The Light-blue Soldier Crab, Mictyris longicarpus, is tropical and is found as far south as the east-coast of Victoria. Large groups may erupt from sandbanks in a horde when the tide reaches a certain low level.

If disturbed, they burrow quickly in a cork-screw motion to cover themselves with sand. Soldier Crabs are the only crabs able to walk straight ahead. Why is this so?

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Dark-blue Soldier Crab, Mictyris platycheles

The Dark-blue Soldier Crab, Mictyris platycheles, ranges from Fraser Island in south-east Queensland through New South Wales to Victoria and Tasmania. It only forms feeding armies on southern shores.

Like its relation, large amounts of sand are processed to extract its micro-algae and detritus food. The processed sand is left as round pellets on the sand.

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References:

Davey, K. 1998. A Photographic Guide to Seashore Life of Australia.
New Holland press, Sydney.