Mud Crabs at Sea

The mud crab Scylla serrata is a large crab that occurs widely throughout the Indo-West Pacific Ocean and Australia.

Its Australian distribution extends from north Western Australia through the tropics to just south of Sydney. In Australia this species is commercially fished throughout its range and the industry is valued at over $13 million in the Northern Territory alone.

Until recently this species was regularly confused with three other very similar looking species of mud crab. This made it difficult to manage any impact commercial fishing may have on the crab populations. By comparing specimens in museum collections, and conducting genetic studies, scientists were able to show that there are four distinct species instead of one.

This mud crab specimen was donated to the Australian Museum by a fisherman who collected it from roughly 100 km off the coast, off Port Stephens, New South Wales. He found the crab floating on the surface of the water near a fishing buoy and was curious about its identity because of its unusual location. It is unknown how it got there but the goose barnacles growing on its shell indicate that it had been at sea for some time. Mud crabs generally inhabit sheltered coastal areas influenced by freshwater run-off. However, females have previously been recorded from deep water over 50 km offshore where it is believed they may spawn. This mud crab is a female and is carrying eggs.

This specimen demonstrates the importance of collections in recording the information used to better understand the diversity, distribution and ecology of organisms.