, and the abdomen usually has a pronounced hump in the middle. This diverse group shows an unusually wide range of variation in body form; there are about 1800 described species.

The Stenopodidea are a very small group (25 species) of tropical shrimps. Like the penaeids, they have claws on their first three pairs of legs, but their third pair is greatly enlarged. The ghost and mud shrimps (Thalassinidea) are an especially problematic group whose taxonomic affinities remain unclear. The Thalassinidea are thin-shelled burrowing forms that often have large claws on the first pair of legs and small claws on the second.

Heavily armored crustaceans with large muscular abdomens are commonly referred to as "lobsters"; again, this category combines groups with little relationship to one another besides their desirability as food. The Astacidea (crayfish and clawed lobsters) infraorder encompasses about 800 species that have large claws on the first pair of legs and small claws on the next two. The infraorder Palinura is a small (130 species) group found primarily in tropical and subtropical waters, which contains the spiny and slipper lobsters along with some deep-water forms that should probably be classified elsewhere. Slipper and spiny lobsters lack true claws on their front appendages and have a distinctive and unusual flattened larval form known as a phyllosoma.

Crabs have the most compact decapod body form. In the true crabs or Brachyura, the abdomen is greatly reduced in size, folded beneath the body, and involved only in reproduction. Only the first pair of legs has claws. This is a very successful group, accounting for roughly half the 10,000 known species of decapods. The Anomura is composed of a wide range of unusual crablike animals, including the hermit crabs, king crabs, sand crabs, and porcelain crabs. It is an almost entirely marine group, with about 1800 species worldwide. Like the Brachyura, the Anomura usually have claws on their first pair of legs; however, anomuran crabs can be distinguished by the fact that their last pair of legs is much smaller and often tucked into the gill chamber. In addition, their second pair of antennae is positioned beside the eyes rather than between them as in the brachyurans.

Anomurans have the widest range of variation in abdominal form of any of the decapods. Hermit crabs have elongate, soft, asymmetrical abdomens that they protect with an empty snail shell. The king crabs are believed to have evolved from hermit crabs; although the abdomen is tucked underneath the body, all retain the asymmetry of hermit crabs and some even have completely soft, unprotected abdomens. At the other extreme, porcelain crabs have thin flat abdomens superficially similar to those of true crabs, but have retained a tail fan and a limited ability to swim.

Distribution

Decapod crustaceans occur worldwide. The marine species are most diverse in the Indo-Pacific region but relatively uncommon in extreme deep-sea and polar waters. Freshwater crabs and crayfish are most diverse in Southeast Asia and North America, respectively; crayfish occur naturally on all continents except Africa and Antarctica. Terrestrial decapods are largely limited to tropical and subtropical regions and occur on all continents except Europe and Antarctica. Approximately 90% of all species of decapods are marine; fewer than 1% are terrestrial.

Habitat

Although there are some species of midwater shrimps that seldom if ever encounter the bottom of the ocean, the vast majority of decapods are associated with benthic habitats. Marine decapods live in all types of habitats from intertidal mud flats to deep-sea hot vents. Such highly structured habitats as rock or coral reefs, seagrass beds, and mangrove forests support the greatest numbers of species, but even featureless sand and mud bottoms support many types that are adept at burying themselves. Most decapods emerge to feed only at night when fewer predatory fishes are active.

Structures to hide in are also important for freshwater decapods; they are most abundant in vegetated areas. Many freshwater decapods construct burrows that allow them to remain in contact with ground water when their pond dries up. Terrestrial decapods are also dependent on water; while some crabs can occur as much as 9 mi (15 km) from the ocean and up to 3,280 ft (1,000 m) above sea level, they all have planktonic larvae and must return to the ocean to breed.

Behavior

Decapods exhibit many complex and even spectacular behaviors. The Caribbean spiny lobster Panulirus argus sometimes migrates toward deeper water in long lines or queues of as many as 65 individuals. The reason for these migrations is not entirely clear, but seems to be associated with avoidance of winter storms. Juvenile red king crabs (Paralithodes

camtschaticus) often gather together into mounds that may contain thousands of individuals, possibly to deter predators.

Many decapods use visual and even auditory signals to communicate with one another. Sounds are usually produced with some type of stridulating surface, and this means of communication is most common in terrestrial and semiterrestrial species. Stridulation refers to sounds produced by rubbing body parts together. Communication using pheromones appears to be common in aquatic species, especially in conjunction with mating. Pheromones are released in the urine via the antennal gland; when crayfish fight they literally blow pheromone-laden urine into the face of their opponent.

