King Crabs (sagar kakra, raj kakra) are actually not at all crabs but aquatic marine arachnids having some similarities with the crabs. They belong to the order Xiphosura (Greek xiphos, sword and uros, tail). Globally there are now four living species under 3 genera. They are Limulus polyhemus living along the Atlantic and Gulf coasts of the US; Tachypleus tridentatus found in Japan, China, and southern Sabah (Malaysia); Tachypleus gigas and the smallest living species Carcinoscorpius rotundicauda in south and southeast Asian countries including Bangladesh, India, Myanmar, Thailand, Malaysia and China. These prehistoric creatures, called the 'living fossil'evolved from trilobites have survived about 550 million years without much morphological changes mainly because of their advanced immune system. Fossils of the King crabs of over 400 million years old like Mesolimulus walchii (fossil found in Germany) and Austrolimus (fossil found in Australia) look almost identical to the living species. This helmet shaped, spear-tailed chelicerates are called Horseshoe crabs because of the resemblance of their dorsal plate (Carapace) to horseshoe. They belong to the class Merostomata, an ancient class of animals evolved in the early Cambrian (570 million years ago) and were a dominant life form in the seas and oceans of the world until the Permian (310 million years ago).
King crab's horse shoe-shaped body is divided into anterior cephalothorax or prosoma and posterior abdomen or opisthosoma, extending posteriorly as long caudal spine or telson. The unsegmented prosoma broadly rounded anteriorly and produced into a distinct rather blunt spinous process in posterolateral margins; strongly convex dorsally with a median ridge and flanked by a pair of submedian ridges. Depressed area between the median and submedian ridges bears reddish patches. Anterior tip of the median ridge bears a prominent, broad tubercle on two sides, the inconspicuous simple eyes. Similar tubercles in the posterior half of the submedian ridges bear the prominent compound eyes at their bases. The median and submedian ridges have a number of backwardly directed microscopic spines, visible to the naked eyes in adults. Posterolateral angles of the prosoma smooth in both outer and inner edges. Opisthosoma broadly attached to the prosoma by a transverse hinge, roughly quadrangular with a distinct posterior concavity into which the spear-like caudal spine/telson fits. Each serrated lateral margin bears 7 fixed and 6 movable spines. Short, sharp, broad based and slightly larger movable spines fit into concavities between two fixed spines. Fixed spines near the base of the caudal spine are comparatively larger and broader than the other fixed spines. But first three movable spines considerably longer than the rest in full-grown female specimens. Median ridge of opisthosoma is devoid of spines, flanked on either side by a row of six-minute inner depressions and an outer row of much bigger depressions. Opisthosoma dorsally much less convex than the prosoma.
Ventrally the prosoma bears a single, prominent, sharp median spine in its anterior half and 6 pairs or tube like distinct appendages surrounding the jawless mouth. Opisthosoma bears a pair of longer posterolateral spine. Six pairs of ventral plate like mesosomatic appendages bear genital pores in 1st flaplike genital operculum and bookgills on 5 other succeeding pairs. Soft sclerites in between the appendages. Caudal spine (telson) roundly convex dorsally and flat to convex ventrally and bears no ventral groove or lateral or dorsal ridges in Carcinoscorpius rotundicauda whereas triangular with a dorsal serrated keel and ventral longitudinal groove in Tachypleus gigas which is quite larger in size. It gradually tapers to its posterior blunt tip; longer than the rest of the body and movably articulated with abdominal notch.
Sexual dimorphism distinct. Males easily recognized by the smaller size than the females and swollen propodus of the first two clawed walking legs serving as claspers during mating.
Colour The typical C. rotundicauda found in Southeastern part of Bangladesh and the Southeast Asia is brown or dark brown depending on the habitats. But the Sundarban's C. rotundicauda is dark green or greenish black while the uniform greyish dorsal and ventral pale greyish colour of T. gigas blends well with the sandy habitat.
