Credit: Ajith Kumara (SriLanka)
Review: Abdel Rahman El Gamal (Founder of the website)
Mud Crab (Scylla serrata) which is also called mangrove crab belongs to the family of swimming crabs (Portunidae). This is an economically important species of crabs and considered highly esteemed as food whereas the flesh from its claws and walking legs is considered a delicacy especially in South East Asia. There is a growing interest in the culture of mud crabs in ponds and cages in a number of countries in especially in the Far East.
Distribution and introduction: The mud crab is widely distributed throughout the Indo-West Pacific, occurring from South Africa to the Arabian Sea and eastward to Japan, Hawaii, Philippines and Australia.
The species has been introduced to several countries in attempts to establish populations of this commercially important species and hence are now found throughout the Indo-Pacific, from Japan, China, Philippines, and Hawaiian Islands to Australia, Indonesia, East and South Africa, the Red Sea, and Gulf of Mexico.
Habitats: Typically mud crabs (mangrove crabs) inhabit mangrove marshes and river mouths in estuarine environments as well as in sheltered coastal habitats. The mud crab lives in soft muddy bottoms in brackish water along the shoreline where they dig deep burrows. They may live in the open sea down to depths of 5 to 10 meters. They freely walk on muddy bottoms and swims in the water.
Description: This relatively large crab can exceed 18 cm in carapace width and according to FAO, a maximum male carapace width of 25-28 cm across the shell and a maximum weight of 2-3 kg. The females are generally slightly larger than males. The abdomen of the males in narrower than the female’s but overall it is broadly triangular. The carapace is smooth and its front is armed with four blunt teeth and each anterolateral border has nine similarly-sized sharp teeth which are projecting obliquely outwards.
As the male crab matures, the claws become very large, powerful and heavy with several well developed spines on the outer surface of carpus and on the anterior and posterior dorsal parts of propodus. The walking legs are smooth whereas the last pair are flattened and paddle-shaped turning into swimming appendages. The first pleopod of the male is stout, tapering and bearing numerous spinules on its outer side.
Individuals of mud crab are grayish green, purple-brown to almost black in color with small irregular white spots on the carapace and swimming legs. Mature females have wide and dark abdomens and dark orange ovaries that fill the cavity under the carapace.
Feeding: While the zoea and the megalopa stages feed on zooplankton, the sub-adult and adult crabs are carnivore and mainly eat mollusks, crustaceans, worms and small crabs; oysters are their preferred food while they rarely feed on fish and plant matter.
Reproduction: Scylla serrata become reproductively mature starting at around 90 mm carapace width, as early as the first year of life. Mating takes place after the female undergoes a pre-copulatory molt.
During copulation which usually lasts 7 to 12 hours, the male crab delivers non-motile spermatozoa that may be retained by the females for up to several weeks or even months after mating before being used to fertilize egg clutches. One female can produce at least three batches of eggs with an interval of about 35-45 days; the number of eggs per batch is up to 2 million eggs and sometimes more.
Berried females carry their bright yellow egg masses attached to the pleopod hairs of the abdominal flap and migrate offshore where the fertilized eggs which hatch in a few weeks into zoea and pass through five zoeal stages, after which they become megalopa that molt once and assume the appearance of a crab. Crab instars to juveniles are found in estuaries, tidal flats and mangroves, where they burrow in mud or sand or seek shelter under fallen leaves, twigs, etc. Crab instars undergo several molts before attaining full maturity.