Mictyris longicarpus, commonly called soldier crabs, "are about 1.5 cm wide. They have a small, round, blue body. They have long jointed legs with purple stripes. It gets its name from two characteristics; its habit of travelling in large numbers, and its ability to walk forwards instead of sideways. "This species travels in large numbers of crabs, which have been named 'armies', This type of crab is the sole member of its species that is able to walk forwards instead of sideways."
Soldier crabs live on the east coast of Australia, They can be found in estuaries, beaches, and intertidal mangroves."Although this species originated in eastern Australia, it also lives in other parts of the country. "The predominant areas stretch between Queensland to New South Wales and again from Victoria to Tasmania." Soldier crabs also live in Singapore, where one type is listed as threatened due to loss of natural beaches.
The soldier crab's breathing adaptations equip it for life both in and out of water. "This crab is well adapted for life out of water; it can absorb air through special parts of its legs which are thinner. It also absorbs water from the sand through silky hairs on the abdomen." Its tiny legs "have some decalcified areas in each which allow them to absorb tiny amounts of oxygen that aids in their respiration process."
Soldier crabs prefer sandy intertidal zones with gentle waves. They dislike sea grass, because it is hard to burrow into. "Soldier crabs are typically found near the mangrove communities, which provide adequate shelter and protection for them. The wet and muddy nature of the sands allows for these crabs to make a quick retreat into a newly formed burrow when threatened in any way....Since these crabs can not burrow down into the ground where sea grass is present, they tend to have a mutually exclusive relationship with seagrass."
To escape predators, the soldier crab burrows into the sediment. As a result of this, the sediment then benefits from increased oxygenization. "When threatened, these soldier crabs begin to burrow themselves into the sandy beaches in a sideways corkscrew-like motion. This motion is mostly in the counter-clockwise direction, but some have been seen to burrow in a clockwise direction. This burrowing into the sand increased the oxygenation within the sediments. This is known as bioturbation."
Soldier crabs scavenge beaches for dead bits of animal or plants, a main source of their diet called detritis. They themselves are prey for many animals. These crabs are shy and hide when they sense footsteps. However, you may be able to quietly observe them once you detect small sandballs they leave behind while eating. "The soldier crab eats the thin coating of detritis on sand grains, Sand grains are scraped up with the downward pointing pincers and brought to the mouthparts that then sift out any tiny food particles. The sifted sand is then discarded in a little ball."