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.
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.
Also Read: How long does it take for a mud crab to grow to legal size? https://www.guidetoprofitablelivestock.com/howtoraisecrabs/How-long-does-it-take-for-a-mud-crab-to-grow-to-legal-size.html
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.
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.
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.
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.
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.