About Crabs

India is one of the few countries in the world that are bestowed with a high degree of terrestrial and aquatic biodiversity. India's marine regimes range from Lakshadweep Islands in the Arabian Sea through the west and east continental coasts to the Andaman and Nicobar Islands in the Bay of Bengal. Each of these is, in turn, is host to diverse ecosystems.

Studies of marine biodiversity in India have a long tradition and cover a wide range of faunal and floral groups. The earliest works on the crabs of Indian Seas were those of Milne Edwards (1834), Henderson (1882) and De man (1887-88 a, b, c). The first comprehensive study of the crabs of west coast was that of Borradalile (1900-1906). Other works of substantial interest are those of Pillai (1951), Chhapgar (1957) and Sankarankutty (1961).

Since 1961, there has not been notable progress in the studies of diversity of crabs of west coast of India, save for occasional reports. Within this period, the taxonomic statuses of several species have also been revised. Hence, there is a need to update the current knowledge of the biodiversity studies of crabs of west coast of India.
This CD presents taxonomic description of 254 species of crabs belongs to 120 genera under 24 families. All descriptions are illustrated with photographs and line drawings. In addition, key to the families, subfamilies, genera and species are provided for easy identification.

BRACHYURAN CRABS

All crabs are decapod crustaceans, which simply means they have 10 legs. Eight of these legs help in the movement (walking legs) and the first two, called claws or chelipeds (chela means 'claw' and ped means 'Foot'), in capturing prey, communication, mating and defense. The nearest relation to the decapod crabs are the anomuran crabs, which have only 3 pairs of walking legs. A typical example for the latter is the hermit crab that has a soft coiled abdomen protected by a snail shell.
Crab features: The 4500 or more species of true crabs belonging to the infra -order Brachyura (brachia means 'gills' and oura means 'tail') are the most advanced of the decapods in that they have the body modified from the shrimp like decapod ancestor. The body is short, wide and flat. The abdomen, once a muscular organ used for swimming, is now simply a flap used to cover reproductive appendages and hold eggs. The uropods, which along with the telson form the tailfan in other decapods, are totally absent in the crabs. The antennae and antennules are greatly reduced, and originate before the eyestalks (unlike in anomurans like Porcelain crabs). Most of the Brachyuran crabs move in a sideways fashion. Males have narrow, pointed tails, whereas females have broader tails capable of holding the eggs. Many crabs use their large and colourful claws to communicate to each other, in combat rituals and attracting females. Thanks to the pair of compound eyes on stalks the crabs can detect movement as far as 20 - 30 m. The 2 short antennae also give them a good sense of smell and taste. The crabs are also sensitive to sounds, which they detect through hairs on their body. Gills are actually structures attached to the walking legs.
Crabs are found in fresh water and marine habitats. Many crabs live in mud. With modified gills, they flourish in muddy sediment- filled waters that would clog the gills of other creatures. Many crabs specialize in intertidal zone, and can be out of water during low tide. Some crabs can live permanently on land. Their gill compartments remain tightly sealed to prevent water loss and the gills are kept wet with body fluids. However, all crabs return to the water to release their larvae.

Crab Kisses

Crabs are usually aggressive towards each other, and males often fight over females. Social and mating behaviors are complex. Many make species - specific sounds by banging on the ground with their pincers or stamping with their feet, or make elaborate pincer movements. Mating occurs only when the female has just moulted and her new shell is not yet hard; the males however are in their hard-shelled form. Thus males may protect a female just before she moults, so he can have first access when she is able to mate, and may continue to protect her till shell hardens. Some males even carry the female around. Usually the eggs are laid shortly after mating, but females can also store sperm for many months. The eggs are fertilized as they pass through the chamber holding the sperm. In most, the females carry their eggs cemented to their underside and protected by the tail, which is folded under the body to form a brood pouch. Some large females can carry as many as 300,000 eggs. The eggs of marine crabs pass through two larval stages in the female's brood pouch before they hatch into tiny free-swimming larvae with long spines, a long narrow abdomen and antennae. They then pass through 3 or 4 distinctive larval stages before setting to the bottom as tiny crabs. Eggs of freshwater crabs are larger and hatch directly into juvenile crabs.

Interesting features

Some Brachyuran crabs have peculiar interesting characters. Family Dromiidae ( Sponge crabs) are the most primitive among the true crabs. They cut sponges from the reef and carry them around on their backs for camouflage. These sponges are held in place by the hindmost legs. The spincers are short and needle shaped, and are permanently bent upwards over the carapace. These crabs also use bivalve shells, sea stars or colonial tunicates in areas where sponges are not found. The crabs belonging to family Calappidae are called as 'Box crabs'. In German these are called "shame-faced" crabs, because they hide their faces behind huge hands, the flattened chelae. The distinctive chelipeds are adapted for peeling open snail shells. The chela also fit together in front of the crab's face to form a water channel (leading to openings in front of the chelipeds) for breathing when the crab buries itself in the sand. Family Leucosiidae; Purse crabs get their name from the females' abdomen, which is rigid and cup shaped, completely enclosing the egg mass while brooding. Species are either globular (resemble marbles with thin legs) or spectacularly eroded (resemble rubble). Family Majidae; the spider and arrow crabs have wide range of occurrence. It is also found in hydrothermal vents.

