Blue manna crabs are scavengers, but also hunt and capture small fish and other marine animals. To catch their prey, they hide and wait for their prey to come close. Then they launch themselves at small fish and other animals that they catch in their strong claws.

During the day crabs hide from predators by remaining buried under the sand with only their eye showing. Animals that like to eat crabs include stingrays, octopus, and large fish.

Crabs mate in autumn. The 180,000 to 2 million orange eggs that each female produces are fertilised after mating. The eggs are then piled up in a spongy mass under the crab's abdomen. The females are now 'berried' or 'in berry', incubating the eggs for up to 18 days depending on the water temperature. A female can spawn more than once in a season, using sperm from one mating.

After hatching, crab larvae develop through a series of stages over about four to six weeks. The crab larvae drift up to 80 km out to sea, moving offshore and along the coast depending on the winds, tidal currents and the Leeuwin Current. By the time they reach inshore waters and enter the estuaries, the later stage larvae begin to look more like the adults and are about the size of a 10 cent coin.

Few of the blue manna crab larvae survive to reach the shallow coastal nursery grounds. The crabs that do survive moult frequently and rapidly increase in size and weight.

Blue manna crabs, like all crustacea must replace their shells to grow, this process is known as ecdysis. When crabs shed their old shell, the new fully formed shell underneath is still soft. During the process the crab takes in lots of water to stretch the new shell before it hardens. Once the shell is hard the excess water can be released, leaving a bit of room in the new shell for the crab to grow. Moulting occurs about every two weeks when crabs are young. This slows down to about once a year as the crab's growth rate lessens with age. Crabs are vulnerable to predators while soft-shelled, and during this time they stay buried in the sand.

Scientific and alternate names
Blue manna crabs (Portunus pelagicus) are also commonly named blue swimmer crabs, sand crabs and sandy crabs.

What they look like
Blue manna crabs are ten legged animals that have a flat body. The soft flesh of the body is protected by a hard, rough shell called a carapace. The carapace has pointy bits on each side. Blue manna crabs have a variety of colours from brown to blue to purple. The claws are long and slender.

Blue manna crabs have been known to reach a carapace width of over 200 mm, a claw span (length) of 800 mm, and weight of over 1 kg.

Where they live
Blue manna crabs live in estuarine and inshore waters throughout south-west Western Australia. Cockburn Sound, Leschenault Inlet near Bunbury, the Peel Inlet near Mandurah, the Swan River, and Geographe Bay are all particularly well known for the number of crabs that can be found there. Crabs can be seen on the sand, in mud and seaweed covered areas.