Experiments have demonstrated that crabs and lobsters are capable of such complex learning as navigating a path through a maze. Crabs that are offered novel items of prey quickly learn the most effective way to feed on them, and decapods have been taught to respond to a specific cue or to discriminate among colors.

The activity patterns of intertidal species are often synchronized with the tidal cycle. For example, a crab may emerge to feed only during nighttime high tides, and this pattern becomes set in the animal's own biological clock. When the crab is placed in captivity away from all tidal influence, its set rhythm of activity can persist for days or even weeks.

Many marine decapods form symbiotic associations with other organisms. Some shrimp set up cleaning stations where fishes line up to be picked over for parasites. Others may live full-time with a fish, building and maintaining a shared burrow while the fish watches for predators. Far more have established less formal associations with larger organisms that offer protection from predators, such as the many kinds of shrimps that associate with sea anemones.

Feeding ecology and diet

Decapods employ a wide variety of feeding techniques, ranging from filter feeding, grazing, and deposit feeding to predation. Some are specialists that use just one of these methods, while others are generalists that make use of several different techniques depending on the circumstances. One of the most common misconceptions about crabs and other decapods is that they are primarily scavengers, since many are harvested from baited pots. Most large marine crustaceans are actually very efficient predators and only scavenge when the opportunity arises. The decapod body plan allows for a great degree of specialization in feeding structures; this specialization is especially apparent in the structure of the claws. Fast, slender claws can be used to snatch elusive prey, while massive, strong claws armed with molars can exert tremendous force on mollusks and other hard-shelled prey. Much of the elaborate ornamentation and other features seen in marine snails and other mollusks is a side effect of predation by shell-crushing crabs, as these groups are engaged in what is essentially an evolutionary arms race.

In many cases decapods have evolved asymmetrical claws, thus providing more than one type of tool for subduing and extracting prey. For example, the pistol or snapping shrimp of the family Alphaeidae have one enormously enlarged claw that produces a loud noise when snapped and can stun or kill prey. Apart from its use as a weapon, however, the large claw is virtually useless for most other functions, and the much smaller claw on the other side is used to convey food to the shrimp's mouth. On the other hand, claws are not essential to predatory decapods: palinurid lobsters and many shrimps lack large claws, but are still remarkably effective predators. Spiny lobsters have exceptionally strong mandibles (jaws) that are used to crush mollusks and other prey, and many shrimp are quite skilled in using their walking legs to envelop worms or smaller crustaceans.

In many tropical areas land crabs play the role of earthworms, as the primary recyclers of fallen leaves and other plant material. They also help to turn over the soil with their burrowing activities. Many marine and freshwater species are herbivorous as well, but most will occasionally ingest animal material when it is available.

A number of anomuran crabs and caridean shrimps are exclusively filter feeders, pulling detritus and plankton out of the water with their maxillipeds, antennae, or modified legs. Many more are deposit feeders, consuming much the same type of material after it has settled out of the water column. Deposit feeding crabs often have downturned claws that are adapted for scooping up sediment so that it can be processed by the animal's mouth parts.

Reproductive biology

With the exception of one species of crayfish, decapods reproduce sexually; and in almost all cases the sexes are separate. A number of caridean shrimps are protandric hermaphrodites, which means that they mature as males first and later change into females; a few species retain male structures and become functional hermaphrodites. Some species of snapping shrimps live in colonies with only one reproducing "queen," much like social insects.

In species where there is intense competition for mates, the claws of males are often proportionately much larger than those of females. An extreme example of this disproportion is seen in the fiddler crabs (Uca spp.), in which one of the claws of the male is useless for feeding and functions only to attract females and duel with other males. Courtship and mating can take anywhere from seconds to weeks, depending on the species. Copulation in penaeid and caridean shrimps is often an instant affair, while mating in crabs is often a long process involving extended periods of guarding before and after mating. In these cases the female crab can only mate while in her soft-shell condition, so she must molt her exoskeleton immediately prior to mating. A female preparing to molt releases pheromones that attract males; the male will embrace and carry the female for days or even weeks preceding her molt. Male hermit crabs are often seen dragging a smaller female about by the shell in anticipation of mating.

