What is Mud Crab ?

Mud crab is an organism under the phylum arthropoda. It is highly popular due to its great demand in the export market. The commercial scale mud crab culture is developing fast along the coastal areas of Andhra Pradesh, Tamil Nadu, Kerala and Karnataka. It is one of the popular sea-foods in many parts of the world particularly in the south-east Asian countries. The mud crab( Scylla serrata), commonly known as “green crab” or “mangrove crab” and locally called “habba kankra ” or “kankra” occurs abundantly throughout the tidal coastal areas of Bangladesh.

Types of mud crabs

Mud crabs in the genus Scylla inhabit coastal areas, estuaries and backwaters.

1.Larger species : The larger species is locally known as `green mud crab‘. It grows to a maximum size of 22 cm carapace width and 2 kg in weight. These are free living and distinguished by the polygonal markings present on all appendages.

2.Smaller species : The smaller species is known as `red claw‘. This grows to a maximum size of 12.7 cm carapace width and 1.2 kg in weight. It is without polygonal markings and has a burrowing habit. Both species have good demand in the domestic as well as in the foreign market.

Habitat of mud crab:

Scylla serrata is common in the mud flats of the littoral and inter tidal zones of the Bay of Bangal.The species hardly occurs in sandy and rocky areas. It is distributed over a wide range of salinity,from 2ppt. to oceanic waters,from the costal to the interior brakish water.They rarely tolerate turbid water. They are essentially euryhaline,but die at once in 70 ppt of salinity. The crabs live in mud burrows,which occur densely in internal mangrove swamps,a little above the low tide mark. Crabs usually take shelter in burrows during the day and during high tides at night they swim around in search of food. About 80% of catch from burrows are males.Each crab burrow is oblique,1-2 m deep and 8-16 cm wide at the opening.

Culture practise

1. In cages: The culture of mud crabs in many countries generally falls under two major categories fattening in cages and growing out in ponds. Most fattening procedure is done in cages made of bamboo.Nets,earthen ponds or ditches in bamboo cages in individual compartments and fattening is done for 15-30 days.Stocking density varies from 30-60 kg/ cage,depending on the supply and depth of the cages. Feeding with trash fish,soft-shelled snails,animal offal and other materials is carried out daily.A few farmers supplements trash fish with fish offal,obtained from fish sellers,or shrimp head,obtained from farmers. However, in grow out system, relatively small sized crabs,caught from wild, which were discarded by the wholesaler(less than 5 crabs/kg) are grown for a langer peripd (one to several months) untill they attain marketable size.

2.In ponds: Ponds for crab culture can be any size but small ponds having an area of about 350 to 500 m^2 are preferred which are more efficiently manageable than the bigger ponds. The depth of the pond should be as mush as 1 to 2 meters,while gradient or slope of the dikes should be step to prevent escaping. After 2 to 3 days of pond preparation, locally caught crab larvae are stocked at a rate of 3-5 crab/m^2 at around 0700-0800h. Animal food,such as trash fish,snail meat,cattle or poultry offal,fish viscera,or kitchen wastes are used as feed.Fresh water soft- shelled snails are the most important food items of crabs.Crabs are fed at a rate of 7-10% total body weight per day.Enough food is given to prevent cannabalism. Water exchange is crab ponds is mostly done by tidal exchange.The amount of water exchange each time depends on tidal fluctuations.

The water quality parameters shall be maintained within the range shown below: Salinity:15-25% Temperature:26-30° C Oxygen:> 3 ppm pH:7.8-8.5

Harvesting Crabs

Crabs weighing of 220-250g and with carapace widths of 12-15cm are harvested in 5-6 months.The harvesting should be made before the crabs come into reproductive conditions to have a high meat yield and secure good price.Harvesting is done with the help of dip net and lift net baited with trash fish.Plastic pipes are placed sometimes on the bottom for harvesting.

Marketing crabs

The marketing system expanded during the end of the eighties and early nineties with the foreign markets opening up.Before that,crabs were maketable only locally.Fishermen usually market their catch 2 tp 3 days after capture.During this time the crabs are kept in their homes or boats, either in water or in case, without water.Male crabs weighing less than 200g and female crabs of less than 150g are rejected and sold locally.Males and females are separated at the district level market or at the fourth or fifth local marketing steps and in some cases, even at the production point. Present International markets for crabs are Singapore, Hong Kong and a few other Southeast Asian countries.More than 95% of export involves live crabs.Processed and frozen export are negligible.

Tanjuma Tasnim Hira

Student of Fisheries at Bangladesh Agricultural University