The Soldier Crab: A common inhabitant of the Intertidal Flats

Reflections

Have you ever walked along the sand when the tide is out and seen tiny piles of sand balls covering the sand bars? Or better still have you seen the battalions of blue soldier crabs making their way around the intertidal flats? Well they are quite a remarkable creature. They are the only crab that walks forward rather than sideways, they breathe air and bury themselves when the tide comes in so they don't end up being a snack for a passing by fish! As air breathers they have developed a unique way of staying underwater - they take their air with them!

The Soldier Crab

The Soldier Crab: Mictyris longicarpus

(Family Mictyridae)

Soldier crabs are an abundant grazing and burrowing species of the intertidal flats and estuaries in the western Indo Pacific Region.  In Australia they are found along the northern coasts from the NSW/Victorian border to southern WA.  They can be a conspicuous part of the intertidal flats with their habit of forming large aggregations or battalions of individuals on sandy tidal flats at low tide. Unlike many other crabs which move sideways, these crabs walk forwards and feed on the rich source of surface organic matter associated with the intertidal flats.

As the tide retreats they often commence feeding as individuals and leave behind distinctive sand pellets.  In some areas these pellets can cover the entire exposed intertidal area. As individual feeding efforts lessen, soldier crabs will often form small groups although sometimes they will merge into large aggregations containing many hundreds of individuals.   As the tide returns individuals burrow into the sand in a characteristic cork screw action. These crabs are air breathers and take their air with them by creating a small air pocket known as an “igloo” within their burrows. This can provide them with enough oxygen until the next low tide when they return to the surface.

The soldier crab finds its food within the top few centimetres of the intertidal flats. They can either feed just under the surface by creating horizontal tunnels (hummocks) or they can form aggregations on the surface and feed.   During low tide soldier crabs can spend up to 4 hours on the surface and can travel as far as 450 metres.  It has been suggested that the crabs move around to avoid depleting one area of food as well as to find suitably moist sand. This moisture is required as they feed by scooping up sediments into their buccal cavity and then adding water so that the lighter materials including organic material float on the water and can be ingested. The remaining sand and sediment is extruded as a pellet.

They can be more than 25 mm across their shell (carapace). Their diet includes fine organic particles including diatoms, algae, gastropod eggs and other minute organisms. 

Bioturbation: Burrowing and Feeding

Impact on sediments

Burrowing – sediment mixing, redistribution of organic matter, change in irrigation patterns and increase of oxygen supply to deeper sediments.

Feeding – intensive surface grazing decreases the populations of meiofauna, microphytobenthos and organic matter, all of which influence the biochemistry of the intertidal flat sediments.

Because soldier crabs can occur in large numbers they can have a significant impact on the ecology and geochemistry of the intertidal flats they inhabit.

Images

 

 
 
 
 
 
Video

This video shows soldier crabs on the intertidal sand flats at Brighton in Moreton Bay.

 

Corkscrew burrowing action of Soldier Crab.