Fertilization can be external or internal, depending on the taxon. Many female brachyuran crabs have internal receptacles for sperm storage where sperm can remain viable for years, and males have evolved a number of different strategies to help insure that it is their sperm that fertilizes the eggs. The male will often stand guard over the female while she is soft to prevent others from mating with her, or block her genital openings with a sort of "chastity belt" to prevent matings with other partners.

In all groups except the Dendrobranchiata the eggs are brooded on the female's pleopods (abdominal appendages) until they hatch. Females clean and aerate their egg masses, and a chemical cue from the eggs stimulates the female to violently shake the mass when the larvae are ready to be released. Parental care generally ends with hatching, although young crayfish often continue to associate with their mother for protection. Some tropical crabs that make use of the freshwater trapped in bromeliad plants practice true maternal care, bringing food to their developing young and protecting them from predators.

Decapods in cold and temperate regions usually release their larvae in the spring to coincide with plankton blooms, while those in the tropics often reproduce year round. In most cases development consists of several planktonic larval stages followed by a stage that makes the transition to a benthic existence; in crabs these are known as the zoea and megalops stages, respectively. The megalops has well-developed pleopods and can swim like a shrimp, but once it finds an appropriate place to settle it molts to the first juvenile stage and the pleopods are lost.

Conservation status

In the year 2002 there were 176 species of decapods listed by the IUCN. Of these, 159 were freshwater crayfish; all but two of the remaining species were shrimps or brachyuran crabs that also live in freshwater. Freshwater species often have very limited distributions, making them especially susceptible to habitat destruction or degradation. Only one marine species was listed, but virtually nothing is known about the populations of most marine decapods, especially those that are not commercially exploited. Three species of crayfish and three species of shrimp were listed as threatened in the United States.

Significance to humans

The order Decapoda encompasses nearly all of the crustaceans that are used for human consumption, and supports many large and valuable fisheries. In addition, penaeid shrimps and crayfish are extensively cultured for food in many parts of the world.

A number of human fatalities have been caused by the consumption of poisonous crabs. Several reef-dwelling species of Indo-Pacific crabs appear to acquire toxins from their food; since toxicity varies with the crab's diet and location, it can be very difficult to determine whether one of these crabs is safe to eat. In other areas, decapods are often host to such schistosome parasites as lung flukes, which can infect humans who eat raw or poorly cooked freshwater crabs or crayfish.

In some rice farming areas land crabs are considered serious pests, both because they eat the plants and because they dig burrows that drain water from the fields. The burrowing activities of thalassinid shrimps have had serious effects on oyster culture; when present in large numbers they loosen the substrate to such an extent that the oysters sink into it and are smothered.

The intentional and unintentional introduction of some decapods to new areas has caused a number of detrimental effects. For example, the accidental introduction of the European green crab (Carcinus maenas) to the eastern coast of the United States has resulted in serious population reductions in the clam fisheries there. Various intentional introductions of freshwater crayfish have resulted in crop damage and threats to native species of crayfish.

Species accounts

Resources

Bauer, Raymond T., and Joel W. Martin, eds. Crustacean Sexual Biology. New York: Columbia University Press, 1991.

Bliss, Dorothy E. Shrimps, Lobsters and Crabs. New York: Columbia University Press, 1989.

Burggren, Warren W., and Brian R. McMahon. Biology of the Land Crabs. Cambridge, U.K.: Cambridge University Press, 1988.

Crane, Jocelyn. Fiddler Crabs of the World. Princeton, NJ: Princeton University Press, 1975.

Factor, Jan Robert. Biology of the Lobster. New York: Academic Press, 1995.

McLaughlin, Patsy A. Comparative Morphology of Recent Crustacea. San Francisco: W. H. Freeman and Company, 1980.

Schram, Frederick R. Crustacea. Oxford, U.K.: Oxford University Press, 1986.

Warner, G. F. The Biology of Crabs. New York: Van Nostrand Reinhold Company, 1977.

Williams, Austin B. Shrimps, Lobsters, and Crabs of the Atlantic Coast of the Eastern United States, Maine to Florida. Washington, DC: Smithsonian Institution Press, 1984.

Dew, C. B. "Behavioral Ecology of Podding Red King Crab, Paralithodes camtschatica." Canadian Journal of Fisheries and Aquatic Sciences 47, no. 10 (1990): 1944–1958.

