Freshwater Crayfish

by John Walsby

14k Drawing of Koura by J. Walsby

Crayfish, or koura, are our best known freshwater animals apart from fish. We have two species in New Zealand. Most familiar and found throughout the North Island and north west of the South Island, is Paranephrops planifrons. A slightly larger and plumper P. zelandicus with hairy pincers, occurs along the eastern side of the South Island.

Crayfish are common in most rivers, streams lakes and swamps. They prefer soft sandy or muddy bottoms for making burrows, but along rocky streams will hide under stones. They are timid and shy away from bright light but you can find them during the daytime in pools that are densely overhung by vegetation that shuts out bright sunlight.

At night when the crayfish are most active, you can spot them with a torch by the reddish gold reflection of their eyes. They are inquisitive and will emerge from hiding to investigate small pieces of raw mince dropped into the water.

Normally the koura walks nimbly but warily over the bottom with its tail tucked underneath the body. But in times of danger, it spreads its broad tail fan and suddenly straightens the tail with an explosive muscular thrust that sends the animal shooting backwards to safety.

The large pincers for attack and defence, on the front walking legs, make the koura look more like overseas lobsters than local marine crayfish. The next two pairs of legs also have small pincers and the koura uses all three pairs for holding and tearing food apart. They are omnivores eating water weed, decaying fallen leaves and any small animals they can catch.

The large pincers crush small invertebrates which are then further torn and shredded by specialised appendages around the mouth. Inside the mouth is a crushing and grinding apparatus that mashes food into a digestible pulp. Their feeding helps to recycle the organic debris in our rivers, breaking down vegetation and dead animals into small particles which are suitable for complete bacterial decay after digestion by the crayfish.

Koura are important prey for eels and trout and also for other freshwater fish like bullies which snap up the juveniles.

Like many crustaceans, female koura carry their eggs under the abdomen attached to hairs on the swimmerettes. In this condition the females are said to be "in berry". Brown when first laid, they soon change to red and are carried around for protection for about four months before hatching. The juveniles stay with or close to the parent for another couple of months.

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