Euastacus

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The Euastacus or spiny cray genus is a group of fully aquatic crayfish species found in freshwater environments in the south east of the Australian mainland. The Euastacus genus is one of two genera of fully aquatic crayfish found in the south east of the Australian mainland, the other genus being the Cherax or smooth-shelled yabby genus. Both genera are members of the Parastacidae family, a family of freshwater crayfish restricted to the Southern Hemisphere.

Euastacus crays are distinguished from the smooth-shelled Cherax species by the short robust spikes on their claws and carapace, and frequently, their larger size. Many Euastacus species grow to a relatively large size, with the Murray River Crayfish (Euastacus armatus) being the second largest freshwater crayfish species in the world. (The largest freshwater crayfish in the world is Tasmanian giant freshwater crayfish (Astacopsis gouldi), found on the Australian island of Tasmania.)

The Cherax and Euastacus genera continues a trend present in many Australian native freshwater fish genera of speciation into generalist lowland and specialist upland species. Generally the Cherax or smooth-shelled yabby species inhabit lowland rivers at low to medium altitudes and swamps and ephemeral waters in inland areas of Australia including the Murray-Darling Basin. Conversely, the Euastacus or spiny cray species are only found in permanent waters and generally inhabit upland rivers at medium to high altitudes in the Murray-Darling Basin as well as many easterly and southerly flowing coastal river systems. The partial exception to this is the Murray River Crayfish which was originally found along the entire length of the Murray River in addition to many upland habitats, and is still found in the middle reaches of the Murray River.

The Cherax genus has a far wider distribution than the Euastacus genus, and is found in many parts of Australia including south-western Australia. The Euastacus genus is restricted to the south-east of the Australian mainland. There is a high degree of endemism in Euastacus species in coastal river systems, with many species restricted to single river or creek catchment. Euastacus species occur in several upland reservoirs.