Measurements of C. rotundicauda Male: prosomal length-6.5-8.9 cm; width-13.0-16.8 cm; opisthosomal length 5.2-6.2 cm; width 8.4-11.0 cm; total length of body (excluding caudal spine) 11.7-15.1 cm; length of caudal spines/telson 13.2-17.4 cm. Length of 1st marginal spine 0.78-0.88 cm. Female: prosomal length 7.9-9.0 cm; width 15.2-18.0 cm; opisthosomal length 5.7-7.1 cm; width 10.4-11.0 cm; total length of body (excluding caudal spine) 13.6-16.1 cm; length of caudal spine 14.4-17.1 cm. Length of 1st marginal spine 0.87-0.89 cm (shortest among the horseshoe crabs).
Measurements of T. gigas: Total body length up to 35 cm (maximum record 50 cm). Body diameter 25 cm. Male: prosoma: length 8.9-9.1 cm; width 17.8-18.2 cm; opisthosoma: length 6.6-6.82 cm; width 10.5-10.9 cm; total length of body (excluding caudal spine) 15.5-15.92 cm; length of caudal spine: 18.2-18.7 cm. Length of 1st marginal spine 1.58 cm. Female: prosoma: length 8.31-9.7 cm; width 17.5-22.2 cm; opisthosoma: length 7.1-8.5 cm; width 97.0-13.8 cm; total length of body (excluding caudal spine); 15.4-20.2 cm; length of caudal spine: 16.0-21.3 cm; length of 1st marginal spine 1.59 cm.
Generally, the life cycle of King crabs consists of egg, larva, juvenile and adult stages. They reach maturity between the ages of 9 and 12 years. Mature male is distinctively smaller in size, with modified first and second walking legs as claspers and shape of genital pores. Spawning occurs during high tide, when they migrate to protected sandy beaches in coves or marshes on islets near Cox's Bazar, Sonadia, Moheshkhali and the St. Martin's Island. Chars of Sundarbans are the main breeding ground of C. rotundicauda where as rest of the coastal localities are shared by both. Spawning season of C. rotundicauda is early spring (from January to March) on muddy and early summer (April to June) for T. gigas on sandy parts of the beach. During spawning, a female C. rotundicauda releases 2,000 to 30,000 eggs to be fertilized by the male and then buried in sand. Diameter of its eggs 2.26-2.48 mm where as eggs of T. gigas about 3.5 mm. The larvae in their large eggs look like a small reddish sphere and take about 6 weeks to hatch and pass through as much as 16 moults before they become adult and reach sexual maturity. In the early stages the larvae look a bit like trilobites and are called as 'trilobite larvae'. The larvae appear like adult but the tail remains rudimentary. They can live for 12 to 19 years.'
King or Horseshoe crabs are basically scavenger animals. They live mostly buried in the mud of seafloor where they hunt for small animals, worms, crustaceans, mollusks (live young clams) and even small fish which they eat. They are predominantly nocturnal. Their breeding habit is quite interesting. The male select a female and cling onto her back by the clawed 1st and 2nd walking and 3rd proximal legs (claspers) for long periods. The female coops out a hole (nest) about 15-20 cm deep in the beaches with the front part of cephalothorax and lays eggs, which are subsequently fertilized by the male. The female then covers the eggs with sand, which protects them from the action of the waves while allowing them to be warmed by the sun. Then they go back to the deep water. These sluggish creatures spend mostly burrowing in mud or sand for feeding. They use the telson for short hops. They are very hardy and can survive weeks out of water and even without food, but their gills must be kept moist.
They live in warm, shallow coastal waters on the sea floor and river mouth, at about 30 m depth but are readily seen when they move into shallow water to spawn and lay eggs on muddy/sandy beaches. Though they are generally considered to be entirely marine they are found entering into rivers in Bangladesh and India. The young crabs dwell on the mud flat for about seven to nine years until a total body length of 15 cm has been reached before they migrate to deeper waters in the sea.