Family Inachidae; These species are generally delicate, having very long thin legs. There are generally no orbital cups for the eyes to retract into, and the first segment of the antenna is usually long and thin. Family Portunidae These are the swimming crabs, swift and aggressive predators that hunt by night using long and powerful pincers. By day they are usually found hiding in crevices or caves. The last two segments of the fifth pair of legs are usually flattened into swimming paddles. Swimming crabs do not swim all the time. In fact, the first three pairs of legs (after the large claws) end in pointed tips, and these are used for walking. Swimming is also done sideways. The commonest crab encountered in the markets is the mangrove swimming crab or mud crab (Scylla serrata). The blue crab (Portunus pelagicus) and three-spotted crab (Portunus sanguinolentus) are some of the prettiest. Crabs of the genus Charybdis have six spines on the sides of the body, and Thalamita have five. An interesting Charybdis species is the Christ crab (Charybdis feriatus), which has a design of a cross on its back, an on either side, looks like an angel with flowing wings. One of the most pugnacious swimming crab is the ring legged crab (Charybdis annulata).

Rainbow swimming crab (Charybdis erithrodactyla) is a beautiful, multi-coloured swimming crab with blue marks on a yellow - orange carapace and legs. It has large chelate legs with numerous spines. The crabs belongs to Family Epialtidae have small eyes without true orbits, and a large beak-like rostrum. The body is smooth and usually angular in someway. The last three pair of legs are prehensile and may have a "thump". These legs are much shorter than the second pair. Most species have been found living in masses of green algae (mainly Halimeda), but also at times in the brown alga Sargassum. Family Xanthidae; members of this family are sometimes called stone crabs, dark fingered crabs or mud crabs, Round crabs, Boxing and Pom - Pom crabs (genus Lybia). These crabs have round or oval carapaces, dark tripped claws or chelae. The smaller species are slow and not likely to pinch, but one should be careful handling the larger ones. Small species are two inches across in size. Several Xanthid crabs are poisonous if eaten. The poison is found in the exoskeleton and is similar in action to the neurotoxin saxitoxin. Family Pinnotheridae ( Pea crabs) are endosymbiotic crabs that live within mussels, Giant clams, Sea cucumbers and Tunicates/ sea squirts. These crabs are commensals, they don't feed on their hosts (feed on detritus) but use them for protection against predation. Family Carpilidae crabs are called as queen crabs. Family Grapsidae; shore, marsh, rock and spray crabs, these crabs are flat with a square, or round carapace, the front is wide, and the large eyes are situated on short stalks and set far apart. All but a few grapsid species live in intertidal habitats, a notable exception are species of Percnon, it can be commonly found (but rarely caught) sub tidally on rocks and coral heads in tropical reef areas. The flattened body may be bottle green (in Metopograpsus) or reddish brown (In Grapsus) and, unlike the arching front of shore crabs, has a straight front with eyes at the extreme angles of the square. They are rock crabs, also called Sally Lightfoot and are very inquisitive. If you remain perfectly still, they will approach nearer and may even crawl over on to your legs. But with the slightest movement, they scuttle away dashing off and hiding in the nearest crevice or run in to the sea if there is no shelter. Family Trapeziidae; the coral crabs. Triangular shaped bodies, and multi-coloured chelae. They are normally found with coral Pocillopora sp. and Acropora sp. FamilyOcypodidae (Ghost and Fiddler crabs) Ocypodids resemble grapsids in shape but their eyes are set close together and at the end of long stalks.The sounds produced by the ghost crab-Ocypode ressembles that of cricket or frog. Stridulating was observed when one animal was forced in to a cavity already inhabited by another. This species is common in sandy beaches where it lives in burrows emerging at night to feed. Ghost crabs have characteristic thickened elevated eyestalks. Fiddler crabs (Uca) with about 65 species are found on the coasts of most warm and tropical Oceans. Fiddler crabs are small, usually only 0.8 - 3.5 cm wide and some Indo -Pacific species are colorfully coral red, bright green and golden yellow or light blue. The males have one very large cheliped often accounting for half of the total weight of the animals. Most species live close to the tidal zones usually in open sunny areas, rarely among mangroves or other plants, but always in place where the flood tide brings detritus and deposites it in a thin layer. Most populations densely occupy the empty tidal areas and shallow polls with as many individuals per m2 Depending on they build vertical burrows 12 - 50 cm deep ground water; above the high tide line burrows may be I m deep.

