CULTURE AND SEED PRODUCTION OF CRABS

Among the marine edible crustaceans, crabs, rank third by virtue of their delicacy and demand for human consumption. In India, 600 hundred species of crabs are available, of which only two species are used to the culture. (Scylla serreta and Scylla tranquebarica). S. serreta is smaller and S. tranquebarica are larger species. Scylla species are generally known as mud crabs or green crabs.

 In malayalam they are known as “patcha njandu,kuzhi njandu and kattu njandu”.

Scylla serreta

S. tranquebarica can be easily distinguished from S.serrata by polygonal marking on the carapace.

 Among the marine crabs, mud crabs are the only species which can remain alive out of water for a considerable period of time. In earlier periods crabs are cultivated along with milkfish, but later on great demand, the monoculture is practiced. Major markets for Indian live mud crabs are in Singapore, Malasia, Taiwan, Hong Kong, and China. Hatchery production technology of mud crabs is presently available in Philipines, Indonesia, China and in India.

Scylla tranquebarica

TAXONAMICAL  POSITION OF MUD CRABS

Phylum: Arthropod

Class: Crustaceans

Order: Decapods

Family: Portunidae

Genus: Scylla

Species: serrata

Parts of a mud crab

HABITAT OF MUD CRABS

Adult crabs are found both in marine and estuarine waters. Mud crabs possess a pair of paddle-shaped swimming legs, which help them for fast swimming. Mud crabs in their megalopa stage enter in estuaries, coastal lagoons, and backwaters, grow fast, attain maturity and become berried. For hatching of the larvae, the berried female migrates into the sea waters. They bury under the sandy bottom. These are common among the mangrove forest.

FEEDING

Crabs generally feed on crustaceans, mollusk, small fish, detritus, and plants.But in culture, 18˚c frozen fishes are given as feed, after thawing the feed is given for crabs. 

IDENTIFICATION OF SEX

The immature and mature males of mud crabs have a slender and triangular shaped abdominal flap on the ventral side of the body. The immature females have a broad and triangular shaped abdominal flap. matured females have a semicircular shaped abdominal flap. The male crabs normally grow faster than females and attain bigger size than the females. Based on the gonadal development, 5 stages of maturity are recorded i.e., immature, maturing, late mature, fully mature and spent.

Male and Female abdominal flap

LIFE CYCLE

Mating occurs soon after moulting of a matured female. The male inserts spermatophores into the spermatheca. When the egg matures, it passes through spermatheca where fertilization takes place internally. The fertilized eggs are shed out through the opening of the vulvae and spawning event takes place. The spawned eggs are placed at the abdominal flap of female crab with the help of ciliary action of pleopods. Generally, the crabs spawn near the seawater lagoons, bays and coastal areas. Scylla serata prefers salinity in the range of 28-35 ppt for spawning.

The life cycle of the mud crab is divided into two phases; 

Early post-larval stage and grow out stage.

The eggs hatches into Zoea, which pass through various stages (zoea; 1-5) metamorphose into megalopa and migrate to brackish water areas.Then they enter into the crab instar stage and the entire process takings a total period of 25 days. It continues to grow to become juvenile, sub-adult until it attains adult stage. Upon attaining maturity and mating they migrate to sea for spawning.

Fecundity

The number of eggs found attached to the pleopods of female mud crabs varied from 0.3 to5 million. Breeding season is throughout the year.

SEED PRODUCTION OF MUD CRABS

SELECTION OF BROOD STOCK FROM WILD:

Healthy broodstock is collected from wild, for the immediate breeding programme, only mature female or berried female is collected from wild.

  1. Brood must have; 

  2. hard exoskeleton,

  3.It should be free from all pathogens,

  4 all the appendages are to be intact, 

  5 no damages on the body. 

Berried female

TRANSPORTATION OF BROOD:

After the collection of berried female, chelipeds are tied with jute fiber or banana fiber. 

Then they are to be kept in the bamboo basket filled with wet seaweeds /cotton wool/paddy straw/mangrove twigs. Finally, the basket should be covered with wet jute cloth with proper ventilation. In case of berried females, the animal has to be brought in wet condition from the collection point and kept individually in the thermocol container. The chelipeds need not be tied with ropes because it may damage the eggs.

