Koura has a hard shell-like skin so it is a crustacean, one of the large group of animals, the arthropods, so-called because their legs and bodies are protected by a jointed hard but flexible body covering.
This covering eventually gets too small for the growing animal. It then splits and is left behind, while the new skin underneath hardens. This is called moulting.
There are two species of koura found only in New Zealand. In the east and south of the South Island and on Stewart Island they are larger (about 80mm long) and have very hairy pincers.
But koura in the North Island and in Marlborough, Nelson and the West Coast of the South Island are slightly smaller (about 70mm long) and have less hairy pincers.
It lives in fresh water such as streams, lakes and ponds, and even in swamps. Koura shelter between stones on gravelly bottoms but they can burrow into muddy bottoms, and will burrow well down into swamps that dry out over summer, to wait until the water returns.
Koura need oxygen like all living animals and plants which they get from the water through their gills, under the thorax where the legs join the body. Water is sucked in, pumped forwards over the gills and out through the mouth. If the gills get clogged they can back-pump to flush out any debris.
Koura use their four pairs of walking legs to get around quite quickly - they do not crawl as their common name, 'crawly', suggests. They also have a sort of reverse gear, to shoot back into shelter when alarmed, by flicking their tail forwards violently. Their first pair of legs, their pincers, are used mainly for catching food, for fighting with invaders, or for waving menacingly at intruders. They can give a painful nip, so be careful.
Koura is a scavenger that feeds on leftovers that float by in the water or settle on the bottom; old leaves, small insects are favourites. It does not go hunting for its food. Once food is caught in the pincers it is torn up, pushed into its mouth to be ground up and separated by a filter system that lets only fine pieces pass through to the small stomach to be digested.
Large trout love koura and so do shags.
Female koura produce eggs between April and December, and most in May and June. She carries the berry-like eggs, between 20 to 200, under the side flaps of her abdomen, when she is said to be 'in berry'. Small koura hatch about 3 to 4 months later, looking exactly like their parents in miniature. They cling to their mothers with their pincers until they are nearly 4mm long, around December of their first year. By their fourth year they are 20mm long and become adults.
Crayfish populations are decreasing in some areas as they are subject to habitat modification and land intensification.
Predation by introduced species has also played a role as has harvest for human consumption in some places. They are listed as a threatened species and their populations are in gradual decline.
You can help
Koura have a special part to play in our freshwater lakes and streams. They are native animals and recycle some of the leftover materials by their scavenging, therefore helping to clean up our streams and lakes in their own small way.
As they are so slow growing and many do not reach adulthood, there are never too many. It pays to take care of them and put the ones we catch back where we found them.
Fence off your waterways or ponds from stock and plant native vegetation along the edge of waters.