Semaphore Crabs range from southern Queensland toVictoria and Tasmania. (QLD, NSW, VIC, TAS)
The Semaphore Crab is robust and barrel-shaped, with large, near equal-sized claws. The long, purple eyestalks are held upright, and reach to near the edge of the carapace when lying in the eye sockets. Adults are about 20 to 25 mm wide across the carapace, which is dark purple or bluish black in colour. The claws are light purple, with white fingers. The young look quite different - a purple carapace, mottled with light-grey markings and light orange-red claws; they can be confused with the Red-fingered Marsh Crabs, Sesarma erythrodactyla, that live in the same habitat.
Ecology/Way of Life:
These communal crabs often group on a soft, stable estuarine mudflat. They occur mainly near the high tide mark, preferring bare mud, but may occur amongst mangroves. Each crab never strays far from its protective, vertical burrow, a central focus for the owner and worth fighting for. As the tide drops, these crabs emerge and begin feeding; they scoop up pellets of mud with the fingers of the claws, and pass them to the mouthparts where food particles are separated. When threatened, these crabs race towards the closest protective burrow, and enter it sideways. They feed mostly on algae, organic detritus and microorganisms on the surface of the mud, as well as dead or sick invertebrates.
As the tide rises, the crab re-enters its burrow, which ends in a chamber. Excavated material is tolled into a ball and deposited around the burrow entrance. This species plugs the burrow with mud as the tide comes in. It is commonly called the Semaphore Crab, because a male will stay still at intervals, raise his body once or more times, while outstretching his claws several times, in a way that resembles someone sending semaphore signals. This distinctive behaviour has a territorial function, and signals to males to remain at a distance, while attracting female crabs to enter their burrows for mating. Other behaviours include fighting in defence of the burrow, during which the claws are held forwards, wide apart and with open fingers.
Interaction with Humans/Threats:
The Semaphore Crab is so common on the mudflats of our estuaries, even in areas of heavy pollution, that they appear to be under no threat from human activity. However, they do accumulate toxins from the surface of the mud while feeding, a fact put to good use recently in Queensland, where the accumulation of toxins in Semaphore Crabs and other species provides an indicator of an estuary's health. Other potential threats include loss of habitat or siltation.
Heloecius cordiformis. The genus Heloecius was described by Dana in 1851 and applied to the species cordiformis by Henri Milne-Edwards in 1837. The genus name is derived from the Greek: helos = marsh and oikos = home, presumably referring to its preference for estuarine mudflats; the species name is from the Greek: chorde = rope, and the Latin: formis = shape.
Bennett, I. (1987), W. J. Dakin's classic study: Australian Seashores. Angus & Robertson. 231 pp.
Davey, K. (1998). A Photographic Guide to Seashore Life of Australia. p. 61. New Holland Press.
Griffin, D.J.G. (1968). Social and Maintenance Behaviour in two Australian Ocypodid Crabs (Crustacea; Brachyura). Journal of Zoology, London. V.156, pp.291-305.
Healy, A. & Yaldwyn, J. (1970). Australian Crustaceans in Colour. A.H. & A.W. Reed, Sydney.
Jones, D. & Morgan, G. (1994), A Field Guide to Crustaceans of Australian Waters. Reed Books 200 pp.
Marine Research Group (1984), Coastal Invertebrates of Victoria, p.123. Museum of Victoria.
Text, map & photograph by Keith Davey.
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