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Decapoda (Arthropoda) one of the specialized orders of the arthropod class Crustacea that includes the familiar shrimps, prawns, lobsters, crabs, etc. The decapods are well-known for their economic importance in the field of fisheries and include many species that are widely used as food items. Being a large group, it cannot be defined precisely by any single morphological feature; however, the members share many characters common to other groups. They have a carapace that covers the cephalothoracic region and the sides of the overhanging carapace enclose the gills within well-defined lateral branchial chambers. Of the eight pairs of thoracic appendages, three pairs form the maxillipeds and remain firmly attached to the mouthparts. All decapods have well-developed gills, arranged in several rows. They have five pairs of thoracic walking legs, the first pair is usually larger and heavier with pinching claws (chelae).

Some common decapods

The order contains over 8,500 species having considerable variations in their body form and size. Many of them are minute, only about 0.5 cm in length, while the Japanese Giant Crab may have a length of the expanded legs as much as about 4 meter. Commonly known as 'spider crab', this crab is the largest among the living arthropods. Although most decapods are marine, many crayfish and prawns are familiar inhabitants of different types of freshwater bodies including ponds, rivers, and lakes. Many species of true crabs have similar habitats. Some crabs and hermit crabs are adapted to terrestrial life and return to seas during breeding season. Many crayfish live in burrows, and members of at least one species of crab live on tall trees throughout their life.

Origin 'Although no fossil evidence of the true decapods were available in the Palaeozoic deposits, fossils of shrimps were recorded in the Triassic and Jurassic rocks. Some lobster-like fossils showing relations with a few living deepwater species were collected from the Triassic deposits. Genuine fossils of lobster and crayfish have been recorded respectively from the Jurassic and Cretaceous deposits. Most of the familiar fossils of the decapods are those of crabs, the stem groups of which evolved during the Jurassic period. Subsequently, during Cretaceous a few more specialized groups appeared. It is believed that during Triassic, representatives of almost all the major groups of decapods had appeared.

The origin and evolutionary history of the decapods have not been adequately traced. The presence of nauplius larval stages in the development of prawns and shrimps indicates their phylogenetic relationship with primitive crustaceans. Undoubtedly, Decapoda as a group, is highly specialized. Their relationship with the Euphausiacean crustaceans cannot be ruled out.

Classification 'The Decapoda is divided into two suborders - the Natantia, containing the prawns and shrimps; and the Reptantia, containing the crayfish, lobsters and crabs. In Natantia the body is generally adapted for swimming, elongated, and tends to be laterally compressed with a well-developed muscular abdomen. The carapace is usually extended anteriorly forming an elongated or short, keel-shaped serrated rostrum. The legs are generally slender. The first two or three pairs of legs are chelate, and one of the chelate pairs may be longer and heavier than the others.

The members of Reptantia are adapted for crawling although some have become secondarily specialized for swimming. The body tends to become dorsoventrally flattened to at least some degree. The legs are usually heavier, and in many species the first pair form powerful chelipeds. When pleopods are present they are not adapted for swimming.

The abdomen of reptantian decapods exhibits various degrees of reduction. The section (Macrura) which contains the spiny lobsters, true lobsters, mud shrimps, and crayfish, the abdomen is well-developed with uropods and bears the full complement of appendages. In some the appendages are reduced. The carapace is always longer than broad and may bear a depressed rostrum.

The true crabs are included in section Brachyura, where the abdomen is greatly reduced and flexed beneath the cephalothorax. All the abdominal segments may be distinct, but more commonly certain segments are fused together. The cephalothorax is as wide as, or wider than its length, giving the body a flattened appearance. The rostrum is small or absent.

The anomurans include highly specialized decapods that occupy a position in between Macrura and Brachyura. The group consists of the hermit crabs and their related forms. In the hermit crabs, the abdomen is not flexed beneath the cephalothorax but is usually modified to fit within the chamber of the empty gastropod shells. The abdomen is asymmetrically developed with a thin soft cuticle. Other anomurans look like true crabs in having the abdomen flexed beneath the thorax.

