Every day in our estuaries the armies are on the move.  Soldier crabs, so called because of their habit of moving together in huge numbers, and because of their blue “uniforms”, patrol the muddy sand flats searching for food.

We probably all have noticed these very common crabs as they quietly click their way across the wet sand or spiral their way to safety beneath the surface.  But they are worth a closer look.

Two species are found in our area, often mixed in the same armies.  Both have almost spherical bodies and long pale legs which can be used to move the animal forward, unlike most crabs which move sideways.  One species (Mictyris longicarpus), found in temperate and tropical areas, has darker purplish joints to its legs and a paler blue body.  The other (Mictyris platycheles) has a darker body, does not grow as large, has prominent purple side-bulges towards the rear, and is more common in southern parts of Australia.

Soldier crabs spend most of their time in burrows just below the surface.  When the tide has receded to a certain level and the sand has reached a suitable sloshy consistency they emerge and move slowly towards the water, feeding as they go.  Sometimes the crabs will make horizontal feeding burrows instead of coming to the surface.

Like all crustaceans, crabs have three pairs of mouthparts.  In soldier crabs the third pair is very wide and broad and hides the others.  The crabs use their flattened claws like scoops to lift the wet sand to their mouth parts where the detritus particles and unicellular algae left by the tide are filtered out and passed to the stomach.  The depleted sand is shaped into balls and left behind.

The soldier crabs are constantly on the lookout for predators.  Their compound eyes are convex giving wide-angle vision which can be more than 180 degrees.  The total image is crude but the crab easily detects movement, including pursuing children.  No doubt there is also safety in numbers with more eyes to keep watch.

When the tide starts to return the crabs move to slightly higher ground and corkscrew their way below the surface where they will be safer from the returning fish.  They dig down with the legs on one side while walking backwards with the legs on the other.  This disappearing act can be done very quickly if the animal is threatened.