Freshwater Crayfish
by John 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.