Among the decapods the members of the section Penaeidea appear to be very primitive. They have the following diagnostic features: the first abdominal segment overlaps the second segment; the third leg is almost always chelate; the pleopods are adapted for swimming; rostrum is long; body typically elongated, and round or laterally compressed. The commercially important prawns and shrimps of Bangladesh, such as the species of Penaeus and Metapenaeus are included in this group. Many of them are deepwater forms, while some are pelagic.

The carideans constitute the most important and widely distributed group. Although they are seen at all levels of the seas and oceans, some are inhabitants of freshwater bodies. In Bangladesh the popularly known Galda chingdi (Macrobrachium rosenbergii) is the largest caridian prawn of the country.

All colours and combination of colours are found in decapods. Many shrimps can adapt body colouration to the background. Body colouration of many species are specially adapted to harmonize certain habitats. Some brachyuran crab species of the genus Dissodactylus are smallest among the decapods and live as commensal on sand dollars (Echinodermata).

Locomotion and habitation 'Locomotion in decapods is effected in two ways, walking and swimming. Different appendages including the legs are used for locomotion. The pleopods are the principal swimming organs in shrimps. Most shrimps are bottom dwellers and swim intermittently. Legs are used for swimming in some shrimps, prawns and in many crabs. Portunids can swim sideways, backward and sometimes forward with great rapidity. Sudden rapid escape movements in shrimps and macrurans are produced by the rapid flexing of uropod. This movement drives the animal backward, sometimes with such force that the animal jumps out of water. Some crabs, including the red crab of Cox's Bazar and Maheskhali sea beaches are among the fastest running crabs; at top of speed, the body is raised well off the ground and only two or three pairs of legs are used.

The decapods include some notable climbers. The coconut crab, Birgus from the Indo-Pacific and the woolly-handed crab, Eriocheir can both climb to the tops of tall trees, using the sharp heavy ends of the legs.

Many decapods can efficiently burrow in sand and mud. Commonly legs are used for burrowing. Almost all the terrestrial and amphibious crab species dig burrows. The burrows of most species do not extend much deeper than 30 cm, and the excavated sand or soil is usually carried to the surface as small balls or pellets.

Many lobsters and crayfish are also known to dig burrows or retreats. Not all retreats are excavated in mud and sand. Natural retreats are used by many species. Different species of shrimps, macrurans, and crabs inhabit in the natural crevices of rocks and corals. The hermit crabs are adapted for living in the shells of gastropods, and the soft naked crab abdomen is twisted to fit spiral of the shell. They however, always use empty snail shells and never kill the original occupant.

Food and nutrition 'Most decapods are predaceous, omnivores, or scavengers. Any small animal - fish, insect larvae, mollusks, and other crustaceans are their preferred food. Food is caught and handled by chelate legs. Some predaceous species have special modifications for capturing prey. Usually the mandibles serve as the chewing apparatus. Filter feeding takes place in many decapods, but detritus rather than plankton is usually the source of food particles.

In the typical decapod digestive tract, a short oesophagus leads into a large capacious cardiac stomach. This stomach communicates with the ventral pyloric stomach through a small, constricted part. A long intestine extends from the pyloric stomach through the abdomen and opens outside on the ventral side of the telson. Most part of the alimentary canal is lined by chitin. The midgut may be extremely short, particularly in crabs and crayfish. The hepatopancreas of decapods is usually a very large compact paired mass that fills much of the cephalothoracic region. The intestinal walls absorb digested materials. Undigested particles pass on into the posterior end of the intestine, where they are gathered together into feces, and voided through the anus.

Circulation and excretion' Decapod heart is a short, muscular saddle-shaped sac located in the thorax and is provided with three pairs of ostia. Five arteries leave the heart anteriorly - an ophthalmic artery, a pair of cephalic arteries and a pair of hepatic arteries. A median abdominal artery extends posteriorly, and a sternal artery arises either from the ventral side of the heart or from the base of the abdominal artery. The blood is an almost colourless liquid in which are suspended a number of amoeboid cells, the amoebocytes. The principal functions of the blood are the transportation of food materials from one part of the body to another, of oxygen from gills to the various tissues, of carbon dioxide to the gills, and of urea to the excretory organs.

