The boardwalk through the mangroves at Cullendulla Creek Nature Reserve, Batemans Bay, is well worth a visit. If you go walking there at low tide a spectacular little crab might catch your eye. Its back (carapace) is bright green and the ends of its claws are vivid orange-red. This is the Red-nippered Marsh Crab (Parasesarma erythrodactyla).

It lives in burrows or under rocks or bits of wood in the upper fringe of the mangroves or the wetter parts of salt marshes. This colourful crustacean is an adult male. The female's claws are smaller and her colouring is much less flamboyant.

Red-nippered Marsh Crabs emerge at low tide to eat. They play a very important ecological role in estuaries because they help convert mangrove leaves into food that other animals can digest. They and a few other crab species tear up fallen mangrove leaves and eat the plant material as well as the bacteria and fungi species that coat the leaves. The leaf fragments they discard break down more rapidly to form detritus (organic particles). Nearly all food webs in our estuaries are detritus-based so these crabs also help most of our common fish species to survive.

Burrows and wet refuges hold water when the surface is dry and are very important for these estuarine crabs. They are places to escape from predators such as birds when the tide is out, and from fish when it is in. The crabs can also drag their food to them and eat in safety.

When the tide is out, the Red-nippered Marsh Crab fills its 'lung-like' gill chambers with water from the burrow and pumps the water over its back. Fine lines of hairs in a grating-like area on each side wall of the carapace help oxygenate the water before it is returned to the gill chambers.

Burrows and refuges also help the male Red-nippered Marsh Crabs establish mating territories. After mating, the female holds the eggs under her abdomen, then migrates to nearer the low tide mark to spawn. The larvae hatch in lower salinity water and are transported by the currents down to saltier parts of the estuaries. The baby crabs go through several moults, changing form quite dramatically as they do so. Later stages stay near the bottom and gradually make their way back up stream. When they have reached their adult form they settle out in the habitats of their parents.