It is important to understand the anatomy of the cultured species in order to be able to identify any possible parasites or diseases affecting the animals. In this series of posts, we will try to explain in details the anatomy of the Scylla species. Mangrove crabs are exoskeletons which will shed their shells in order to grow.
All the swimming crab species including Portunidae (eg: blue swimmer crabs) and Scylla species has a carapace, two claws, three pairs of walking legs and one pair of swimming leg. The most obvious external feature defining this crab is the carapace. The carapace is a flat and broad shell protecting all the internal organs of the crabs. Carapace width (CW) is one of the indexes used to monitor and measure the size and growth of crabs, it is measured between the lateral spines of the carapace. Along the front margin of the carapace, there are a total of 24 spines, with 6 spines in between the eyestalks and 9 spines on each side of the carapace.
Right in the middle of the carapace lies the pair of stalked eyes separated by 9 spines. A pair of antennae and antennules originating from below the eyestalks can be seen as a pair of long and short segmented appendages respectively. Beneath all that is the mouthparts of the mangrove crab, which are composed of three pairs of maxillipeds for further breaking down of food pieces and swallowing. All these are the sensory organs for the crabs, which they use to locate food, mates and detect predators. Residing in the mangrove areas meaning they will be submerging in murky waters most of the time and the eyes are not very useful in this habitat. So, they detect chemicals or cues externally by using their antennae covered in special chemosensory hairs, moving them to create currents, trapping the water or air in the spaces between the hairs and allow them to detect changes around them. Similar sensory hairs could also be found on the legs.
Scylla species has a distinctive pair of big claws, which contributes to 20-40% of their overall weight, with the claws of females appearing to be smaller than the males. These claws are what make mangrove crabs one of the most demanded seafood in Asian countries. The chelipeds are used to cut or crush their food or prey such as cockles. The claw or chela is a part of the chelipeds, with “teeth” similar to molar teeth in humans is used for crushing shells, called the “crusher claw” whereas the other claw with sharper “teeth” is used for cutting the flesh called the “cutter claw”. On the claw, there is a movable tip called dactylus and immovable tip called pollex. The second segment of the cheliped is the propodus, the third segment is the elbow or carpus. On the propodus and carpus, we will find the presence of spines that we use to identify the different Scylla species of mangrove crabs. There are three pairs of walking legs used for accessing areas on land and the fourth pair of flattened legs are used for swimming. Each walking legs has four segments namely the dactyl being the furthest from the carapace, propodus, carpus and merus. The dactyls also have chemoreceptors that the crabs used to find food and detect cues around them.
The underside of the mangrove crab is the body segments and the abdominal flap that is flexed underneath the body. It is actually the tail during the planktonic stages of the crabs, being reduced and folded under its body. The abdomen shows shapes that differentiate the separate sexes. Males have a sharp triangular shape abdomen, whereas immature females have a broader triangular flap. Matured females’ abdominal flaps appear to be an inverted ‘U’ shape, darkened and with setae (or small hair) around it. Flexing the abdomen exposing the inside of the abdominal flap is the transparent intestine that lies along the ventral midline and ends at the anus, which excretes the faeces from the internal of the crabs.
Hiding underneath the abdominal flap in males are two pairs of pleopods, which they use to transfer the sperm into the female during copulation. Female crabs have paired biramous pleopods on their abdominal segments (Ee et.al., 2004), which consist of setae where the females release their eggs onto it, consistently ventilating the eggs by creating movement using abdominal flap and pleopods. The pleopods are made up of a few segments including the coxa, basis, exopods and endopods. The coxa is the first segment and is connected to the abdomen by the articulating membrane.
It is important to understand the anatomy of the mangrove crabs prior to stocking them so that we could have biosecurity control over crabs that might be a potential biohazard to the farm. A lot of the disease such as shell disease and parasites which can all be observed on the carapace of the mangrove crabs.