1.Morphology of Mud Crab (Scylla serrata)
The body shape of the mangrove crab is relatively short with a reduced abdomen. The short body is caused by the fusion of the head and thorax to form a cephalothorax and is covered by a carapace.
The carapace is shaped like a hexagon, slightly rounded or oval, the size of the right chela is longer than the left chela, the last pair of legs is flat and adapted for swimming.
The anteroteral side of the carapace has nine spikes of almost the same size. The distance between the eye sockets (orbital) is wide, the front has six spines, and has a bulging cheliped propodus segment.
The last pair of pedestrians (pleopod V) is flattened at the last section (propodus and dactylus). The clamp (pleopod I) has a large part of the propodus with a smooth surface. Mud crabs have 6 spines between a pair of eyes, and 9 spines beside the left and right eyes (Karim, 1998).
Mud crabs have a pair of claws, in adult male Cheliped crabs (legs with claws) can reach a size of 2 times the length of the carapace. In addition, mangrove crabs are known to have 3 pairs of walking legs and a pair of flat-shaped swimming legs.
The color of mangrove crabs varies from purple to green and blackish brown (Siahainenia, 2008). Polygonal patterns are clearly visible in almost all parts of the body. The spines on the forehead of the carapace are wide, high and slightly blunt, triangular in shape. The four spines in the middle are almost the same length so they look even.
Female Crab – Rounded Surface Areas at the Abdominal Segment.
Male Crab – Narrow and Triangle Shape Abdominal Segment
Male mud crabs are different from female crabs. Male crabs have a narrow abdominal segment, while the females are larger. The belly of the female crab is in the shape of a bell (stupa) while the male is in the shape of a monument. The difference in pleopod function is located below the abdomen. Pleopod in male crabs functions as a means of copulation, while in females as a place to attach eggs (Moosa et al, 1985). Male crabs have a slightly pointed abdomen that resembles an isosceles triangle, while adult female crabs are slightly rounded and broad.
2.The life cycle of Mud Crab (Scylla serrata)
Female crabs mature at carapace widths between 80-120 mm, while male crabs mature physiologically when the carapace is 90-110 mm wide.
Mangrove crabs that are ready to mate will enter the mangrove forests and ponds. The mating process begins with the male crab coming to the female crab, then the female will be hugged by the male crab using his two large claws.
The male crab then climbs the carapax of the female crab. After mating takes place, female crabs will slowly enter the mangrove waters, ponds, to the shore, and then to the middle of the sea to spawn (Kasry, 1996). Meanwhile, male mangrove crabs will remain in the waters of mangrove forests, ponds or between mangrove roots.
Female crab immigrate to the beach while carrying fertilized eggs that are attached to the pleopod. The eggs will hatch in a few weeks. After the eggs hatch in marine waters, the eggs will enter the larval stage I level (zoea I) which will continue to molt (moulting). The larvae are then carried by currents into coastal waters until they reach zoea V (pascalarva) stage, approximately 18 days.
Furthermore, zoea V stage will experience skin change to megalopa (11-12 days). At the megalopa stage, its body shape is similar to an adult crab but still has a tail. After the Zoea V stage, then enter the juvenile stage which is also called the young crab stage which is born in the shape of a crab with complete body organs.
Remark: The crab molding and change it’s outer part of shell.
3.Habitat of Mangrove Crabs (Scylla serrata)
Remark: Areas highlighted in dark spot are mangrove forests in Indonesia.
This type of animal usually prefers muddy places in mangrove forest areas. Distributed crabs are limited to littoral areas with a depth range of 0 – 32 meters.
During the day, juvenile-grade crabs are rarely seen in mangrove areas, preferring to immerse themselves in mud. Mud crabs are euryhaline or can live in waters with a wide salinity range, namely 5 – 40 ppt. During growth, mangrove crabs like water with a route between 5 – 25 ppt.
Therefore, young crabs are found on the coast or river mouths which have relatively low salinity. Young crabs are also found in rivers far from the sea with a salinity of about 5 ppt. Crabs don’t like cloudy water instead it need clean, pollutant-free water.
4.Feed and Eating Habits
Adult mangrove crab are omnivorous scavenger. At the time of larvae, mangrove crabs eat plankton, and when juvenile like detritus. Adult crabs love fish, shrimp, and mollusks, especially shellfish. Mud crabs also like leaf especially mangrove leaves.
This type of crab consumes feed ingredients from plants that contain lots of fiber. According to Anderson et al. (2004) digestibility of crab on fiber and all vegetable feed raw materials is very high, ranging from 94.4 – 96.1%.
The results showed the presence of cellulase enzymes in the digestive tract of mangrove crabs which was thought to be a contribution from the microflora of the digestive tract.
The presence of cellulase enzymes allows mud crabs to digest dietary fiber. Mud crabs are nocturnal animals that are active at night in search of food.
Meanwhile, mangrove crabs during the day will hide in holes, under rocks, or between mangrove roots.