Crayfish are heavy bodied crustaceans with an enlarged, pincer-like first pair of legs and are known by many common names depending on the area they are found. In New South Wales and Victoria they are called yabbies, in Western Australia the Koonac, Gigly and the Marron, while Queensland has the Redclaw.

What are Crayfish?

Crayfish belong to a group of animals called Crustaceans and are part of the phylum Arthropoda. Other Arthropods are insects (Insecta), spiders (Arachnida), and centipedes and millipedes (Myriapoda). All Arthropods have a hardened outer shell, called cuticle (made from calcium carbonate) that acts as a skeleton. Crustaceans are distinguished from the other arthropods by their two pair of antennae - an outer pair called antennae, and an inner pair called antennules. Crayfish bodies are divided into three parts; the head, the thorax (the section that contains the legs), and the abdomen (or the tail, which is the edible part of many large crustaceans including crayfish, prawns and lobsters). Crayfish, like many crustacean have a carapace or shield which projects backwards from the head and covers all of the thorax The carapace has two functions; firstly it protects the delicate feather-like gills which branch off from the base of the legs, and secondly it provides a water channel that is a constant flow of oxygenated water to pass over the gills and enables the crayfish to breathe.

Family Heritage and the Relatives

All freshwater crayfish in Australia belong to the Family Parastacidae.

Australia's crayfish fauna are divided into nine genera (or groups of species) which include over 100 species. The three most common and widespread genera are Cherax, Euastacus and Astacopsis. These are all medium to large crayfish that are found in streams, lakes and swamps. Cherax species are the best known crayfish and occur over the widest range, from northern, eastern and south-western Australia and are distinguished by having smooth bodies and claws. Cherax are also the main species used in the rapidly growing aquaculture industry.

They have a relatively rapid rate of growth and greater tolerances of larger temperature ranges and water conditions than many other crayfish. The three main species used in yabby farming and found in pet shops in Australia are the Dam Yabby Cherax destructor, the Marron Cherax tenuimanus from Western Australia, and the Queensland Red Claw Cherax quadricarinatus.

The Yabby

The yabby (Cherax destructor) has the largest range of all Australian crayfish. It occurs across most of Victoria, western New South Wales, south-western Queensland and eastern South Australia. They are probably the most hardy crayfish. They can tolerate poor water conditions and long periods of drought by burrowing deep into the river bed or dam wall in order to stay moist. This species was named destructor because of the damage its burrowing caused in dam walls and levee banks. Yabbies range in colour from brown, green to pale blue with mottled claws.

The Queensland Red Claw is characterised, as its name suggests, by a bright red patch on the outside of the large claw. The Marron is a dark brown or black or sometimes a bright blue colour. The striking blue form of the Marron is being selectively bred and are becoming increasingly popular as pets.

Other Crayfish

The second most widely distributed genus is Euastacus. This genus occurs from north Queensland throughout eastern and southern New South Wales, most of Victoria and southern South Australia. These crayfish are commonly referred to as freshwater lobsters, spiny lobsters or spiny crayfish because their bodies are covered in large spines, particularly on the tail and the claws. Probably the most heavily armoured and widespread Euastacus species is Euastacus armatus or the Murray River Crayfish. Its range includes central and southern New South Wales, northern Victoria. This species has a green to green-brown body and large white spines on the tail and white claws. Euastacus species generally prefer well oxygenated water and are often found in cooler, faster flowing mountain streams and rivers. The different species come in an array of colours from the white claws and green body of Euastacus armatus, the blue and white Lamington Plateau Crayfish Euastacus sulcatus to Euastacus suttoni with dark red, green or black body and red or orange claws, to Euastacus spinifer the Large Sydney Crayfish with a dark green body, red tipped spines on the abdomen and blue tinged claws.

The third genus Astacopsis is found only in Tasmania and includes Astacopsis gouldi or the Giant Tasmanian Crayfish and is not only the largest crayfish in the world but is also believed to be the largest freshwater crustacean. Although large animals are now rare, specimens have been recorded to weigh in excess of 6 kg, and measuring over 400 mm in length with claws longer than 150 mm.

