Crab Farming At Home
Crab Farming At Home
The Biology Of The Mud Crabs
Distribution and habitat
Both the species of mud crabs are found in the shallow coastal waters, lagoons, estuaries, backwaters, brackishwater lakes, mangroves and inter-tidal swamps of east and west coasts of the mainland and the creeks and bays of Andaman and Nicobar Islands. They prefer sandy or muddy slush bottom. Both the species remain buried under the substratum during the day and are active in the night.
While the large? species (S. tranquebarica) remains buried under the substratum, the smaller species (S. serrata) usually makes burrows at the bottom and in the embankments of brackishwater canals and fish farms. Both the species migrate into brackishwater area during their postlarval stage (megalopa stage), Early juveniles abound the intertidal region, while the adults occupy the deeper portions of tire estuaries. After attaining maturity, adults migrate, especially the berried females to tire sea for breeding. S. tranquebarica is free living and frequents open areas of estuaries, whereas S. serrata is more common in mangrove areas.
Sexual identity
Sex can be identified in juveniles measuring above 35 mm in carapace width (CW) by the shape of the abdominal flap. In male, the abdominal flap is slender and triangular (Plate 1 D), while it is broad and triangular in immatured (Plate 1 E) and semi-circular in matured and berried females (Plate 1 F). In both sexes, the abdominal flap in live crabs, 7s-folded firmly against the ventral side of the body.
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Food and feeding
Both tlie species of mud crabs are carnivorous. They feed on slow moving and bottom dwelling animals such as bivalve molluscs, small crabs and dead and decayed animal materials. These crabs are also called as scavengers. In fact, they cannot catch a live and moving prey.
Moulting
The growth in mud crabs is manifested with the shedding of outer shell. Before moulting, a new exoskeleton shell is fomied below the old, hard and dead shell. During the moulting process, the old shell is cast off. The formation of new shell and casting of old shell is called as moulting process, which requires energy. The increase in size of the crab after moulting takes place due to the absorption of water by the tissues of the body and thus the moulted crabs are larger in size.
Since (lie moulted crabs have utilised stored energy for moulting, they weigh less and they contain more of water. The newly moulted crabs with watery meat and soft exoskeleton are called as "water crabs". Such "water crabs" remain defenceless and become easy prey to other animals, particularly other hard mud crabs. The newly formed shell of the moulted crabs gets hardened after 3-4 days after moulting. The frequency of moulting is more in juveniles and sub-adults, while it is less in adults. The hard shelled crabs are called as "meat crabs", which fetch a higher price.
Growth
In tlie experimental field culture, the early juvenile crabs (15 to 60 mm in CW/3 to 20 g in total weight) grow at a rate of 7 to 12 mm / 3 to 13 g per month, while juvenile crabs (61 to 80 mm / 25 to 70 g) exhibit a monthly growth of 11 to 12 mm / 45 to 97 g. In the sub-adult and adult stages, the monthly growth works out to 8 to 10 mm / 100 to 130 g. The larger species (S. tranquebarica) attains a maximum size of 220 nun / 2.5 kg and the smaller species (S. serrata) 140 nun / 0.5 kg in the wild.
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Maturity
Attainment of maturity in females can be easily noticed by the change in the shape of abdominal flap, from triangular to half-round/horse-shoe shaped (Plate 1 E & F). For males, there is no external character to identify the matured ones. The size at maturity for female is about 120 mm for larger species (S. tranquebarica) and 83 nun for smaller species (S. serrata). After the onset of maturity, the development of ovary takes place internally. Initially, the colour of the ovary is bright orange which changes to deep yellow before extrusion of eggs. The inner ovarian development can be determined by pressing down between carapace and abdominal flap. The eggs, if matured, are visible by their yellow colour.
Mating
The copulation takes place between a hard male and a freshly moulted soft female. Prior to copulation, a hard male climbs over the back of hard female crab, clasping her by Ms chelipeds and first two pairs of walking legs. Tltis formation is called as "doubler’ or "premating embrace", which lasts for 2-3 days. The pair separates when the female reaches the verge of moulting. The female moults and it is called as "pre-copulatory moult".
The male assists the female during the pre-copulatory moulting. A few hours after moulting, the male embraces the soft female again for the actual mating. The male gently turns the female over on her back using Ms chelipeds, while the female unfolds her abdominal flap and holds the male in position. The copulation lasts for 6-8 hours. During tire copulation act, the male deposits spermatophores in tire seminal receptacles of the female.
How Long Does It Take For A Mud Crab To Grow To Legal Size?
Suitable site
Ponds can be constructed in tide-fed estuaries, backwaters and creeks. The crab ponds are established in traditional fish/shrimp farms, by converting one portion adjoining the brackishwater canal, as shown in Plate 2 A & Fig.2 A., which would help increase the overall income of traditional fish/shrimp farmers.
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Grow-out pond
Earthen ponds of 0.1 to 0.4 lia in size and rectangular in shape having a sandy or muddy or clay loamy bottom soil are constructed with a minimal digging, which provides ample soil for bund construction. The wider axis of the pond may face the backwater canal in order to liave a greater tidal effect through a traditional wooden sluice.
Pond preparation
Before stocking, the ponds are prepared by removing unwanted organisms by netting. To prevent the escape of stocked crabs from the pond, fencing with suitable materials such as casuarina poles, bamboo split matting, G.I.chicken wire mesh, nylon netting and asbestos sheet to a height of 1 m is erected either in the inner edge of the pond (Plate 2 B & Plate 3 A & B) or on the top of the earthen bund.
The fencing may be positioned at 45 degree angle towards the inner side of the pond, which will prevent the climbing and escape of small crabs. Since mud crabs are highly cannibalistic, earthen and PVC pipes and wom-out tyres may be placed as hide-outs/shelters to reduce the fighting among the normal hard crabs and mortality of the soft "water crabs". A row of earthen mounds may be constructed in such a way that they remain submerged during the high tide and exposed during the low tide, in order to serve as natural habitat.
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Availability of stocking material
Since there is no dependable hatche it technology available for mud crab seed production, tlie stocking material at all sizes has to be collected from wild using nylon drag net, cast net, lift net and bamboo trap. The young crabs abound in estuaries, mangroves, backwaters, brackishwater lakes and creeks. The other source for small crabs and "water crabs" in live condition is from the commercial catches.
Stocking
Crabs with a size range of 8G to 100 g may be stocked at a rate of 1 to 5 per square metre, depending upon their availability.
Feeding
The reared crabs are fed once in a day, preferably in the late evening either with trash fish or molluscan meat (bivalve/gastropod) at a rate of 5 to 10 % of stocked biomass, depending upon the observed feeding intensity and the size recorded at regular and periodical sampling of the reared crabs.
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