ACCLIMATISATION: 

Once the broodstocks are brought to the hatchery, the animals are carefully removed from the transport container to allow them in an empty basin. Then clean sea water is to be added slowly for acclimatization or it is advised to acclimatize them in clean water at collection point itself in order to maintain fertilized eggs in a healthy condition

QUARANTINE SECTION: 

For removing the pathogens the animals are treated with formalin @150 ppm dosage for 30 minutes.

STOCKING: 

After acclimatization and quarantine section the berried crabs are directly stocked into the black colored FRP spawning tanks of 250-300 liters capacity. Covered with a lid and filled with cleaned and disinfected sea water with aeration. Black tanks are always preferred and should be kept in a spawning room with individual spawner.

WATER QUALITY MANAGEMENT:

Salinity-28-32ppt

pH - 7.5-8.5

Temperature – 28 - 32⁰C

Dissolved oxygen - > 4.0 mg/liter

BROODSTOCK FEED:

Fresh fish, squids, bivalves, mussels etc are given as food. 

The mature broodstock can be fed with 5-10% of body weight (40% in morning and 60% in evening).

 The frozen fishes are thawed first before feeding.

INDUCED BREEDING:

 In crabs, gonadal maturation can be induced by eyestalk ablation. During this process, one of the eyes is ablated to remove gonad-inhibiting hormone-secreting gland situated in the eyestalk. 

Ablated eyestalk has to be treated with iodine ointment for healing the wound and preventing the infection. Spawning would take place between 2 and 8 weeks depending upon the ovarian development.

LARVAL DEVELOPMENT: 

The eggs hatch into Zoea. It has five stages (Zoea 1, 2 3, 4, 5). After hatching of eggs the spawner is shifted into another tank. Before collecting the larvae, the aeration is switched off and the incubation tank is covered with a lid, leaving a small opening. Since the larvae are photo-tactic in nature an electric light is kept near the opening of the lid in the corner to attract the larvae. The accumulated larvae are collected by fry bowl or using siphon net. The collected larvae are transferred into larval tanks. Rearing of larvae is continued in the same tank till it reaches the Zoea – 5 stages. (stocking density 70 numbers/liter).Once the larvae cross zoea 5 sage, its metamorphosis into megalopa stage. (Stocking density 1-2 no./liter). To avoid cannibalism certain hideouts are provided.

Temperature 25-32⁰C, Ph-7.5-8.5 and DO- >4mg/liter are provided for larval tanks

FEED FOR LARVAE: 

For early zoea stages, rotifers (Brachionus plicatilis) are given as feed. After 7 days rotifer with microalgae is provided. Day 8 onwards artemia are given as feed. As an alternative, egg custard, fish meat, squid meat etc., can be given. Megalopa develops into crab instar, and then it develops into sub-adult and adult.

NURSERY REARING:

Hatchery produced crab instars 0.3-0.5cm carapace width are stocked in HPDP Happas in brackish water ponds. Where the salinity range is 15-35ppt. Hideout is provided to avoid cannibalism. The minced fish meal can be given as food. Based on the feeding and management nursery days may vary from 30-40 days to attain the “matchbox” crab let.

CULTURE OF CRABS:

Nursery-reared crablets are used to grow out culture. Normally 6-7 months are required to attain a marketable size of average 500 gms. Another way used to grow out culture is growing of juveniles sizes in small brackish water pond for two months to attain the average size of 50-60gms. 

 The size of the grow out pond can vary from 0.25-1 Ha, with proper inlet and outlet water management. Soil texture with a high slit is not suitable for crab farming. Minimum 1-meter water depth is required. Stocking density 1 juvenile crab/m². Stocking density 1 juvenile crab/m². Stocking of crablets can yield an average survival of 60% and stocking of juveniles yield 80% survival rate.

PEN CULTURE: Crablets can also be cultured in pens with average stocking density is 1no/m². The advantage of pen culture is easy growth, faculitate stocking of different sizes of crabs in different pens which may ultimately increase the survival and yield. Crabs generally feed on mollusks, fish, other crustaceans and annelids.