The Semaphore Crab, Heloecius cordiformis, is a very common, equal-handed, purple hued crab from muddy estuaries from southern Queensland to Tasmania.

It is a very common crab underneath the mangroves. In some areas the whole area has so many burrows that the mud is turned over and aerated in a process called bioturbation.

The waving of its claws, called chelae, gives it the name Semaphore Crab. The waving is to repel other males and entice female crabs to enter the male's burrow.

When alarmed the Semaphore Crab races across the mudflat to quickly disappear into its burrow. If it can't find its own burrow, any burrow will do.

Young Semaphore Crabs are quite different in colour to their adults. Their carapace is mottled with grey and their chelae are light orange in colour.

Sometimes they are confused with juvenile Red-fingered Marsh Crabs.

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References:

Davey, K. 1998. A Photographic Guide to Seashore Life of Australia.
New Holland press, Sydney.