The remaining genera contain small species which have relatively restricted distributions. An interesting genus found only in Queensland is Tenuibranchiurus. This genus includes the world's smallest crayfish, Tenuibranchiurus glypticus, which does not exceed 30 mm in length.

Crayfish Habits

Crayfish, can be found in a variety of freshwater habitats such as rivers, streams, dams, lakes and swamps. They are principally vegetarians and are generally most active at night (nocturnal) where they spend their time foraging on the stream bed, eating water weeds and the decaying roots and leaves. Crayfish are, however, opportunistic omnivores, which means they can eat almost anything including meat.

All crayfish have the ability to burrow. Some genera are more adept at burrowing than others. The genus Engaeus or Land Crayfish has become so specialised at burrowing that the animals spend their whole life cycle in a burrow and have adapted their bodies to live in an enclosed space. These specialisations include a reduced tail, eyes and antennae and reduced body size, usually not exceeding 45 mm in length. The burrows can be as long as 10 m in order to reach the water table and are often recognised by mud chimneys at the entrance to the burrow. These chimneys can range from a few centimetres to 40 cm in height.

Keeping Crayfish

Crayfish make interesting and industrious pets and can be purchased from most pet shops. The only species sold as pets are the Dam Yabby, the Marron, and the Red Claw. These are hardy species that require little maintenance and will live from two-five years. To set up an aquarium for crayfish you need a good aeration system, 3-5 cm of sand, some small pebbles and some large hollow or cavernous rocks which the crayfish can hide in during the day, and plenty of long waterweeds.

Points to Consider When Setting up your Aquarium

Keep the water clean by changing it regularly (every two to three months) and use the recommended water conditioners each time the water is changed.

Do not allow uneaten food to stay in the aquarium for more than a couple of hours, as it will quickly turn the water bad.

Use a good quality aerator and filter. Good aeration is vital to sustaining your crayfish. Under-gravel filters, however, are not recommended for crayfish because their burrowing usually uncovers the filter and reduces its efficiency.

Do not over-stock your tank because crayfish are very territorial and will kill or damage each other if placed in too small a tank. Each animal requires about 30-40 cm area with places to hide from the other crayfish.

Crayfish like all arthropods shed or moult their outer skin in order to grow. This happens every three to four weeks in small crayfish (4-5 cm). The length of time between each moult increases as the animal get older to once a year in full grown animals. Most Cherax species take about one and half years to reach maturity (15-20 cm). During the moulting phase the crayfish will stop eating and reduce activity, until on the day of the moult it will appear motionless. Crayfish moult their shell, (ecdysis), by splitting their tail along the back and then flicking the old shell off. The head and claws are removed last Once the shell is removed, crayfish are very soft and will hide until they have expanded into their new shell and the shell has hardened. It is advisable to leave the old shell in the tank as the crayfish will eat it as a source of calcium. Adding a small amount of calcium carbonate to the water will help make a stronger shell.

Feeding your Crayfish

Crayfish are principally vegetarian and will survive well on a diet of waterweed and almost any thinly cut vegetables such as pumpkin, potato, celery (including leaves), and apples and other fruits. You can also give them small amounts of meat or fish but it is not necessary. The important point to remember is not to overfeed crayfish. Crayfish only have a small stomach and will stop eating when full, leaving the remainder of the food. They do not need to be fed every day. A good feeding schedule would be one to two small pieces of food every two to three days. Remember to remove any left-over food after two hours.

References

• Carrol, P. N. (ed). 1980. A yabbie pot pourri. Hawkesbury Agricultural College, Richmond.
• Jones, D. & Morgan, G. 1994. A field guide to Crustaceans of Australian Waters, Reed (William Heinemann Australia), Sydney.
• Merrick, R. 1993. Freshwater Crayfishes of New South Wales. Linnean Society of New South Wales, Sydney.
• Williams, W.D. 1980. Australian Freshwater Life. The Invertebrates of Australian Inland Waters. Macmillan Company of Australia, Melbourne.