The group of invertebrates known as crustaceans is a diverse group of animals. The name crustacea comes from their hard 'crusty' outer shell. It includes not only the familiar crabs seen on the shore but also the mud-dwelling crabs of the mangroves, rainforest crabs, giant and colourful freshwater crays of the Wet Tropics, barnacles, the delicious prawns we eat, the krill of the oceans consumed by the biggest whales and even the slater (or wood louse) - the humble garden creature that rolls up in a ball when you disturb it.
Crustaceans are part of the arthropod group - those animals with an exoskeleton and segmented legs. This group also includes the spiders and myriapods (centipedes and millipedes) and insects. Some crustaceans have great economic importance to the region such as the banana and king prawns or the huge mudcrabs (Scylla serrata).
Some brief facts and figures for all crustaceans are listed below:
The most spectacular crustacean of the Wet Tropics has to be the Mt. Lewis spiny crayfish (Euastacus fleckeri). It is the deepest royal blue in colour with brilliant red claw tips and spines and is most impressive with a body length of 30cm! This is one of only a few species in the same genus in the Wet Tropics and it is confined to clear, cool, mountain streams above 700 metres. These large crayfish grow very slowly so a large specimen is more than likely over ten years old.
The mudcrab or mangrove crab (Scylla serrata) grows to a considerable size – and is not to be confused with the small, varied crabs seen around holes in the mud (such as ghost crabs or fiddler crabs with one large colourful arm). The mangrove crab reaches over 2 kg in weight and bears stocky claws which have enough crushing power to break through the dense telescope whelk shells commonly seen in muddy areas - imagine what they could do to the fingers or toes of a careless crab seeker. These crabs are eagerly sought as a delicacy and rumour has it that they are one of the finest eating shellfish to be found anywhere.
Timid and noctural, the smooth crayfish (Cherax cairnsensis) lives in rocky, rainforest streams in both lowland and upland areas. This 7cm long cray is not a fussy eater, eating just about anything plant or animal that it can find.
Any visitor to idyllic beaches while the tide is out will notice the little balls of sand arranged around little holes below the high tide mark. A great place to see an entire beach covered in millions of these little sandy balls is Four Mile Beach at Port Douglas. As the tide retreats, thousands of little ghost crabs emerge from their homes and start their feeding. As the 'housemaids' of the beach, they feed off microscopic material in between the sand grains, cleaning the sand as they go. As the sand is stripped of any nutritious bits, the crabs gather it into a ball and toss it over their backs. There are patterns to their work - some species make a pile of the filtered sandy balls and others arrange them in rows which radiate out from their burrow.
Fiddler crabs are instantly recognisable for their one oversized and colourful arm which is used for communication purposes such as courtship displays and territorial warnings. The fiddler crab and other small crabs of the mangroves live in burrows in the mud and are easily seen congregating and shuffling about during low tide. They serve to clean up the leaf litter by dragging it down into their holes where the leaves can be eaten as well as any mould which grows on them in the moist environment.
Lacking much colour, but possessing long, spindly 'arms' is the long-clawed freshwater prawn (Macrobrachium lar). Their body length is 15cm and only the male has the extended claws. This prawn is found in clean, clear easterly flowing streams in high rainfall areas. Its juvenile develops in brackish water while the adult spends its life in freshwater. This species has traditionally been one of the many important food resources for Rainforest Aboriginal people.