Evolution of Brachyura

Brachyura are derived monophyletically from extinct Pseudopemphix like Glypheoidea in early Jurassic time. The recent Homolodromia still resembles the earliest crabs. Dorippidae & Calappidae appeared in the lower cretaceous. The family Leucosiidae is known only evolved from Calappidae. The Xanthidae were well established as a highly differentiated family at the beginning of the tertiary, together with the related Geryonidae, Goneplacidae & Pinnotheridae. Portunidae Grapsidae, Xanthidae and Ocypodidae became divergent in adaptation, diversified in early tertiary time and continue to flourish.

Potential value

The Crab meat is very tasty and nourishing. Crab curry is a reputed cure for asthma. Similarly soup made from the swimming crabs Portunus sanquinolentus and P. pelagicus is commonly used by people just after recovery from malaria and typhoid. Scylla serrata serves as a cure for diarrhorea and dysentery. The carapace of Uca spp. is crushed and given as paste for hyperacidity. All the edible crab meats are rich in vitamins and are good for colds, asthma, eosinophil, primary complex, wheezing etc. They are believed to stimulate brain cells. Crab shells are also used elsewhere for preparing suture threads. The advantage of this thread is that it will not cause allergy and will dissolve in due course. Ointment prepared from crab shells, heals the wound quickly and avoids scars.

Several species of crabs are known to be toxic. The crabs reputed to be poisonous are: Ranina ranina. Daldorfia horrida, Thalamita prymna, T. danae, Carpilius convexus, C. maculatus, Demania splendida, Pilumnus vespertilio and Ocypode ceratophthalma. In addition to these, 3 species of crabs of family Xanthidae namely Zosymus aeneus, Platypodia granulose and Atergatis floridus are known to be deadly poisonous. The causative poisonous toxins were identified as either saxitoxin analogs or tetradotoxin in Zosymus aeneus and palytoxin in Demania spp. Isolation of tetradotoxin facilitated better understanding of sodium -potassium transfer across the cell membrane and also neuro-muscular functions. Saxitoxin appeared to be a promissing hypertensive drug. Screening of other brachyuran crabs of India will be helpful in identifying many sources for such and also other compounds.

Socio Economic values

Annual landings of crabs in the Indian waters are estimated to be 13,000 tonnes. Live crabs, cut crabs and meat are exported from India to countries like Japan, USA, France, Hong Kong, Malaysia and Singapore. During the period 1989-94, the country has exported live mud crabs (Scylla serrata) to the tune of about 630 tonnes valuing at Rs. 2.58 million. The export recorded for crabs is tremendous ranging from 36 tonnes in 1987 to 725 in 1993-94. Live mud crabs are exported to Singapore and Hong Kong, which fetch better prices than the swimming crabs Portunus spp.

Indirect economic value

All the brachyuran crabs are indirectly important as they play a dominant role in the marine food web. Few supply the local protein demand and are of medicinal importance. The faeces of all the crabs consist of carbon, nitrogen, phosphorus and trace metals which form a rich food for other consumers. The crabs and their larvae are consumed by many predators and omnivorous fishes and they play a vital role in the transfer of energy through the food chain. Thus are of immense help in recycling nutrients which enhance the richness of the soil by "ploughing".

Socio-cultural value

Crabs have been associated with religion since first century. Romans worshiped the crab as Sea God and named the crab as Neptune. The crab Charybdis feriatus has been called as symbol of cross by christians from coastal areas of India. They do not eat this crab and let it free to swim in the sea after fishing. The crab meats are generally accepted as having good medicinal properties. The abnormal growth of body cells (due to cancer/tumour) resembles the nesting ground of Cancer spp. So the generic name is used to denote this killer disease. Portunus sanguinolentus, which has three reddish round spots each encircled by whitish ring is considered to be the manifestation of Lord Siva who has three eyes.

Scientific Value

Many highly active biocompounds have recently been isolated from reef brachyurans associated with antimicrobial, antileukemic, anticoagulant and cardio active properties. Similarly most of the brachyuran crabs are economically important. Their contribution to the local protein demand and the medicinal importance has been already discussed. A few of them are labelled as 'Pests' causing damage to coastal plants and culture ponds. However, the smaller intertidal crabs which are not of direct economic value have been used in the preparation of high energy yielding, cheaper artificial pellet feeds for the cultivation of edible varieties of sea food. Crabs are always the chosen test animals since their tolerance level is very wide. Brachyuran crabs have been used as tools in the elucidation of physiological mechanisms, fertilization, regeneration and cell association and mechanisms of drug action.