Scylla serrata (often called mud crab or mangrove crab, although both terms are highly ambiguous, as well as black crab) is an economically important species of crab found in the estuaries and mangroves of Africa, Australia and Asia. In their most common form, the shell colour varies from a deep, mottled green to very dark brown.
Scylla serrata in Bangladesh: Culture technology for Crab (Scylla serrata) : Habitat and Diet :Scylla serrata is common in the mud flats of the littoral and inter-tidal zones of the Bay of Bengal. The species hardly occurs in sandy and rocky areas. Mud crabs are extremely tolerant of salinity and temperature variation, and can survive in a salinity range of 2 to 50 ppt and temperatures of 12 to 35°c. It is not known to what extent they are affected by salinity variation but in the case of temperature, it is known that activity and feeding falls off dramatically below 20°c. Scylla serrata is mainly active at night. Although many occupy burrows in the intertidal zone, the majority of adults live sub-tidally, when they bury in the mud during the day. They emerge at night and forage for food, covering about 500m each night. They feed on slow moving (or stationary) bottom-dwelling animals such as bivales, snails, other crabs, hermit crabs, and worms. They rarely eat fish under natural conditions since they lack the ability to catch them..They are attracted to a wide variety of baits, including fish, but this does not mean that fish form part of their normal diet.
Reproduction : Mud crabs reach sexual maturity at between 18 and 24 months and mate in the warmer months. The mature females(known as jennies) release a chemical attractant, or pheromone, into the water which attracts the males(known as bucks). The successful male picks up the female and carries her around for several days until she moults. Copulation can occur only when the female is in the soft shell condition. The male deposits a spermatophore, or packet of sperm, inside the females reproductive opening. Where it is stored until the developing ova are ready to be fertilized. Following mating the females migrate offshore to spawn. Eggs are released in batches of two to five million at a time. The eggs are carried beneath the females abdomen which has to be folded outward to accommodate the large egg mass. Eggs hatch in 2 to 4 weeks and a zoea larval stage emerges. These zoea larvae are sensitive to high temperatures and low salinities, and therefore cannot exist in estuaries, they require marine conditions. There are four zoalstages, they give rise to a megalopa stage. Appendages on the abdomen enable the megalopa to swim and return to the estuaries. Larval life lasts about a month. Once back in the estuary, the megalopa to change into juvenile crabs which settle down in sheltered areas such as between mangrove roots or in seagrass beds.
Lifecycle of Crab : 1.Zoea – A mud crab begins life as a larva called a ‘zoea’, which hatches from an egg. It is about one millimetre long, has undeveloped limbs, and floats with the plankton. 2.Megalopa – A zoea grows by ‘moulting’ (shedding its shell) four times over 12 to 15 days. During the fifth moult, it transforms into a ‘megalopa’, which has functional claws. After a week or so, it moves inshore and settles to the seabed. After a few days, it moults into a stage one juvenile crab. 3.Juvenile crab – A miniature version of the adult, it is about 4 mm wide. About a month after hatching, when 10 – 20 mm wide, it moves to an estuary and settles in a sheltered area. 4.Young adult – The crab reaches sexual maturity at 18 to 24 months. 5.Mating – In the warmer months, mature females release a ‘pheromone’ (chemical attractant) into the water to attract males. Once paired, the male climbs on top of the female, clasps her with his hind legs, picks her up and carries her around for up to four days. He releases her when she begins to moult, and after she has shed her shell, he turns her upside down to mate. He deposits a capsule of sperm inside her reproductive opening, where it’s stored for months until her developing ‘ova’ (eggs) are ready to be fertilised. After mating, the male flips the female upright and holds her under him for a few more days while her shell hardens. 6.Spawning and hatching - The female migrates offshore to spawn. The fertilised eggs are released in batches of two to five million. After digging a hole in the sand or mud with her abdominal flap, the female releases her eggs into it. Using her swimming legs, she gathers the eggs up to carry them under her abdomen. The eggs hatch in two to four weeks. The lifecycle then begins again.
