The Murray River crayfish (Eustacus armatus) is the second largest freshwater crayfish in the world, and the largest in the genus Eustacus. Large white claws and a particularly spiny abdomen distinguish the Murray River crayfish from other crayfish species. The hard plate, or carapace, just behind the head of the Murray River crayfish is dark to medium green or brown and can occasionally have a slight blue tinge. The segments of the abdomen also have a blue to green tinge and the abdominal spines are pale orange, cream or white. Smaller individuals may have green and yellow mottling on their claws rather then the characteristic white colouration
Like all crayfish, the Murray River crayfish has a tough exoskeleton for protection, as well as four pairs of walking legs and four pairs of small swimming legs called swimmerets. The tail of the Murray River crayfish acts as a directional aid and is also used for propulsion, enabling the Murray River crayfish to quickly propel itself backwards to evade predators.
The Murray River crayfish has eyes on the ends of stalks that project away from its head. In addition, the Murray River crayfish also has a pair of antennae, one of which is long and whip-like.
There is little obvious difference between male and female Murray River crayfish, but when directly compared, the female has a wider abdomen.
The diet of the Murray River crayfish is typical of many crayfish eating small aquatic species including insect larvae, water snails and tadpoles, as well as plant material including roots. The Murray River crayfish typically waits in the opening of its burrow in the river bank with antennae protruding to detect passing prey. Prey is grasped using its long, thick pincers and is dragged back into the burrow to be eaten.
The Murray River crayfish reaches sexual maturity between five and nine years old. In late May, the female Murray River crayfish lays between 200 and 1,200 eggs which attach to the fine hairs on the swimmerets. The eggs hatch in October, and the first few days of the juveniles life are spent under the protection of the females body. The young crayfish are then left to fend for themselves, remaining hidden in the riverbed until November or December. The Murray River crayfish will moult ten times in its first year.
Like other crustaceans, the Murray River crayfish can detect chemical cues released from injured crayfish or other species, or from predators, alerting them to potential danger.