The now-depleted crew of the Vulcan Spirit had never encountered, nor even heard of, soldier crabs until they were discovered during a day trip from Bundaberg to the estuary of the Burrum River. These are amazing animals, living in vast numbers on and in the intertidal mud. Each crab is about the size of a child’s marble, with a blue carapace (back), and in close-up looks like this:

 

 

This is Mictyris longicarpus, endemic to the east coast of Australia. It lives on algae and detritus that it sifts from the fine-grained estuarine sand, leaving behind tell-tale balls of processed sand.

 

 

They gain their name from their habit of moving around in seemingly organised groups of hundreds or thousands of individuals, often with larger crabs at the front apparently leading the army (it is now thought that they are at the front simply because their legs are longer!). With crabs’ usual keen eyesight they can spot movement from several metres away, and the entire army will wheel about and march off away from the threat. If the threat chases them or approaches too close the whole lot ‘dig in’, using a unique-amongst-crabs corkscrew movement to quickly twist themselves beneath the surface of the sand – the entire army disappears in seconds as if by magic. After a few minutes they re-emerge, again in unison, and resume their march:

 

 

But crabs cannot see a motionless watcher, and would happily march up to and over my feet. Extraordinary.

It is possible to herd these crabs by careful movement, like a sheepdog. Go too fast and they disappear, but walk slowly and they will move just like sheep. Here is Ali scattering crabs away from the waterline in front of her: