Soldier Crabs are a phenomenon. They are not very big or very special to look at but the behaviour that they exhibit is spectacular. For some reason that scientists do not fully understand, these tiny crabs (about 1 centimetre across the carapace) will suddenly emerge from the wet sand of the lower shore in specific tidal conditions, gather in vast numbers, and march across the beach. Millions of them. Armies of them. Not scuttling randomly sideways like normal crabs but walking forwards. Then they disappear again just as quickly by cork-screwing themselves down into the wet muddy sand. If you stop and stare at the surface sediments, you can see the sand grains heaving as the little creatures excavate burrows below, presumably extracting microscopic food particles from sand they eat, and bringing new deposits or casts to the top.
While I was walking along Myall Beach near Cape Tribulation in Queensland, I was thinking that the vast sandy shore was a bit disappointing from the marine invertebrate point of view. Then I suddenly became aware of extensive dark shadows moving across the surface – looking rather like wind-blown sand. As my eyes focused on these erratically-moving darker patches, I realised that they were battalions of moving crabs. If I stood still long enough, more small crabs would emerge from the wet sediments at my feet, as if on some unknown cue, congregate together and march away, joining the horde and gathering new recruits on the way.
The crabs probably belong to the Family Myctyridae but I don’t think they are Myctyris longicarpus on the basis that the carapace colouring and markings are different in the specimens I photographed. The Soldier Crabs are abundant in this location because the tidal flats are enriched with nutrients brought down from the mountains by a river that flows through the tropical rain forest – reaching the beach as Myall Creek (complete with crocodiles).
The short video clips attached to this post – best viewed without sound because the wind blowing over the microphone is a bit intrusive – demonstrate some of these Soldier Crab habits.
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