Australian Soldier Crab (Mictyris Longicarpus)
They feed on detritus, any small organisms, such as diatoms, gastropod eggs or nematodes that are found in the sand, leaving rounded pellets of discarded sand behind them. They start to feed within fifteen minutes of emerging from their sand holes… feeding may last from one to two and a half hours, then they aggregate into armies, with the largest at the front.
Soldier crabs are mainly seen at low tide before the crabs dig into the sand to wait till the next tide. Much of their time is spend buried in the sand and only emerging to the surface a few hours before low tide, some stay submerged for the entire tidal cycle.
The number of crabs which can emerge at a time is influenced by temperature, wind and rainfall, with the different sexes responding differently. For example, one day, nearly all the males emerge for the day, then the next day, there will be a mixture of male and female crabs.
Upon emergence, the crab performs ‘the most aerobic grooming performance’… ‘In less than a second the crab falls onto its back, to remove any sand, then flips upright again in a half a somersault.’
They live in sandy estuaries, beaches and intertidal mangroves, where massive groups of crabs seem to emerge from nowhere all at the same time.
By M.A.Loveday (Lecturer and Guide on Wildlife/ Lover of Nature)
References Wikipedia & personal observations.