Diesel, R. "Maternal Care in the Bromeliad Crab, Metopaulias depressus: Protection of Larvae from Predation by Damselfly Nymphs." Animal Behaviour 43, no. 5 (1992): 803–812.

Skinner, D. G., and B. J. Hill. "Feeding and Reproductive Behaviour and their Effect on Catchability of the Spanner Crab Ranina ranina." Marine Biology 94, no. 2 (1987): 211–218.

Zhang, Dong, et al. "Mating Behavior and Spawning of the Banded Coral Shrimp Stenopus hispidus in the Laboratory." Journal of Crustacean Biology 18, no. 3 (1998): 511–518.

Blue Crab Home Page. July 2001 [25 July 2003].

Crayfish Home Page. 24 Nov. 2002 [25 July 2003]. .

Fiddler Crabs (Genus Uca. 31 March 2003 [25 July 2003]. .

[Article by: Gregory C. Jensen, PhD]

One of the more highly specialized orders of the class Crustacea. This order includes the shrimps, lobsters, hermit crabs, and true crabs. The order is so diverse that satisfactory definition is difficult, but a few characters are common to nearly all decapods. The most obvious is a head shield, or carapace, which covers and coalesces with all of the thoracic somites and which overhangs the gills on each side. The first three of the eight pairs of thoracic appendages are specialized as maxillipeds and closely associated with the true mouthparts. The gills are usually well developed and arranged in several series.

Decapods vary in size from less than 1/2 in. (1.25 cm) in length to that of the giant Japanese crab, the largest living arthropod, a spider crab which may span more than 12 ft (3.6 m) between the tips of the outstretched claws. Although most decapods are found in the sea, they are by no means restricted to that habitat. Crayfishes are well-known inhabitants of freshwater streams and ponds, as are several kinds of shrimps and some true crabs. A number of crabs and hermit crabs have become well adapted to a terrestrial existence far from water; they return to the sea only seasonally to hatch their eggs. Many crayfishes burrow in the ground, and one species of crab spends its entire life in the tops of lofty trees.

In a general way, the decapods may be divided into two groups: the long-tailed and the short-tailed forms. The long-tailed species, such as the shrimps and lobsters, have a more or less cylindrical, or laterally compressed, carapace that often bears a head spine or rostrum. The large, muscular abdomen permits the shrimp, or lobster, to dart quickly backward out of danger, but the succulence of this tail makes the animals prey to numerous enemies, including humans. In the short-tailed species, the crabs, the carapace is often broadened and flattened dorsally and usually does not form a rostrum. The much-reduced and feeble abdomen is tucked under the thorax, where it serves the female as a brood pouch for eggs.

Physiology

The nervous system is somewhat variable. Although usually fewer, there may be as many as 11 ganglia, 5 thoracic and 6 abdominal, in the ventral nerve chain posterior to the subesophageal ganglion. The sense organs include eyes, statocysts, olfactory filaments, and tactile setae. The eyes are well developed in most decapods, but they may be reduced or entirely absent in species living in the deep sea, in caves, or in burrows. Luminous organs of various kinds occur in a number of deep-sea decapods, and some shrimps have the ability to eject a luminous fluid.

As in most crustaceans, the alimentary tract is a more or less straight tube divided into three parts: the foregut, the midgut, and the hindgut. The first and last have a chitinous, and sometimes calcified, lining. The foregut, in most decapods, is dilated to form a stomach. Its anterior, or cardiac, portion is at least partially lined with movably articulated, calcified ossicles, controlled by a complex system of muscles called the gastric mill. The posterior, or pyloric, part is provided with hairy ridges which combine to form a straining apparatus. The midgut is variously furnished with blind tubes, and a mass of minutely ramified tubules, known as the hepatopancreas or liver, empties through one or more ducts into the anterior end of the midgut.

The heart is short, polygonal, and situated under the posterior part of the carapace. It has three or more pairs of venous ostia, and two pairs of arteries lead from it, in addition to the median anterior and posterior arteries. The blood collects in a venous sinus in the midventral line, passes to the gills, and then back to the pericardial sinus surrounding the heart.

In practically all decapods, respiration is accomplished by a series of gills, attached to the lateral walls of the thoracic somites and to the basal segments of the thoracic appendages. A continuous current of water is drawn through the branchial chamber and over the gills by the fluttering motion of the outer lobe of the second maxilia. The current usually moves forward, but this direction may be periodically reversed in burrowing species. In those crabs best adapted for life on land, the action of the gills is supplemented by the lining membrane of the branchial chamber. This chamber is covered with minute villi and is unusually well supplied with blood vessels, and thus functions as a lung.