Culture Method : In the culture of crabs different method are practices. Among of them the most common method practises in crab culture in Bangladesh are as follows A. Culture practise in cages B. Culture practise in pond C. Gher culture
A. Culture practise in cages :
- Mud crab has been incidental product of culture operations meant to raise milkfish, prawn, and other fin fishes, in Southeast Asian countries.
- The culture of mud crabs in many countries generally falls under two major categories 1.fattening and hardening in cages and 2.growing out in ponds.
- Most fattening procedure is done in cages made of bamboo.
- However, nets, earthen ponds or ditches are also used.
- Crabs of about 150-200g are reared in bamboo cages in individual compartments and fattening is done for 7-15 days.
- Stocking one crab in each chamber 30-60 per cage depending on the size of the cage.
- Feeding with trash fish, low cost cultured fish, fish offal from market, soft shelled snails are used.
- Kitchen offal is offered to the crabs while trash fish and animals offal become scarce.
- However, in grow out system relatively small sized crabs, caught from wild which are grown for a longer period.
B. Culture practise crabs in pond:
- Pond for crab culture can be any size but small ponds having an area of about 350-500m2 are preferred which are more efficiently manageable than the bigger ponds.
- The depth of the pond should be as much as 1-2m while gradient of the dikes should be steep to prevent the crab escaping.
- Pond dikes are made of mud blocks and fenced with bamboo mats of 1.5 to 2m high.
- Mats should be installed from 30cm deep into the mud to avoid crab escaping during low tide by borrowing through dikes.
Pond are treated with lime at about 350-600kg/ha and kept exposed to the pond about 5-7 days.
- Then water is drained into the ponds until a 1.5m depth is attained.
- Water exchange is done daily or periodically by pumping or tidal effect.
- After two to three days of pond preparation locally caught crab larvae are stocked at a rate of 3-5 crab/m2
- Animal food, such as trash fish, snail meat, cattle or poultry offal, fish viscera, kitchen wastes are used as feed.
- Crabs are fed at a rate of 7-10% total body weight per day.
- Enough 100g is given to prevent cannibalism.
C. Gher culture Gher selection and preparation:
- Emphasis should be given in selecting suitable site for crab farming.
- Sandy-loam or muddy areas are recommended.
- Gher size of 0.2 to 1.0 ha is considered good for management.
- Scope of exchanging water should exist.
- Dykes must be protected with bamboo, brick, strong netting or concrete panels. A sand bottom inhibits burrowing.
- Naturally collected fry can survive up to 5 days in air and thus makes transportation of fry very easy.
- Stocking densities vary from 5000- 10,000/ha.
- Average weight should vary from 25 g to 30 g.
- Sex ratio should be maintained at 9:1 (female: male).
Feeding:Like prawn or shrimp, crabs also take food at night. During tide, they also collect food in day time. Feeding rate should be 5% of the total crab biomass. Many ingredients may use directly or in mixed form for feeding them. Some commonly used feeds are muscle of snails, mussels; flesh of trash fishes (e.g. eel, tilapia etc.); shrimp or heads of shrimp after beheading; cattle or poultry offal; fish viscera; kitchen wastes etc. Freshwater soft-shelled snails are the most important food items of crabs. Enough food is given to prevent cannibalism.
Harvesting: The culture period is dependent on the initial size of crab stocked and size desired at harvest. Crabs weighing of 220-250 g and with carapace widths of 12-15 cm are harvested in 5-6 months. But better management provides opportunity to harvest crabs in only three to four months. Mud crabs are very hardy and can survive several days out of water if kept moist. Cast nets and bamboo baskets are usually used for harvesting and marketing respectively.
Hardening crabs :
- Mud crabs have high demand in export and domestic markets.
- At present newly moulted soft male crabs fetch very low price when compared to the hard shell mud crabs.
- Hence, these water crabs are used for hardening and after sufficient hardening they are harvested and sold at high price.
