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Portunus pelagicus (Family Portunidae)

Blue Swimmer Crab

Distribution:

The Blue-swimmer Crab is distributed from East Africa, through the Indian and Pacific Oceans to Japan, the Philippines and Tahiti Island, Australia and to the northern island of New Zealand. It has also invaded the Mediterranean through the Suez Canal. It is found all around the Australian coastline in sheltered bays and inlets. (QLD, NSW, VIC, TAS, SA, WA, NT)

Features:

The Blue Swimmer is one of the best known edible crabs in Australia. Its carapace is very broad, 2-2.5 more wide than it is long. It grows to 300mm. width across the carapace. The carapace is covered with granules, varying from coarse and more widely spaced, to finer, closer ones. At the carapace front, there are four acute, tooth-like bumps of which the outer one is larger and more prominent. In juvenile crabs, these are lower and more rounded. At the region where the front merges into the sides (the anterolateral angle) the first tooth-like bump is larger than those coming after, but they become more spine-like in shape. The ninth spine is very large and projects straight out on each side. The rear of the carapace is smoothly curved. Its long walking legs are partially flattened, especially the hind pair that are very flattened into a paddle-like shape. The hind pair of flattened legs are used for both swimming and digging backward into sand for protection.

In adult males, the very large chelipeds are 2 - 3 times as long as the carapace. The chelipeds are massive, very spined and ridged. The undersurface is smooth. The wrist has inner and outer spines. The width it can reach with its outstretched claws is very impressive and the grip of its chelae is even more impressive. An adult male can reach out up to five times its carapace width. It can also move very fast and can be quite formidable.

The colour of the Blue Swimmer Crab is mainly purplish-brown and blue with paler mottling on the upper surface. It is white below. The legs are blue or lilac. The female Blue Swimmer, known as a 'Jennie' is smaller and more brownish in colour than the male.

Ecology/Way of Life:

Blue-swimmer Crabs are the most common members of the Portunid family in the shallow water surrounding Australia. They are not shore crabs, but visit shallow waters and the intertidal, when the tide is high, hunting for prey. When they are juveniles, they settle in shallow sandy habitats and grow rapidly. They reach maturity after one year.

Blue Swimmers are active, fast and powerful carnivores. They prefer mud and Zostera seagrass habitats, and may also be found in shallow pools and under stones. They occur from the intertidal region at high tide, down to 2 to 68 metres in depth. They come to the water surface after sunset. They are powerful, purposeful swimmers, usually moving along with the water current.

Interaction with Humans/Threats:

Blue-swimmer Crabs are commercially fished in South Australia and in southern Queensland. For many, the Blue Swimmer Crab is Australia's most tasty crab. There has been, and continues to be, a lot of research undertaken on this species because of its commercial worth to the fisheries. This research needs to continue, as anecdotally Blue Swimmer Crabs do seem to be getting smaller and less frequent in estuaries surrounded by human habitation. If one is kept in an aquarium, there is nothing else alive in the tank that is safe from its voracious appetite. It is a powerful, efficient and ruthless hunter that will eat most live things.

Other Comments:

Portunus comes from the Latin word portare, meaning to carry, but also has the meaning of an imposing bearing, or the position of a ported weapon. Tunus may come from the Latin word tunica, that is a Roman shirt-like undergarment, or a close-fitting soldier's coat. This gives the impression of a well-armed soldier, which the Blue Swimmer Crab is, if you have ever tried to grab a live one. Pelagicus comes from the Greek word pelagos, meaning the sea.

Further Reading:

Bennett, I. (1987). W. J. Dakin's classic study: Australian Seashores: a guide to the temperate shores for the beach-lover, the naturalist, the shore-fisherman and the student. p.229, Angus and Robertson.

Davey, K. (1998) A Photographic Guide to Seashore Life of Australia. p.50, New Holland Press, Sydney.

Edgar, G.J. (1997). Australian Marine Life: the plants and animals of temperate waters. p.210, Reed.

Jones, D. and Morgan, G. (1994). A Field Guide to Crustaceans of Australian waters. p.157, Reed.

Acknowledgments:

Text, map and photograph by Keith Davey.

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