If you spend any time at all sitting or standing quietly near a mangrove mudflat when the tide is out you will notice that the surface is alive with crabs. Nearly all of them will be semaphore crabs (Heloecius cordiformis).

Semaphore crabs are small. Their barrel-shaped, mottled purple carapace (the shell that covers their backs) is only 25mm across on the biggest of them. But what they lack in size they make up for in numbers and character.

These crabs believe in high density living. The shallow burrows are everywhere but the crabs seem to know the limit so that the whole substrate does not collapse. Even so it can look like there are more crabs than burrows and predators pick off the unlucky ones who remain on the surface.

Long eye-stalks give the crabs 360 degree vision. They can see above as well, although mainly for detecting movement. Eye-stalks are very handy for spotting predators but not so convenient for living in burrows. Semaphore crabs have solved this problem by having their eye-stalks fold sideways into grooves in the carapace.

Burrows are not only essential as refuges from predators. They also hold water to moisten gills when the tide is out, and are mating territories. The crabs defend their burrows, but will use others in an emergency such as if they are out feeding and are threatened. When the tide comes in or when it gets dark the crabs return to their burrows and plug the hole with a ball of mud.

Semaphore crabs feed when the tide is out. They pick up mud with their claws, sift out edible particles such as detritus, micro-algae and micro-organisms with their hairy mouthparts and leave small balls of mud.

Juvenile semaphore crabs have small claws that are pale orange near the tips but the adult males have the most character. They defend their burrows and try to attract mates to them by signalling with their large mauve claws. The display involves the crab raising its body as far as possible then jerking its claws up and down. It may not be making a semaphore signal but that is how the crab got its common name.

Apparently the females are not keen on inviting the males into their burrows. Sometimes mating takes place outside the female-owned burrows but more often the male manages to attract the female into his burrow to mate.

Like many other estuary crabs, the female releases the fertilised eggs into the water. The larvae spend some time as part of the plankton before the survivors settle out as miniature crabs in suitable habitats.