BIO­DI­VER­SITY

Science Illustrated - - CONTENTS -

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.

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