The antennal gland is the chief excretory organ in all decapods. Maxillary glands are often present in the larval stages, but they never persist in the adult. The antennal gland is compact and more complex than in any other crustacean group. It consists of three divisions: the saccule, which is usually partitioned or ramose; the labyrinth, a spongy mass with a complex system of canals; and the bladder with a duct leading to the exterior.

The sexes are separate in most decapods, although protandric hermaphroditism occurs in some shrimps. The testes most often lie partly in the thorax and partly in the abdomen, and they are usually connected across the midline. The ovaries resemble the testes in shape and position.

Development

In only one group of shrimps are all of the presumably primitive larval stages represented. The first stage is a typical crustacean nauplius with an unprotected oval body bearing a median eye and three pairs of appendages. This stage is followed by a metanauplius in which four more appendage rudiments appear. The third pair of the original appendages becomes less of a swimming organ and more like a pair of primitive mandibles. In the third stage, the protozoea, the seven pairs of appendages become more highly developed, a carapace covers the anterior part of the body, the abdomen is clearly formed though unsegmented at first, the rudiments of paired eyes appear, and the heart is formed. In the next stage, the zoea, the eyes become movable and the carapace develops a rostrum. All thoracic appendages are present, at least in rudimentary form, and those of the abdomen appear, especially the uropods. In the last larval stage, the mysis, the well-developed thoracic appendages replace the antennae as the chief swimming organs. The abdomen increases markedly in size and takes on a form similar to that of the adult.

Phylogeny

No true decapod fossils are known with certainty from Paleozoic deposits, but decapod shrimps are not uncommon in Triassic and, especially, Jurassic rocks. Lobsterlike forms related to Recent deep-sea species also appeared in the Triassic; true lobsters are known from the Jurassic, and crayfishes from the Cretaceous. Crabs are well known as fossils, with primitive groups appearing in the Jurassic; several more specialized groups are found in the Cretaceous, and all of the major existing groups were represented in the Tertiary.

The evolution of the decapods has not been satisfactorily worked out and may never be known completely. The presence of a nauplius larva in the development of some shrimps suggests a link with the primitive crustaceans, but it can hardly be doubted that the Decapoda, as a group, are highly specialized. They are certainly closely related to the Euphausiacea, and fusion of the two groups has been suggested by some authors.

Classification

The classification of the 8500 living species of decapods presents a difficult problem that has not yet been entirely solved. The chief disparity seems to lie between the shrimps (Natantia) and all of the other decapods (Reptantia). The classification listing is

Order Decapoda

     Suborder Natantia

          Section Penaeidea

          Section Caridea

          Section Stenopodidea

     Suborder Reptantia

          Section Macrura

          Section Anomura

          Section Brachyura

Penaeidea

The primitive section of the Decapoda, penaeideans, is distinguished by the following characters: The pleura of the first abdominal somite overlap those of the second; the third legs are nearly always chelate, but are no stouter than the first pair; the first pleopods of the male bear a complicated flaplike appendix, the petasma; and the females have a spermatophore receptacle, the thelycum, on the ventral surface of the posterior thoracic somites. The best-known penaeideans, including most of the commercially important shrimps or prawns of the warmer seas (Penaeidae), live on muddy bottoms in shallow or moderate depths. However, they also occur both in the deep sea and as pelagic organisms in the mid-depths. There are more than 300 species of living penaeideans.

Caridea

This is the largest and most diverse group of shrimps and prawns. The pleura of the second abdominal somite overlap those of the first; the third legs are never chelate; there is no petasma or thelycum. The sexes can usually be distinguished by the presence, in the male, of two stylets on the inner edge of the inner branch of the second pleopods. Carideans occur in all parts of the sea, often in association with other marine animals. Members of at least two families, Atyidae and Palaemonidae, are also widespread in fresh water. The edible shrimps and prawns of northern Europe and of northwestern North America, of the genera Crangon and Pandalus, belong to the Caridea, as do the large fresh-water prawns of the tropical genus Macrobrachium. Most shrimps and prawns are favorite foods of fishes. Some carideans maintain “service stations” where they remove parasites from the outer skin, mouths, and gills of the fish. More than 1500 species of Caridea are known.

