A very militant crustacean
Few Australians growing up either near the beach or as part of a family that takes regular holidays to the coast, can have missed these iconic crabs.
In armies of a few thousand, they appear at low tide and march along beaches and tidal flats, sifting detritus from the sand. If approached, they dig down with surprising speed and an odd “corkscrew” motion, quickly disappearing.
For many of us, soldier crabs are just part of the beach experience - like sand and waves and irritating gulls stealing all your fish and chips.
And yet these crabs are quite unique. Mictyris species crabs are among the few adapted to walk forwards, instead of only sideways. They spend most of their lives buried in the sand, emerging only for a few hours at low tide. But not every crab comes out every day - sometimes the army consists of males only, other times females join the march.
One behaviour few people may have observed is the soldier crab’s remarkable grooming routine. When they first emerge, sand can remain on their carapace. So in the space of less than a second, they rapidly flip through a half-somersault to shake the sand off.
After an hour or two walking the beach and eating bits of biological leftovers they find at the waterline, it’s time for the crabs to bury themselves again. Weirdly, the crabs often dig one burrow... then immediately leave it and dig another. They also sometimes engage in dominance disputes and displays, though this amounts to little more than males standing in front of each other and raising their claws. The smaller crab is the “loser”.
Like all true crabs, soldier crabs are part of the crustacean infraorder Brachyura, which means “short tail”. As a species, they have little to fear from predators thanks to their huge numbers, but they do fall prey to another familiar Aussie beach critter: the ghost crab.