Hickman's Pea Crab
This species ranges from Victoria (Bass Strait) and Tasmania across southern Australia to Shark Bay in Western Australia. (VIC, TAS, SA WA)Features:
The Pea Crab is a small, globe-shaped crab, with no sculpturing on its carapace. It has a narrow front, and small eyes. The carapace is poorly calcified and appears to be quite soft compared to that of other crabs. Female Pea Crabs reach a size of about 10 mm, measured across the carapace. Male Pea Crabs, however, are much smaller in size, and are rarely found. The abdomen of the female is very large, soft and smooth. The eyes are small and not visible from above; they possess little pigment and may not be very functional. The feeding claws are small, weak and equal sized, but are still stronger and more robust than the walking legs, which appear to be almost useless. Pea Crabs are pale cream, fawn or dark-brown in colour, although females bearing eggs are darker.Ecology/Way of Life:
Female pea crabs are found only inside the body or body cavities of other animals in a commensal relationship with the host animal (see topic on symbiosis). Sometimes these relationships can form chains. Thus Herbert Hale, formerly of the South Australian Museum, described how one researcher found a Pea Crab inside a Spiny Oyster. This Pea Crab had been attacked by a parasitic barnacle, to which was attached a parasitic isopod crustacean. Hickman's Pea Crab may be found within the mantle flaps of bivalve molluscs, most commonly the Edible Mussel, Mytilus edulis planulatus, and also within the rectum of sea urchins, and occasionally in univalve molluscs. Other bivalve hosts are: the Queen Scallop, Equichlamys bifrons; the Spiny oyster, Spondylus tenellus; and Cotton's Bearded Horse Mussel, Modiolus cottoni. The rate of parasitism is high, with up to 100% of hosts in some locations. Little is known about how Australian pea crabs feed, but it may be similar to a European pea crab which scrapes its hair-fringed claws across the gills of its host, thus collecting the mucus produced by the host, and the edible plankton trapped in within it. Little is known of mating systems in these crabs. A female with eggs beneath her abdominal plate is about 5.5 to 9.5 mm across her carapace. While she spends most of its life sheltered within the host animal, the much smaller males appear to be free swimming. The carapace of the male is about 4.5 mm in diameter. Once the eggs are laid, the female protects them underneath her large cup-shaped abdominal plate.Interaction with Humans/Threats:
No threats have been recorded for this species.Other Comments:
Pinnotheres hickmani was described by Eric R. Guiler in 1950; the genus name comes from the Greek pino- = a pen shell of the family Pinnidae, and -teres, (from terein), to guard, referring to the sentry-like presence of these crabs in mussels and other bivalves; the species name honours Vernon Victor Hickman, the eminent Tasmanian zoologist.Further Reading:
Davey, K. (1998). A Photographic Guide to Seashore Life of Australia. p. 67, New Holland Press.
Edgar, G.J. (1997). Australian Marine Life: the plants and animals of temperate waters. p. 218, Reed.
Griffin, D.J.G. and Yaldwyn, J.C. (1971). Port Phillip Survey 2. Part 5: Brachyura (Crustacea, Decapoda). Memoirs of the National Museum of Victoria. 32: 43-63. (as Pinnotheres pisum).
Hale, H.M. (1976). The Crustaceans of South Australia. Pts. I and II. p. 172-4. South Aust. Govt. Press. (as Pinnotheres subglobosa).
Jones, D. and Morgan, G. (1994). A Field Guide to Crustaceans of Australian Waters. p. 202, Reed.
Marine Research Group of Victoria. (1984). Coastal Invertebrates of Victoria: an atlas of selected species. p. 116, Museum of Victoria.
Pregenzer, C. (1979). A redescription of Pinnotheres hickmani (Guiler) and comparison with P. novaezelandiae Filhol and P. pisum (L.) (Decapoda: Brachyura). Crustaceana (Suppl.)5: 22-30.Topics: Sexual dimorphism Symbiosis
Text, map and photograph by Keith Davey.Sponsorship welcomed:
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