Yabbie crayfish  (Cherax destructor)

Classified as Vulnerable (VU - A1de) on the IUCN Red List 2003 (1).

This smooth-shelled crayfish usually varies in colour from olive-green to brown, but can also be blue, yellow, red or black depending on the habitat, location and individual (3). The head and internal organs of all crayfish are protected by the carapace and the six segments of the abdomen are individually encased with a flexible membrane between them to allow movement. Crayfish have a pair of large claws at the front end, followed by four pairs of walking legs and then four pairs of small swimming legs called swimmerets. These swimmerets are covered with fine hairs to which the female attaches her eggs. A central tail flap is surrounded by four other flaps that are used to move the crayfish rapidly through the water, as well as curling up to form a brood chamber. There are two eyes on the end of eyestalks, but the senses of touch and taste are far more important, and are perceived using a pair of large feelers (or antennae) and a pair of small, fine, centrally located feelers (or antennules) (3).

The yabbie crayfish is widely distributed throughout Australia, being present in most of Victoria and New South Wales, as well as southern Queensland, South Australia and parts of the Northern Territory (3).

This semi-aquatic freshwater crayfish is found in several habitats including low-lying swamp ground, streams, rivers and dams. It is dependent upon high oxygen levels in the water and ample vegetation. Muddy or silty-bottomed waterways provide murky water which provides some predator protection. Water temperatures of around 20 – 25 °C are ideal, but the yabbie crayfish can tolerate temperatures down to 1 °C and as high as 35 °C by entering partial hibernation (3).

Reproduction is brought about by increases in day-length and in water temperature, with mating begins in the spring once the water temperature has risen above 15 °C. Females will spawn twice or more each season, producing up to 1000 eggs per spawning when fully grown. The male yabbie positions a spermatophore between the female’s fourth and fifth pairs of walking legs, and the female breaks this open and fertilises her eggs with the contents. The small, green, oval eggs are then attached to the swimming legs where they take 19 – 40 days to hatch, depending on the water temperature (3).

The hatchlings grow through three larval stages, moulting between each. Young yabbies moult every few days, pumping water under the new, soft shell to make room for growth. Once fully grown, the yabbies moult just once or twice a year. Freshly moulted crayfish are exhausted and vulnerable to predation due to the lack of protective covering (3). They may also loose legs during the moult - these are usually regenerated (2).

The yabbie crayfish is omnivorous, feeding primarily on rotting vegetation, but is somewhat opportunistic, eating anything it comes across, including, on occasions, other yabbie crayfish. Cannibalism is not a normal state, however, occurring usually when there is insufficient natural food or when there are overcrowded conditions. They are nocturnal, being most active just after dusk and just before dawn. Predators include cormorants, herons, ibises, Murray cod, and Callop. Small, larval crayfish are also vulnerable to attack from other invertebrates (3).

Degradation of native vegetation and water pollution as a result of fertiliser and insecticide run-off from agricultural farms, as well as increase predation and competition from introduced non-native species put pressure on both the yabbie’s ecosystem and the yabbie itself (4).

The Australian Fisheries Management Act of 1994 designated the yabbie’s ecosystem as an Endangered Ecological Community, requiring vegetation management, run-off control and extensive surveying, and without continued conservation efforts is under threat of irreversible degradation (4).

For further information on this species see:

State of Victoria, Department of the Natural Resources and the Environment (September 2004):
http://www.growfish.com.au/Grow/Files/fn082.pdf

• Antennae: pair of sensory structures on the head of invertebrates.
• Carapace: the top shell of a turtle. In arthropods (insects, crabs etc), the fused head and thorax (the part of the body located near the head) also known as ‘cephalothorax’.
• Nocturnal: active at night.
• Omnivore: an organism that feeds on both plants and animals.
• Spermatophore: gelatinous jelly cone with a sperm cap deposited by a male during courtship and picked up by the cloacal lips of the female.

1. IUCN Red List (September, 2004)
   http://www.redlist.org
2. Shrimp, Crabs and Crayfish UK (September, 2004)
   http://www.shrimpcrabsandcrayfish.co.uk/Shrimp.htm?crayfishaustralia.html~mainFrame
3. State of Victoria, Department of the Natural Resources and the Environment (September, 2004)
   http://www.growfish.com.au/Grow/Files/fn082.pdf
4. New South Wales Fisheries Scientific Committee (September, 2004)
   http://www.fisheries.nsw.gov.au/fsc/recommend/Darling-FR22.pdf