LARVAL REARING OF MUD CRABS
The mud crab is a very good alternative to shrimp in aquaculture and is a candidate species in the diversification of brackish water aquaculture activities. Mud crabs are in great demand in many countries, especially the South-east Asian countries. The natural resources of mud crabs have been seriously depleted in many countries and there is an urgent need for the promotion of crab culture. The sustainability of aquaculture, however, depends on the availability and continuous supply of seed and feed, for which a strong technological backup is required.
In crabs, fertilisation is internal and after fertilisation, the mother crab lays between 10 and 20 million eggs, depending on size, which it carries on its abdomen. Such crabs are known as berried crabs. The eggs are brick red in colour immediately after spawning. They take 3-5 days for initiation of cleavage and embryonic development. As development proceeds, the brick red colour of the egg changes progressively to orange red, yellow, yellowish brown, pale brown and, ultimately, to brown by the time of hatching.
Berried female crabs are available in the coastal zones and also near the estuarine river mouths during high tides. It is very important to select the mother crabs carefully for larval rearing. Mother crabs with undeveloped egg mass which is brick red to orange red in colour are most suitable for rearing in the laboratory. Crabs in advanced stages of egg development are not so suitable because the developing eggs may get damaged during transportation and are also prone to bacterial and other infectious diseases, leading to mortality of the eggs and mother crabs.
Mother crabs should undergo prophylaxis before being transferred to the rearing tanks. A water column of 60-90 cm is recommended in the rearing tanks. The crabs should be fed with species of squid or fish once a day, followed by thorough water exchange.
Regular monitoring of the developing eggs should be carried out in order to assess the health condition of the embryo and the mother crab. A healthy developing egg looks bright and glossy. The egg mass should be uniformly coloured, which is an indication of good health of the eggs and also, their stage of development.
When the egg mass turns brown, the mother crab should be transferred to a hatching tank. Hatching usually takes place in the early hours of the day. The egg hatches to the zoea larva which, when healthy, swims on the water surface. Usually 7-20 days are needed for embryonic development and hatching. However, this is entirely dependent upon the temperature and salinity of the medium.
There are eight zoea stages in the development of Scylla oceanica and S. tranquibarica, which have different morphological features. Usually Stage I-III zoea cling to the abdomen of the mother. The first five stages are non-feeding stages and develop at the expense of the remaining yolk. Feeding commences in the Stage VI zoea. Within a span of 10-15 days, the zoea larvae metamorphose to the megalopa stage. The duration of development is strongly dependent on ambient temperature and it was found that a water temperature of 29-30°C is ideal for the development of mud crabs.
The hatchery design plays a crucial role in determining the survival rate of the zoea larvae. Hatchery tanks with continuous flow of water are recommended. The feeding schedules and the quantum of feed are important points for consideration in the success of the hatchery. A survival rate of 50-60% can be attained in the zoea larvae by carefully maintaining the feeding schedules and the water quality parameters. Bacterial and ciliate infections are a common occurrence in the larval development of mud crabs and proper care should be taken to prevent the infections. In both types of infections, the larvae settle to the bottom of the tank and do not feed whereas actively feeding larvae would swim in the water column.
Changes in developmental stage can be observed under a microscope. Under good feeding conditions, the stomach and the intestine are filled with food material. The zoea larva in the initial stage measures about 1 mm, at Stage V it measures about 2 mm and at Stage VIII its size increases to 2.6 mm. At this stage, the head is rather large and the width of the abdomen increases considerably. The larvae also develop stalked eyes and resemble the megalopa larvae closely.
The megalopa larvae of mud crabs are mainly deposit feeders and maximum survival (around 90%) can be attained by planning their diet judiciously. The megalopa is the ideal stage for stocking in nursery ponds and it takes 30 days to grow to a body weight of about 30 g. The juvenile crabs are suitable for transfer to culture ponds for further growout until the attain a marketable size of 500 g.
(Excepts from article by D.E. Babu, K. Rama Rao, D.V.S.N. Raju, B. Jayant Kumar, M. Ratna Raju. Infofish International, n°2, March/April 2000)