“Blue swimmer crab (Portunus arnatus)
Blue swimmer crabs (also known as sand crabs) are specialised for swimming in the water, with their back pair of legs being paddle shaped. When they swim they use these ‘paddles’ to pull themselves sideways through the water. When they are not swimming, they usually are buried in the sand, with just their eyes protruding.
As a crustacean, blue swimmer crabs moult their shells in order to grow. The crab will moult the old shell and enter a soft bodied stage. During the soft bodied stage, the crab will swell up with water to expand its body size, and will grow the new hard shell over the top. Following the growth of the new shell, the crab will gradually replace the water with body tissues. This process will repeat several times throughout the life of the crab.
Blue swimmer crabs are a popular food species of crab and are quite popular with fisherman. In Queensland, as well as most of Australia, there are fishing regulations in place to ensure that these crab stocks are sustainable, so please always follow these laws. The minimum size limit for blue swimmer crabs is 11.5cm carapace width, and females are not allowed to be taken and must be thrown back. To tell the difference between males and females, the best way is to look under their abdomen (or underside). There is a visible ‘flap’ on their underside, which is thin in males and wide in females. Please also be responsible with your crab pots, as they are responsible for the death of many turtles that get stuck trying to eat the animals inside. Always check crab pots regularly and never discard of old crab pots into the ocean. These two photos were taken underneath the Tangalooma Jetty.”