- The exoskeleton of soft shell crabs can be hardened in 4-7 days by stocking in culture ponds/cages provided with adequate feeds.
- Before this operation the culture pond bottom is prepared by adding sufficient quantities of lime and care is taken of ponds bunds are without any hoes and crevices.
- Crabs are stocked at a rate of 1 crab/m2 or 1 crab /chamber of a cage.
- Crabs are fed with trash fish, tilapia or silver craps@8-10% of their body weight twice in a day.
- Two pieces of fish is provided for each crab to prevent cannibalism and fighting each other for food.
Fattening crabs :
- Increasing demand for egg-bearing female crab has stimulated berried female crab fattening in Paikgacha, Rampal and Satkhira region.
- Egg production in mud crab, as in other decapods crustaceans, may be accelerated by eyestalk ablation.
- Crab fattening is carried out in brackish water ponds or in shallow lagoons using floating cages or pens.
- Female crab without eggs, are fattened for 12-15 days until they develop gonad.
- The cage may be of bamboo, polyethylene net, galvanized ware net or plastic.
- Farmers make the bamboo cages, while the other are commercial products available in local markets.
- The bamboo cage measure 7.0×3.0×1.0 feet and is divided into 9.6 compartments. Each compartment accommodates one crab.
- The cage is covered using woven split bamboo.
- Compared to other culture practises. The stocking rate is relatively high, at 1 crab/compartment.
- This implies that 40 crabs can be stocked in one sq.m.cage.
- Crabs are fed with trash fish, tilapia or silver carp@8-10 per cent of their body weight twice a day.
- When market price is low farmers reduce the food ration for daily to delay the gonad development.
Harvesting crabs :
- Crabs weighing of 220-250g and with carapace widths of 12-15em are harvested in 5-6 months.
- The survival rate varies with the culture practise, ranging from 50-70 %.
- The harvesting is done in various categories like crabs with full of gonad and hard shell to have the high quality meat and secure good nice.
- Harvesting is done with the help of dip net and lift baited with trash fish.
- Plastic pipes are placed sometimes on the bottom for harvesting.
- When the crab hides in the pipes, it is lifted and harvesting is done.
- For effecting harvesting, during construction
of the pond itself, a water inlet tank of 1.5 m2 is made in the centre of the pond.
- When water enters in the tank, crabs congregate there and are collected.
Marketing crabs :
- The trading pattern of the mud crab involves a sense of intermedianes between the catcher/farmer and the consumer or exporter.
- The marketing system expanded during the end of the eighties and early nineties with foreign markets opening up.
- Before that, crabs were marketed only locally.
- However, as domestic consumers are generally poor, the price was much lower than in the export market.
- Fishermen usually market their catch 2 to 3 days after capture.
- During this time the crabs are kept in their homes or boats.
- Male crabs weighting less than 200g and female crabs of less than 150g are rejected and are sold locally.
- The present domestic market is based mainly on this supply.
- Male and females are separated at the district level market or at the fourth or fifth local marketing shop and in some cases, even at the production point.
- The marketing trend is for improvements in both quality and price.
- Present international markets for crabs are Singapore, Hong Kong, Malaysia, and a few other Southeast Asian countries.
- More than 95% of export involves live crabs, processed and frozen exports are negligible.
The natural range of Scylla serrata is in the Indo-Pacific. It is found from South Africa, around the coast of the Indian Ocean to the Malay Archipelago, as well as from southern Japan to south-eastern Australia, and as far east as Fiji and Samoa. The species has also been introduced to Hawaii and Florida.
The crab is large in size and has two distinctive large claws that are used for fighting, hunting, and movement. Their shell is generally seen as shades of dark green. The crab is also identified with two adapted appendages located behind the smallest legs and are used for swimming short distances to escape predators.
The crabs diet consist of other crab, fish, and dead matter. The crab normally relies on its camouflage and fast snapping claws to catch and crush its prey to death. Although big the crab is uncharacteristically fast when it needs to be; this animal hunts mainly at night when most fish are in their sleeping state.
