A freshwater crayfish from different parts of WA can vary widely in terms of appearance, with various crayfish sometimes being called a yabby by mistake. Taxonomists, who classify plants and animals, look for certain identification keys when examining freshwater crayfish. 
In the South West, a scientist's first step is to count the number of keels on a crayfish's head. Keels are the long ridges that run across the top of an animal's head. If four keels are found then a koonac or yabby species is likely. If the claws are broad and oval shaped, the animal could be one of two koonac species. If the claws are big and elongated, and there are obvious hairs inside the "elbow", then one of two yabby species is likely.
Where there are five keels on the head and a small spine is found on the long spike that sticks out from the head between the eyes, then it's likely a marron or gilgie. If there are no spines on the middle panel of the tail, one of two gilgie might be identified.
 Marrons have two small spines on the middle panel of the tail, with smooth marron featuring no hairs on their heads while the heads of Margaret River hairy marron are covered with short hairs.
SOURCE:   Department of Fisheries WA.
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