Soldier crabs (FAMILY MICTYRIDAE) are a unique group of crabs that live on tidal mudflats in sandy estuaries and mangroves. In Australia they are found from Western Australia around the top end of Australia to Wilson’s Promontory in Victoria. They are also found on islands of the western Pacific Ocean including New Caledonia and also South-East Asia.
They have several adaptations that make them different from most other crabs. Instead of flattened bodies their bodies are spherical and they walk forward instead of sideways. They emerge from their holes in the sand at low tide and move in large groups of dozens or even thousands of individuals. They move together in the same direction creating the illusion of moving sand. As they move they place sand into their mouths with their pincers eating any organic matter contained amongst the grains leaving behind small, round pellets of sand now devoid of organic material.
Being active at low tide gives them certain advantages over crabs that are active in the aquatic environment created by the high tide. The aquatic crabs suffer from the predations of fish like toadfish as well as shorebirds like herons, oystercatchers and whimbrels. The main predators of soldier crabs are birds. As the high tide returns the crab digs down into the sand with a corkscrew motion.
Adaptations to Amphibious Existence
Apart from obvious external structural differences there are some internal differences that enable them to be successful in their particular niche.
a)Within the thorax is a chamber containing the gills and the lungs. The gills occupy the lower part of the thoracic cavity. The upper part of the cavity contains a complex set of tiny tubes, which are highly vascularized, to allow efficient exchange of the gases carbon dioxide and oxygen. These crabs obtain about 90 % of their oxygen from their lungs. Access to air is facilitated by a hole in the forward part of the carapace (shell). This allows the animal to breathe both in and out of water.
b)With the onset of high tide they begin burrowing downward in the sand. As they dig the animals create a small breathing cavity around themselves which is four to five times as large as the crab itself. They maintain the cavity’s structure by placing sand above them like a plug. This plug prevents water from entering the cavity. The crabs burrow down to a depth of about 30 cm. At high tide they may be covered by as much as 2 metres of water in the Sydney region. When mating, the male and female will spend an intimate few hours in the cavity.
c) Another behavioural adaptation is the group association (flocking instinct). These large groups may provide a protective function in allowing confusion of predators by providing too much choice the way herd animals do in Africa. The hesitation created in the predator may give the whole group an opportunity to escape simultaneously by burrowing, thus improving the chances of survival.
ACKNOWLEDGEMENTS
Maitland, D. & Maitland, A. (1991) Penetration of Water into Blind-ended Capillary Tubes and its Bearing on the Functional Design of the Lungs of Soldier Crabs Mictyris longicarpus, School of Biological Science, UNSW
Australian Museum Online
Australian Government – Dept. of the Environment and Heritage