The crabs main predators are; stingrays, eels, bigger mud crabs, octopi, and humans. To hide from most of these predators the crab burrow in mud or sand and spends most of the day in these burrows. Other defenses against its predators is its unique thick shell that is resistant to crushing. Its signature claws can also reach nearly allover its body making it difficult to grab.
A study on tidal flats in Deception Bay in Queensland found juvenile crabs (20–99 mm or 0.8–3.9 in carapace width) were resident in the mangrove zone, remaining there during low tide, while subadults (100–149 mm or 3.9–5.9 in) migrated into the intertidal zone to feed at high tide and retreated to subtidal waters at low tide. Adults (150 mm or 5.9 in and larger) were caught mainly below the low tide mark, with small numbers captured in the intertidal zone at high tide.
These crabs are highly cannibalistic in nature; when crabs undergo moulting, other hard-shelled ones sometimes attack the moulting crabs and devour them. The females can give birth to a million offspring which can grow up to 3.5 kg (7.7 lb) in size and have a shell width of up to 24 cm (9.4 in) wide.
Aquaculture and consumption
Interest in the aquaculture of this species has been high due to the high demand/price for them, high flesh content, and rapid growth rates in captivity. In addition, they have a high tolerance to both nitrate and ammonia (twice that of the similar sized Portunus pelagicus), which is beneficial because ammonia-N is often the most limiting factor on closed aquaculture systems. Their high ammonia-N tolerance may be attributed to various unique physiological responses which may have arisen due to their habitat preferences. However, their aquaculture has been limited due to the often low and unpredictable larval survival. This may be due to inadequate nutrition, disease, "moult death syndrome" (due to their highly cannibalistic behaviour during the megalopa stage), inadequate protocols (e.g. suboptimal environmental conditions), or a combination of all.
S. serrata can be kept easily in home aquaria when smaller, but will outgrow small setups. They are very active and will eat almost any conventional sinking pellets; they also consume some small fish pieces and vegetable matter. They are tolerant of most water conditions and are generally a very hardy and entertaining species.
Generally cooked with their shells on, when they moult their shells, they can be served as one of many types of soft shell crab. Some consider them to be among the tastiest of crab species, and demand for them is large in South and Southeast Asian countries, where they are often bought alive in the markets. In the northern states of Australia and especially Queensland, mud crabs are relatively common and generally prized above other seafood by the general public.
Mud crabs can be killed humanely by placing them in a freezer for up to two hours before cooking.
- "Species Fact Sheets: Scylla serrata". Food and Agriculture Organization. Retrieved September 2012.
- J. Masterson (December 1, 2007). "Scylla serrata". Indian River Lagoon Species Inventory. Smithsonian Marine Station at Fort Pierce. Retrieved September 11, 2012.
- B. J. Hill, M. J. Williams and P. Dutton (1982). "Distribution of juvenile, subadult and adult Scylla serrata (Crustacea: Portunidae) on tidal flats in Australia". Marine Biology 69 (1): 117–120. doi:10.1007/BF00396967.
- N. Romano & C. Zeng (2007). "Effects of potassium on nitrate mediated alterations to osmoregulation in marine crabs". Aquatic Toxicology 85 (3): 202–208. doi:10.1016/j.aquatox.2007.09.004. PMID 17942166.
- N. Romano & C. Zeng (2007). "Acute toxicity of ammonia and its effects on the haemolymph osmolality, ammonia-N, pH and ionic composition of early juvenile mud crabs, Scylla serrata (Forskål)". Comparative Biochemistry and Physiology A 148 (2): 278–285. doi:10.1016/j.cbpa.2007.04.018. PMID 17540593.
- Dominic Mapstone. "Live Mud Crab - Scylla serrata". Retrieved 5 December 2009.
|Wikimedia Commons has media related to Scylla serrata.|
- Australia: Queensland government page on mangrove crab aquaculture
- Cycle of the mud crab
- Life Cycle of the Mud Crab - Fishnote No.11 March 2007, Northern Territory