Antennal or green glands are the excretory organs of decapods and reach their highest degree of development in this group. There are many species that can tolerate either very brackish or freshwater, but must return to salt water to breed. In these forms the antennal gland is known to play a role in osmoregulation.

Reproduction and development' In most decapods the sexes are separate, only some are known to be hermaphroditic. The hermaphroditic species are usually protandric, behaving as a male in the early part of their life. Subsequently female organs are developed. Considerable sexual dimorphism is present in many species. It is particularly striking in fiddler crabs, in which one claw of the male is very large, in contrast to the two small claws of the female. These crabs go through elaborate courtship behaviour before mating. Courtship and copulatory behaviour are particularly interesting in the anomurans.

Egg laying takes place shortly after copulation in forms with no seminal receptacles, in others egg laying may not take place until some time later. Penaeid shrimps shed their eggs directly into water or carry them only briefly. In all other decapods the eggs on extrusion are typically attached to the pleopods with a cementing material. Fertilization is perhaps internal in brachyurans, but in most decapods the eggs are probably fertilized at the moment of egg laying.

In some prawns and shrimps the eggs hatch as nauplius, metanauplius or protozoea larvae, but in almost all other marine decapods hatching takes place at the protozoeal or zoeal stage. Some larval stage has interesting morphological features and recapitulates their ancestral groups.

Economic importance 'The decapods like the bivalve mollusks have importance as a source of human food, and the two groups together comprise the so-called shell fisheries. Throughout the world, the shrimps, lobsters and portunid crabs are caught extensively for human consumption. Although marine shrimps are collected from the Bay of Bengal by trawling, at present the tiger shrimps (Bagda, Penaeus monodon) is being cultivated in about 1,45,000 ha of coastal and tidal lands in ghers and ponds at Satkhira, Bagerhat, and Cox's Bazar (including Chakaria and Teknaf) under the monoculture, polyculture, and integrated with paddy in Khulna area and salt in Chakaria area. Prawn (Galda, Macrobrachium rosenbergii) culture in about 30,000 ha in about 60,000 ponds and paddy fields are being practiced mainly in Bagerhat, Khulna, Jessore, Patuakhali, Feni, and few other places. Annual farmed shrimp and prawn production is about 28,514 m tons (headless), which brings about Tk. 16,150 million as foreign exchange.

Crab fishery is now an important economic activity in Bangladesh. The major species for export is the mud crab, Scylla serrata. Crab fishers collect the crabs from the estuary and intertidal zones, particularly of the Sundarbans mangrove ecosystem using small fishing boats, lines (baited) and hooks. They also use different types of nets and traps. Most of the harvested crabs after proper grading are exported to Singapore, Taiwan, Hong Kong, China, and Japan. Bangladesh earns about US$ 6.0 million annually by exporting about 1,500 m tons of live mud crabs.

Decapoda (Mollusca) An order of the class Cephalopoda comprising the sepia, cuttlefish and the squids. They are all marine, spindle-shaped and have become adapted for a swimming existence. The head projects into a circle or crown of 10 prehensile tentacles or arms, two of which are much longer than the rest. The two long tentacles have suckers near the distal end, the other four pairs bear sucker along the entire length. The eyes are highly developed.

Squid, Psychroteuthis glacialis Cuttlefish, Sepia aculeata

Although the majority of the decapods are medium-sized, ranging between 15 and 100 cm in length including the tentacles, some species attain giant proportions. The giant squids of North Atlantic Architeuthis may attain a total body length of about 20 m. The body of cuttlefish such as sepia tends to be short, broad and flattened. They are slow swimmers than the more streamlined squids. The fins of the decapods function as stabilizers, but they also may be used for steering and propulsions. They feed on small fish, crustaceans and other small decapods, and in turn furnish food for cod and other large fish. Squids are important to the diet of sperm whales.

Seven species of squids and two species of cuttlefish have been recorded from the Bay of Bengal. These animals have no importance as fisheries item in Bangladesh. However, people of some southeastern Asian countries and also of many parts of the world, including Europe eat them. The powdered dry shell of the decapods is used as toothpowder. The sepia-ink has a great demand to the artists. [SM Humayun Kabir]