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Petrolisthes elongatus (Family Porcellanidae)

Elongated Porcellain Crab, or Half Crab


The Elongated Porcellain Crab has a restricted distribution in south-eastern Australia. It is found in Vic and Tas, as well as New Zealand. (VIC, TAS)


The Elongated Porcellain Crab looks like a flattened large armed crab. Its smooth carapace is almost oval in shape but is slightly longer than it is broad, with only faintly marked regions and no spines. The appearance is of a very flattened and very wide crab.

The abdomen is closely pressed against the underside of the thorax, so that the creature looks like a little crab. Although crab-like, porcellanids differ from the true crabs, the decapods, by having a small last pair of walking legs, a small tail fan, has its abdomen folded under the carapace and has very large flattened chelae. It grows to a length of 8-10 mm and a carapace width of 7 mm. Its elongated flattened arms make it seem up to four times wider. The antennae are very long, but not very hairy. Male Elongated Porcellain Crabs are larger than the females are. The Porcellanidae family have more genera and species in the tropics.

The Elongated Porcellain Crab is green-brown in colour with brownish coloured chelae.

Ecology/Way of Life:

The Elongated Porcellain Crab occurs under rocks near low-tide level and below to 12 metres, on medium to high-energy coasts. It lives under stones on reefs. It is well camouflaged and although common on some shores, is not often seen. It is abundant in sheltered bays and reefs around Tasmania. They are filter feeders and feed by trapping food particles on long combs of hair, the third maxillipeds, that is part of their mouthparts, rather than use their antennae. They throw these hairy nets forward to capture passing planktonic food.

Interaction with Humans/Threats:

The Elongated Porcellain Crab has a very restricted range, only occurring on Victorian and Tasmanian shores in Australia. If there was a large environmental disaster in the Bass Strait region, such as a major oil spill, this could impact on significant populations of this species.

Other Comments:

Petrolisthes elongatus, Milne-Edwards, 1840. Petrolisthes comes from the Greek word petra, meaning a rock, and listhes, may come from the Greek word lissos, meaning smooth. Elongatus comes from the Latin word elongare or elongatum, from longus, meaning long.

Further Reading:

Davey, K. (1998) A Photographic Guide to Seashore Life of Australia. p.46, New Holland Press, Sydney.

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

Healy, A. and Yaldwyn, J. (1970). Australian Crustaceans in Colour. A. H. and A. W. Reed.

Jones, D. and Morgan, G. (1994). A Field Guide to Crustaceans of Australian waters. p.129-30, Reed.


Text, map and photograph by Keith Davey.

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