Stenopodidea

This is a small group of shrimps which superficially resemble the Penaeidea. The third pereiopods are chelate but are much longer and stouter than the first pair; the pleura of the second abdominal somite do not overlap those of the first; and there is no petasma or thelycum. All of the 20 or more living species are marine; some are closely associated with other animals, such as sponges.

Macrura

This long-tailed group of the Reptantia includes the deep-water and fossil eryonids, the spiny lobsters, the true lobsters, and the mud shrimps (Fig. 1). The abdomen is extended and bears a well-developed tail fan. There are about 700 known living species.

Reptantian decapods. (a) Spanish lobster, Scyllarides aequinoctialis. (b) Mud shrimp, Callianidea laevicauda. (c) Eryonid, Polycheles crucifer. (d) Spiny lobster, Panulirus interruptus. (e) Crayfish, Orconectes limosus. (Smithsonian institution)

The Eryonidea are rather thin-shelled, blind inhabitants of the depths of the sea. The carapace is flattened dorsally and considerably expanded laterally. The first four, or all five, pairs of pereiopods are chelate, with the first pair much elongated. There are about 40 living species; the group was probably more numerous in ancient seas.

The Scyllaridea include the spiny lobsters or langoustes (Palinuridae) and the Spanish, or shovel-nosed, lobsters (Scyllaridae). They are heavily armored like the true lobsters but are distinguished by the absence of a rostrum and of chelae, except occasionally on the last pereiopod of the female. They are abundant in shallow and moderate depths of warm and temperate seas, where they are often of considerable commercial importance. There are about 85 Recent species.

The true lobsters and crayfishes, Nephropidea, also have a firm shell but are also characterized by a rostrum and by chelae on the first three pairs of pereiopods, with the first pair being noticeably larger than the others. Most lobsters (Nephropidae) are found in cool seas or in the cool, off-shore waters of the tropics. The most familiar lobsters (Homarus) are those found along the Atlantic coasts of Europe and North America. The Norway lobster of Europe (Nephrops) is also of some commercial importance but is smaller and less meaty. The crayfishes (Cambaridae, Astacidae, Parastacidae) are widespread in fresh waters of the temperate regions of all continents except Africa. They are of commercial importance in southern Europe and Australia. More than 300 living species of lobsters and crayfishes are known, more than half of them being from the fresh waters of the United States.

The mud shrimps, Thalassinidea, are usually thin-shelled, burrowing crustaceans with large, chelate or subchelate first pereiopods, and no chelae on the third pereiopods. More than 250 living species are found in shallow and deep seas throughout the world, especially in tropical and warm temperate regions.

Anomura

This intermediate, and possibly unnatural, group lies between the Macrura and the Brachyura. It includes the galatheids, the porcelain crabs or rock sliders, the hermit crabs, the king crabs, and the mole crabs or hippas. In nearly all of these diverse crustaceans, the first, and sometimes the last, pereiopods are chelate or subchelate. The abdomen is usually bent forward ventrally or is asymmetrical, soft, and twisted. There are about 1300 Recent species.

Brachyura

These are the true crabs (Fig. 2). The abdomen is symmetrical, without a tail fan, and bent under the thorax. The first pereiopods are always chelate or subchelate. The true crabs are as numerous as all other decapods combined, numbering nearly 4500 Recent species. They are divided into four subsections.

Common brachyuran crabs. (a) Gymnopleuran crab, Raninoides louisianensis. (b) Dromilid crab, Dromia erythropus. (c) Mask crab, Ethusa mascarone americana. (d) Box crab, Calappa sulcata. (e) Purse crab, Persephona punctata. (Smithsonian Institution)

The Gymnopleura include about 30 species of primitive burrowing crabs; their carapaces are more or less trapezoidal or elongate, the first pereiopods are subchelate, and some or all of the remaining pereiopods are flattened and expanded for burrowing.

The Dromiacea is a primitive group of about 200 species. The first pereiopods are chelate; the last pair is dorsal in position and modified for holding objects, such as sponges, tunicates, and bivalve mollusk shells, over the crab.

In Oxystomata the first pair of pereiopods are chelate and the last pair are either normal or modified as in the Dromiacea. The oxystomes include the mask crabs (Dorippidae), the box crabs (Calappidae), and the purse crabs (Leucosiidae). There are nearly 500 Recent species.

