crab culture

Reproduction and Life Cycle of Mud Crab

The male can only mate with a female while her shell is soft, a period of about 48 hours after moulting. The male crab can sense when the female is about to moult and pairs up with her. He climbs on top of the female, clasps her with his legs, and carries her underneath him for up to four days. The male releases the female when she begins to moult and, when the process is complete, turns her upside down to mate.

When the female mud crab is ready to spawn, she creates a shallow hole in the sand or mud with her abdominal flap. She then releases her eggs into the depression and immediately begins to gather them up with a pair of her smaller legs.

The amount of eggs is 8,50,000-15,00,000 in every 200-250g of female crab. Mud crab eggs are bright orange when released and turn almost black by the time they are ready to hatch. The eggs are about 0.3 mm in diameter and a single batch may contain over two million eggs. While mud crabs produce large numbers of offspring, the mortality rate of larvae and juveniles is high. Hence, few crabs survive long enough to mate.

About 12 days after spawning, the eggs hatch, releasing the planktonic larval stage known as the zoea. Hatching is believed to occur offshore. The eggs take one to two hours to hatch, during which time the female stands on the tips of her legs and moves her abdominal flap back and forth in a motion that helps free the zoea.

At the first planktonic stage, the animal is just over 1 mm long. It increases in size by moulting four times over the next 12 to 15 days. During the fifth moult, the larval crab transforms into a megalopa with relatively large working claws. This stage lasts seven to nine days, during which the animal moves inshore and settles out of the plankton. The megalopa later moults into a stage one crab.

Stage one crabs are only about 4 mm wide, but frequent moults mean that they grow very quickly. It is thought that wild mud crabs grow to around 10 cm in width in about a year and become sexual mature in about two years. The typical life span of a mud crab is thought to be three to four years.

Site selection for crab culture

Choosing a site for crab farming is not difficult. First, there should be adequate supply of estuarine water because good and stable salinity is conducive to growth. Smaller ponds are advisable since they are easier to manage. Make sure the soil is clay or clay loam. This kind of soil is capable of retaining water. If possible, the site should be free from floods. The change of salt water easily available crab culture area. All the year round should be maintaining salinity above 5ppt. This area also free form animal and birds.   For easy harvesting, the site should have good drainage.

Pond preparation

After repairing of dyke and gates, the ponds were allowed to sun and dried up to the bottom soil cracks. As per the experimental design for Treatment-2 ponds were encircled with bamboo slits made fence with a area of 40 m² each. The bamboo slits was pushed into the mud up to 50 cm depth to prevent escape burrowing of crabs. The depth of water is also important. Advisable depth is one meter to prevent exposure of cultured crabs and stop them from boring holes through the dikes.

The area of pond should be 0.2-1.0 hector and depth 1-1.5m is suitable for crab culture.

Steps of pond preparation

Drying out the pond

 The drying of the pond bottom is the most practical cheap and effective method of eliminating undesirable species in pond prior to the culture period. Drying oxidizes harmful chemical substances especially sulfides and facilitates mineralization of organic matter. The pond is dried until the soil cracks or when it is firm enough to hold one's weight without sinking more than 5 cm on walking over the surface.

Leaching

When the pond soil is found to be acidic, it is normally “leached. This is done by flushing and washing the pond bottom with water to leach away undesirable metallic compounds like aluminum, iron and excess sulfur ions.

Tilling

Tilling or ploughing of bottom soil improves soil quality by exposing subsoil to the atmosphere thereby speeding up the oxidation process and the release of nutrients that are locked in the soil.

Suitable Place create

 In every full moon and new moon lunar cycle crab increased their size by moulting. So they hide their body to escape from predators. In natural environment they hide mangrove types of trees to escape predators. On this condition they are very weak and shell is soft. In cultural pond used bamboo fence, small size of PVC pipe and earthen pot to create suitable place for moulting crab to escape from predators.

Required soil quality of the farm

• Sandy clay to clayey-loam soils.
• Clay or loam-based soil containing more than 90% clay is preferable.
• pH between 6.5 to 8.5 preferable.
• Total nitrogen 0.1%; total phosphorus 0.1%; organic carbon> 1.0%; free CaCO₃ 5%.
• Organic compound should be contains 7-12%

Required water quality of the farm

·         Water should be free from agricultural and industrial pollution.

·         The pH of the water preferably ranges from 7.5 to 8.5.

·         The dissolved oxygen level is preferably not lower than 4 ppm.

·         The salinity of the water for shrimp farm should be from 5 to 25ppt.

·         The temperature of the water body should be 22 to 30^oC.

Water supply and fertilizer used

For prevent undesirable species on the time of water supply used 0.25mm mesh size net. Initial time water supply should be maintaining 30cm. After 7 days mustard oil cake used 500kg\hectore. After 4 days inorganic fertilizer used TSP and Urea is 3:1 at the rate of 35kg\hectore. Then gradually increase water level of cultural pond and maintain 1-1.5m.

Stocking
Naturally collected fry can survive up to 5 days in air and thus makes transportation of fry very easy. Stocking densities vary from 5000- 10,000/ha. Average weight should vary from 25 g to 30 g. Sex ratio should be maintained at 9:1 (female: male).

Feeding
Like prawn or shrimp, crabs also take food at night. During tide, they also collect food in day time. Feeding rate should be 5% of the total crab biomass. Many ingredients may use directly or in mixed form for feeding them. Some commonly used feeds are muscle of snails, mussels; flesh of trash fishes (e.g. eel, tilapia etc.); shrimp or heads of shrimp after beheading; cattle or poultry offal; fish viscera; kitchen wastes etc. Freshwater soft-shelled snails are the most important food items of crabs. Enough food is given to prevent cannibalism

Harvesting
The culture period is dependent on the initial size of crab stocked and size desired at harvest. Crabs weighing of 220-250 g and with carapace widths of 12-15 cm are harvested in 5-6 months. But better management provides opportunity to harvest crabs in only three to four months. Mud crabs are very hardy and can survive several days out of water if kept moist. Cast nets and bamboo baskets are usually used for harvesting and marketing respectively.

Handling

Adult crabs in captivity are tied with dried nip strings. Both pincers are tied close to the abdominal cavity to prevent crawling. When transported, proper handling is important. Place them in baskets to avoid getting trampled or crushed.

Conclusion
 Crab is an important fisheries item which command good demand in the markets especially in foreign markets. This species is of high price too. Our country is highly suitable for crab culture. As, most of the crab grow naturally in the coastal gher, and since, shrimp culture practice in Bangladesh is now posing a great threat to environment due to degradation of soil fertility and productivity; crab should be cultured on more commercial basis. Suitable cost-effective technique should be introduced and proper marketing channel should be developed. Efforts are very much needed for the establishment of hatcheries of crab for artificial breeding. Sufficient extension works are also required regarding this issue.