Some of these creatures are very important commercially and form the basis of large fishing enterprises. Prawns lobsters , crabs, crayfish, Balmain and Moreton Bay bugs, shrimp and scampi; all these mouthwatering morsels belong to this group. These are all large active crustaceans with big muscles that comprise the meat we love to eat. Decapods, however, are a very diverse group found in all aquatic environments at all depths and a few such as the Coconut crabs spend most of their lives on land. Feeding includes all strategies imaginable; suspension feeding, predation using large nippers, herbivory, scavenging, etc. There are small planktonic species , benthic species dredged up from the ocean depths and the giants of all crustaceans, the lobsters and spider crabs. Many have weird planktonic larvae that bear little resemblance to the adults. 1200 species have been described in Australia alone. Five or six species of prawns are fished or farmed commercially.

• To be a crustacean with 5 pairs of thoracic legs, ie. 10 footed or
• A first pair of legs that is usually specialised with claws or nippers which can sometimes be very large
• A distinct shield like cover (carapace) over the back and gill chamber
• Eyes that are usually prominent and on stalks.
• A muscular abdomen, ending in a flap or telson ,for swimming (as in prawns and lobsters ) or reduced to a tiny tucked in flap (in crabs).
• To have various larval stages includes a planktonic zooea with stalked eyes and 3 pairs of legs.

Lobsters, prawns or shrimps, crabs, spider crabs, hermit crabs, Balmain Bugs, Moreton Bay Bugs, crayfish, rock lobsters, scampi.

The bodies of decapods are extremely varied, but there are four general types: lobsters, prawns, crabs and hermit crabs. Many are very colourful while some edible forms such as prawns and rock lobsters assume bright colours when cooked!! Prawns are free swimming and are sometimes found in great shoals which yield tons of catch for trawlers. Lobsters and crayfish ( including freshwater yabbies) are powerful swimmers both forwards and backwards. They often live alone in crevices and cracks, but can be lured out with bait such as meat or fish. Crabs generally walk (sideways) but swimming crabs such as the Blue Swimmer have the last or 5th. pair of legs modified as paddles for weak swimming. Hermit crabs have soft unprotected abdomens which they hide inside shells discarded by gastropod molluscs. One group of crabs called sponge crabs camouflage themselves by placing small pieces of sponge or seaweed on the carapace which to look like a walking garden! Larval forms of decapods look very different from the adults. Crabs have a zooea larva often with hugely elongate head spines. Lobsters have a flattened leaf-like phyllosoma larva which lives in the plankton and is so different from adult lobsters that for many years it was thought to be a different creature entirely.

Decapods live in all seas at all depths. Many are live in freshwater or estuarine water and a few species are largely terrestrial, but they need to return to water for breeding. Some lobsters are known to migrate to deeper water at certain times of the year, in long lines with each lobster close on the tail of the one before it. Commensal crabs such as the pea crab found living inside edible mussels are common, but there do not seem to be any parasitic decapods. On coral reefs small female gall crabs called Haplocarcinus marsupialis live on a branch of coral which grows over the crab, enclosing it in a calcareous pouch. The male is much smaller and can enter the pouch to mate.

All decapods have numerous pairs of limbs modified as specialised mouthparts. Next time you are in a Chinese restaurant with a tank of crabs (often giant Tasmanian deepwater crab Pseudocarcinus gigas ) and lobsters, observe the constant movement of mouthparts and nippers. The presence of large nippers indicates a prey catcher (or larger food items) while prawns lack these and are filter feeders. Mud crabs and ghost shrimps are suspension feeders, driving a water current through their burrows and removing small particles from the sediment. Other species are scavengers, herbivores or detritus feeders, and are important in recycling organic matter.

Decapods are not just tasty items for us, many fish enjoy them too! Supply of green prawns for bait is an important commercial activity. Larger invertebrates such as octopuses enjoy crabs, leaving only an empty skeleton after the meal. All sorts of decapods living in the water column are preyed on by fish and squid. Schooling behaviour of prawns is probably a defence against every individual being eaten by such predators. Trawling by man is more thorough!!

Decapods have separate sexes which you can see. Males prawns have specially modified limbs or claspers to grasp the female during release of sperm. (It's an easy party trick to learn to sex prawns by the presence of the male petasma). Female crabs have a broader abdominal flap than males which is used to brood eggs. Most decapods copulate, mating couples apparently finding each other with the help of released chemicals such as pheromones. Fertilisation is external but many decapod females brood the eggs on their abdomen until hatching. A female brooding eggs is said to be "in berry", and should not be collected in this condition. Larvae hatch out as free swimmers in the plankton, usually called zooea, and go through several changes before turning into miniature adults. Rock lobsters, Australia's commercial crayfish, spend 18 months as phyllosoma larvae, then another year as a prawn-like puerula larvae. After 2 years tiny juveniles develops which take 5-6 years to reach maturity and mate. Information like this is essential to make regulations to protect commercial species from over exploitation and to try to develop aquaculture farms for suitable species. For many varieties this information on life histories is unknown.

Most decapods are free-living , a few are commensal, living in close associations with other species such as the coral gall crab. None are parasitic.

Decapod are favourite foods of man all around the world. In Australia, the western rock lobster Panulirus cygnus supports the largest single species fishery valued at $200 million in 1988-9. It operates out up and down the whole west coast of Western Australia, and exports live animals to Japan and mostly frozen tails to the U.S.. They are caught in baited traps called lobster pots. Similar species are caught in N.S.W. , S.A. and Vic.. Tropical rock lobsters including the Painted rock lobster are eaten from the reefs but will not enter traps so are caught by hand by divers. The prawn fishing industry in Australia is valued at $250 million. Many different species are trawled including Tiger prawns, Greasybacks, Banana prawns and some Coral prawns. Each has its own season and conditions. Some species are now being grown in aquaculture farms. Deepwater trawls off N.W. Western Australia have recently yielded good hauls of shrimps, a different species to the familiar prawns. These shrimps produce fewer eggs than prawns so there is a risk of over-fishing. Blue Swimmer crabs are a popular commercial species. These are caught in estuarine areas where they go to mate. The Mud or Mangrove Crab is also popular eating, being caught by drop net and pot. The giant southern or Tasmanian deepwater crab, Pseudocarcinus gigas, is caught in South Australia, Western Australia and Tasmania by cray-pots or trawling. It weighs up to 15 kg, one of the largest crabs in the world. It is very active underwater but on shore it is unable to lift its huge nippers due to their weight. It is said to be moderately common. Some human fatalities have occurred from eating coral reef crabs. This has not yet been recorded in Australia although the same species of crabs are known here. Freshwater crayfish are found all over Australia. They are variously called yabbies, marrons or gilgies. Some are now commercially farmed , sometimes introduced into farm dams as an alternative source of protein. The giants of this group are the Spiny freshwater crayfish such as the Murray River Crayfish but these have been seriously over-fished.

The Coconut or Robber Crab is found on islands across the Indian and Pacific oceans. It lives on land, breathing with small gills in a chamber lined with spongy tissue rich in blood vessels. They eat coconuts and have been observed tearing the husk from a coconut then carrying it up a tree and dropping it to the ground several times until it breaks, allowing the crab to tear into the flesh. They make good eating and can be caught by girdling a tree with grass so the descending crab thinks it has reached ground. The crab lets go, falling to the ground and stunning itself. They weigh up to 2.5 kg. and are immensely strong. These crabs must return to the sea to spawn where a typical zooea larva develops into a post larval form that migrates back to land. The ancient zodiacal sign Cancer is based on a crab.