Most of the crabs, more than 3700 living species, belong to the Brachygnatha, in which the last pereiopods are rarely reduced or dorsal in position. To this group belong the swimming crabs (Portunidae). Also worthy of mention are freshwater crabs (Potamidae, Pseudothelphusidae, and Trichodactylidae), found in tropical and some temperate regions, usually in areas not inhabited by crayfishes. The crabs of the genus Cancer are large and abundant enough to be of commercial importance in northern Europe and North America. The rock, or Jonah, crabs of New England and the Dungeness crab of the Pacific Coast are familiar edible crabs of this genus. The ubiquitous little mud crabs (Xanthidae) are well known to every visitor to rocky shores. One of them, the stone crab (Menippe), is highly esteemed by connoisseurs of seafood. The pea crabs (Pinnotheridae) are often found in oysters and other bivalve mollusks. The square-backed crabs (Grapsidae) are characteristic of warm, marshy areas but are not restricted to that habitat; they mark the trend toward the true land crabs (Gecarcinidae), the depreciations and intrusions of which are familiar to all who live in the tropics. The ghost crabs (Ocypode), which scuttle almost unseen over sandy beaches, and the fiddler crabs (Uca) of muddy shores, are closely related. The end of the list is reached with the spider, or decorator, crabs (Majidae), slow-moving animals that often conceal themselves by attaching seaweeds and various sessile animals to their carapaces. One of them (Macrocheira) is the largest arthropod now alive. See also Arthropoda; Crab; Crustacea.

An order in the class of Cephalopoda. All of its genera are composed of species with ten legs, e.g. the ten-legged squid, cuttlefish and calamaries.

For other meanings of decapod, see decapod.

Decapoda
"Decapoda" from Ernst Haeckel's Artforms of Nature, 1904
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Subphylum: Crustacea Class: Malacostraca Superorder: Eucarida Order: Decapoda Latreille, 1802
Suborders
Dendrobranchiata Pleocyemata See text for superfamilies.

The decapods or Decapoda (literally means "ten footed") are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers.

Contents 1 Anatomy 2 Classification 3 See also 4 References 5 External links

Anatomy

Main article: Decapod anatomy

As their name implies, all decapods have ten legs; these are five pairs of thoracic appendages on the last five thoracic segments. The front three pairs function as mouthparts and are generally referred to as maxillipeds, the remainder being pereiopods. In many decapods, however, one pair of legs has enlarged pincers; the claws are called chelae, so those legs may be called chelipeds. Further appendages are found on the abdomen, with each segment capable of carrying a pair of biramous pleopods, the last of which form part of the tail fan (together with the telson) and are called uropods.

Classification

Classification within the order Decapoda depends on the structure of the gills and legs, and the way in which the larvae develop, giving rise to two suborders: Dendrobranchiata and Pleocyemata. Dendrobranchiata consists of prawns, including many species colloquially referred to as "shrimp", such as the Atlantic white shrimp. Pleocyemata includes the remaining groups, including true shrimp.

The following classification to the level of superfamilies follows Martin and Davis ^[1], with some changes based on more recent morphological and molecular studies ^[2][3][4].

