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Mictyris longicarpus (Family Mictyridae)

Light-blue Soldier Crab

Distribution:

Light-blue Soldier Crabs occur from Shark Bay in central Western Australia, across tropical northern Australia and south along eastern Australia to eastern Victoria. (WA, NT, QLD, VIC, TAS)

Features:

The carapace (shell) of the Light-blue Soldier Crab is globular in form, about 12 to 15 mm in width, and longer than wide. The distinctive reddish area on each joint of its spindly legs is only partly hardened with calcified shell material, allowing these crabs to absorb some oxygen through their leg joints. Soldier crabs hold their feeding claws vertically in front of its face rather than horizontally. These crabs are unusual in their leg structure, which enables them to walk forwards rather than sideways, as found in all other crabs.

Ecology/Way of Life:

Light-blue Soldier Crabs are best known in Australia for their habit of forming huge armies that swarm over intertidal mud or sand flats when the tide reaches a certain low level, a behaviour that gives rise to their common name. This spectacular behaviour is part of a series of activities linked to the tidal cycle, feeding and competition for space. It begins between high tides, when the crabs push to the surface, raising small sand hummocks from which the crabs emerge, which takes from a few minutes to an hour. After cleaning the sand from themselves, the crabs feed for 10-15 minutes. While feeding, the whole population begins to trek towards the water, taking 15 minutes so to reach it. Once they reach moist sand they increase their feeding activity greatly. Each animal scoops up and eats large quantities of sand to extract small amounts of algae and other edible organic matter from the sand. The sand is ejected from the mouth and deposited in tiny round pellets called pseudo-faeces, which are clearly visible on the sand. At this time, large groups of crabs accumulate to form "armies", until many hundreds are together, scuttling along in formation, the whole group moving together in unison. They may cover 500 m in this phase, which may last from 30 minutes to 2 hours. In the return phase, the armies move towards higher ground, and begin to break up. There they dig into the damp sand with a corkscrew motion, which helps to turn over and aerate the sediments, and is accompanied by much threat behaviour between males. This whole activity cycle exposes the crabs to a range of predators, including Straw-necked Ibis, Mangrove Kingfishers, White Egrets, toad fishes, ghost crabs and a sand snail.

Mature crabs seem to prefer clean, sandy environments, whereas younger crabs are more common in sand with some mud. The female crab releases her eggs to be fertilised by sperm she has held since mating, and then retains them in a spongy mass until they hatch.

Interaction with Humans/Threats:

No direct threats have been identified for these crabs. However, loss of habitat, particularly mangroves, would reduce the available organic matter, and have a marked effect on this species.

Other Comments: Mictyris longicarpus was described and named by Peirre Andre Latreille (1762-1803), one of the foremost workers on crustaceans and insects of his time. The origin of the genus name is uncertain, but may come from the Greek: miktos = mixed, and tyrbe = confusion, probably in reference to the milling armies formed by these crabs; the species name is formed from Latin: longus = long and Greek: karpos = wrist, relating to the long first segment of the legs. Further Reading:

Cameron, A.M. (1966). Some aspects of the Behaviour of the Soldier Crab, Mictyris longicarpus. Pacific Science 20: 224-234.

Bennett, I. (1987) W.J. Dakin's classic study: Australian Seashores. p. 227. Angus & Robertson.

Davey, K. (1998) A Photographic Guide to Australian Seashores. p. 68. New Holland Press.

Edgar, G.J. (1997) Australian Marine Life: the plants and animals of temperate waters. p. 217. Reed.

Jones, D. & Morgan, G. (1994) A Field Guide to Crustaceans of Australian Waters. p. 192. Reed.

Acknowledgments:

Resources by Keith Davey.

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