Invertebrates - 'Crustaceans' Hard Shelled Creatures of the Wet Tropics

The group of invertebrates known as crustaceans is a diverse group of animals. It includes not only the familiar crabs seen at the shore but also the mud-dwelling crabs of the mangroves, 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 Leader (King) prawns which please our tastebuds or the huge mudcrabs (Scylla serrata) which delight the palates of those posessing a wallet which is equally well fed!

Invertebrates nature's recyclers and 'housemaids' - many of the crustaceans feeding mainly on detritus, rotting leaves and scavenging protein when they can in the form of dead fish or other animals. They perform a very necessary task in the environment which we should appreciate. There are exceptional species, though, that possess colours on the fantastic scale, such as the first crustacean profiled 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 30 cm (1 foot)! 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 (2,300 feet). These large crayfish grow very slowly so a large specimen is more than likely over ten years old.

 
Mudcrabs, Mangrove Crab

The mudcrub referred to here is a specific species (Scylla serrata) which grows to considerable size - not the small, varied crabs seen around holes in the mud (such as the Fiddler crabs with one large colourful arm). The Mangrove crab reaches over 2 kg (4.4 pounds) 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 - although Alaskan King crab and Maine Lobster fans might have something to say about that!

 
Ghost crabs

Any visitor to the area's 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 billions 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 '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 with some species making a pile of the filtered sandy balls and others arranging them in rows which radiate out from their burrow.

 
Long-clawed Freshwater prawn

Lacking the colour but possessing long, spindly 'arms' is the Long-clawed Freshwater Prawn (Macrobrachium lahs).

The body length is 15cm (6 inches) 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 resource for rainforest Aboriginal people.

 
Smooth Crayfish

Timid and noctural, the Smooth Crayfish (Cherax cairnsensis) lives in rocky, rainforest streams in both lowland and upland areas. This 7 cm long cray is not a fussy eater, taking just about anything plant or animal that it can find.

 
Fiddler Crabs

These crabs are instantly recognizable for their one oversized and colourful arm which is used for communication purposes (such as courtship displays and territorial warnings).

The Fiddler 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.

 
If you would like to learn more about crustaceans, visit the Museum of Victoria's Crab Gallery pages.