Whiteleg shrimp, Litopenaeus vannamei

Spotted cleaner shrimp, Periclimenes yucatanicus

California spiny lobster, Panulirus interruptus

Australian land hermit crab, Coenobita variabilis

Blue crab, Callinectes sapidus

Lyreidus tridentatus

Order Decapoda Latreille, 1802

• Suborder Dendrobranchiata Bate, 1888—prawns
  • Superfamily Penaeoidea Rafinesque, 1815
  • Superfamily Sergestoidea Dana, 1852
• Suborder Pleocyemata Burkenroad, 1963
  • Infraorder Stenopodidea Claus, 1872
  • Infraorder Caridea Dana, 1852—true shrimp
    • Superfamily Procaridoidea Chace & Manning, 1972
    • Superfamily Galatheacaridoidea Vereshchaka, 1997
    • Superfamily Pasiphaeoidea Dana, 1852
    • Superfamily Oplophoroidea Dana, 1852
    • Superfamily Atyoidea de Haan, 1849
    • Superfamily Bresilioidea Calman, 1896
    • Superfamily Nematocarcinoidea Smith, 1884
    • Superfamily Psalidopodoidea Wood-Mason & Alcock, 1892
    • Superfamily Stylodactyloidea Bate, 1888
    • Superfamily Campylonotoidea Sollaud, 1913
    • Superfamily Palaemonoidea Rafinesque, 1815
    • Superfamily Alpheoidea Rafinesque, 1815
    • Superfamily Processoidea Ortmann, 1890
    • Superfamily Pandaloidea Haworth, 1825
    • Superfamily Crangonoidea Haworth, 1825
  • Infraorder Eryonoidea de Haan, 1841
  • Infraorder Achelata Scholtz & Richter, 1995
  • Infraorder Astacidea Latreille, 1802—lobsters and crayfish
    • Superfamily Enoplometopoidea de Saint Laurent, 1988
    • Superfamily Glypheoidea Winkler, 1883
    • Superfamily Nephropoidea Dana, 1852
    • Superfamily Astacoidea Latreille, 1802
    • Superfamily Parastacoidea Huxley, 1879
  • Infraorder Thalassinidea Latreille, 1831
    • Superfamily Thalassinoidea Latreille, 1831
    • Superfamily Callianassoidea Dana, 1852
    • Superfamily Axioidea Huxley, 1879
  • Infraorder Anomura MacLeay, 1838
    • Superfamily Lomisoidea Bouvier, 1895
    • Superfamily Galatheoidea Samouelle, 1819—squat lobsters
    • Superfamily Hippoidea Latreille, 1825
    • Superfamily Paguroidea Latreille, 1802—hermit crabs
  • Infraorder Brachyura Linnaeus, 1758—crabs
    • Section Podotremata Guinot, 1977
      • Superfamily Cyclodorippoidea Ortmann, 1892
      • Superfamily Homolodromioidea Alcock, 1900
      • Superfamily Dromioidea de Haan, 1833
      • Superfamily Homoloidea de Haan, 1839
      • Superfamily Raninoidea de Haan, 1839
    • Section Eubrachyura de Saint Laurent, 1980
      • Superfamily Dorippoidea MacLeay, 1838
      • Superfamily Calappoidea Milne Edwards, 1837
      • Superfamily Leucosioidea Samouelle, 1819
      • Superfamily Majoidea Samouelle, 1819
      • Superfamily Aethroidea Dana, 1851
      • Superfamily Hymenosomatoidea MacLeay, 1838
      • Superfamily Parthenopoidea MacLeay, 1838
      • Superfamily Retroplumoidea Gill, 1894
      • Superfamily Cancroidea Latreille, 1802
      • Superfamily Corystoidea Samouelle, 1819
      • Superfamily Portunoidea Rafinesque, 1815
      • Superfamily Bythograeoidea Williams, 1980
      • Superfamily Xanthoidea MacLeay, 1838
      • Superfamily Bellioidea Dana, 1852
      • Superfamily Potamoidea Ortmann, 1896
      • Superfamily Pseudothelphusoidea Ortmann, 1893
      • Superfamily Gecarcinucoidea Rathbun, 1904
      • Superfamily Cryptochiroidea Paulson, 1875
      • Superfamily Pinnotheroidea de Haan, 1833
      • Superfamily Ocypodoidea Rafinesque, 1815
      • Superfamily Grapsoidea MacLeay, 1838

See also

• List of Atlantic decapod species

References

1. ^ Joel W. Martin and George E. Davis (2001). An Updated Classification of the Recent Crustacea. Natural History Museum of Los Angeles County. 
2. ^ Dixon, C. J., F. R. Schram & S. T. Ahyong (2004). "A new hypothesis of decapod phylogeny". Crustaceana 76 (8): 935–975. doi:10.1163/156854003771997846. 
3. ^ Porter, M. L., M. Pérez-Losada & K. A. Crandall (2005). "Model-based multi-locus estimation of decapod phylogeny and divergence times". Molecular Phylogenetics and Evolution 37: 355–369. doi:10.1016/j.ympev.2005.06.021. 
4. ^ Peter K. L. Ng, Daniele Guinot & Peter J.F. Davie (2008). "Systema Brachyurorum: Part 1. An Annotated checklist of extant Brachyuran crabs of the world". Raffles Bulletin of Zoology Supplement No. 17: 1–286. 

External links

Wikimedia Commons has media related to: Decapoda

Wikispecies has information related to: Decapoda

• Decapoda at the Open Directory Project

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)