In Bangladesh C. rotundicauda is found along the estuarine and continental shelf environments and often caught in the fishing net. It is common along the coast of Cox's Bazar, St. Martin's, Sonadia and Moheskhali islands and rivers and chars of the Sundarbans. T. gigas occurs in the intertidal zones of Cox's Bazar, Teknaf and St. Martin's Islands and absent in the Sundarbans.'
Prior to the latter half of the twentieth century, the commercial fishery caught horseshoe crabs for use in fertilizer and livestock feed in America. In Southeast Asian countries like Thailand, Malaysia, Vietnam and China now horseshoe crab eggs are local delicacy while in the western world it is harvested for its blood. They are also used as a bait for fishing eel and conch shell. Their blood enzyme has long been used in the pharmaceutical and biomedical industries to test the safety of intravenous drugs as well as prosthetics (antibiotics) for bacteria to save human lives and is now also used to check medical equipment for endotoxin contamination.
In the Sundarban area fishermen use its tail (telson) to cure pains (like rheumatism) by tying the lace made of its hollow tail and string. Malformed crab embryos found in the polluted places are used as a bioindicator for analyzing seawater pollution. The tribal Maghs in Moheskhali and Sonadia Islands eat unlaid protein rich eggs of King crab directly fried in oil. Shore birds also attend the feast at full moon night in March-May when the larvae and juvenile come out of sand nest and move towards the sea through beaches.
King crabs play an important role in the ecological equilibrium of certain coastal environments as a scavenger or omnivore. In some coastal ecosystem, many species of migratory and resident birds, wild boars (in Sundarbans), turtles, crabs and fishes (sharks, eels, sea-bass) prey on its eggs, larvae, juvenile or even the whole animal itself. In fact, the health of the king crab is critical to the health of both ecosystem and humans. T. gigas is ecologically segregated from C. rotundicauda being absent in the muddy area of Cox's Bazar and the Sundarbans coast.
Though king crabs are not yet threatened in Bangladesh, world-wide they are listed as a vulnerable species because of their decline due to over harvesting as well as environmental degradation caused by habitat destruction.
These prehistoric creatures are very important animal from phylogeny and evolutionary point of view as they have affinities with large groups of invertebrates. The C. rotundicauda found in the eastern part of Bangladesh coast (Chittagong, Cox's Bazar, Teknaf and St. Martin's Islands) differs from the same species found in the Sundarbans (western Bangladesh coast) in body colour and morphometry. The eastern king crab is similar to its typical Southeast Asian relative by the shape of the body, round tails, male claws and chromosome number (32). But the Sundarbans king crab is' (i) larger than the typical one in body size, and almost similar in size to T. gigas of the Bay of Bengal, which is always larger than the C. rotundicauda in other parts of Asia. However, (ii) the lateral marginal spines of abdomen in the Sundarbans king crabs are relatively shorter (0.69 cm) than those of the typical C. rotundicauda (0.81 cm), (iii) the colour of the Sundarbans king crab is dark green or greenish black whereas the typical one is brown or dark brown; (iv) the carapace of the Sundarbans king crab is thicker and swollen than the others, especially in the abdomen region and (v) the entire body shape of Sundarbans king crab is more circular than the typical one. Though genetically they are of the same molecular patterns and chromosome number, due to the differences in morphometric characteristics and geographical isolation, the Sundarbans king crab can be considered as a subspecies of C. rotundicauda or at least be called by the name 'Sundarbans King Crab' for its exclusive occurrence in this largest mangrove heritage of the world. [Abu Tweb Abu Ahmed]
Bibliography Chowdhury, SH and Hafizuddin, AKM 1980. Horseshoe crabs (Chelicerata: Merostomata) occurring along the Southeast coasts of Bangladesh. Bangladesh J. Zool. 8 (1): 5-13; Christianus, A. and Hajeb, P. 2003. The horshoe crab, a living fossil. Fish Mail. Malaysia Fisheries Society, 12 (4): 3-6; Itow, T, Misra, JK and Ahmed, ATA 2004. Horseshoe crabs (King crabs) in the Bay of Bengal, South Asia, Shizuoka University. Bull. Fac. Educ., Nat. Sci. Seri. 54: 13-30.