Museum Notebook: Elusive koura wai Māori need all protection, fierceness they can muster

A koura wai Māori exoskeleton with its tail folded underneath its body. Whanganui Regional Museum Collection 1802.8278

In the lower storage area of Whanganui Regional Museum, in a plain brown box labelled Crustaceans, are the remains of a small but ferocious-looking creature.

Black body-armour. Bulging eyes. Two spiky weapons with blade-like pincers out the front. Four legs on each side for rapid movement. A tail that can be tucked away underneath.

It is the exoskeleton of a koura wai Māori, a native freshwater crayfish. There are four koura in the box. They are fragile and some legs and antennae have broken off.

Live koura can regrow a leg if they lose one. As they grow, they moult their exoskeleton and the new and perfectly formed, but temporarily vulnerable, koura emerges, with its new shell gradually hardening into protective armour.

These elusive little critters need all the protection and fierceness they can muster. Introduced wildlife such as rats, stoats, hedgehogs and trout will devour them. Eels and shags gobble them up. Kiwi sometimes use their long beaks to fish for them. Generations of humans have tracked koura down in their quiet, watery hideaways and scooped them out, boiling them alive for a tasty snack.

Being nocturnal, very well camouflaged and usually well hidden under logs, branches or other vegetation, koura are quite hard to spot unless they move. When disturbed, they use a quick flick of their tail to shoot backwards out of danger. They can also defend themselves with their waving spiky pincers which can inflict a painful nip.

A koura wai Māori exoskeleton with its tail showing. Whanganui Regional Museum Collection 1800.460

Australia has more than 140 species of freshwater crayfish. Aotearoa has just two: Paranephrops planifrons, widespread throughout the North Island and north and western South Island, and Paranephrops zealandicus, in the east and southern South Island. The southern species is declining and at risk. Neither is protected. Apart from the Taupō and Rotorua Lakes regions, where koura harvest is restricted to local iwi, and in conservation areas such as national parks, it is still legal to catch and eat koura.

Compared to some Australian species such as Cherax cainii, the smooth marron, which can reach a hefty 2.2kg, koura are small and very slow-growing. Baby koura spend most of their first year clinging onto the underside of their mothers. When they are just four millimetres long, they drop off and begin looking after themselves. It takes four years for a koura to reach two centimetres - just half a centimetre per year. At that point they are adults and can begin breeding.

A watercolour painting of a koura wai Māori by John Tiffin Stewart, 19th century. Whanganui Regional Museum Collection 1805.83.8

Koura growth rates depend on environmental conditions such as temperature and availability of food. As omnivorous scavengers, they will eat anything that drifts into their territory, including their own juveniles. Koura prefer cool, pristine water, so riparian planting to shade the water and keep silt out of streams, and controlling water pollution, are important in helping these fascinating little crustaceans survive and flourish again in our waterways.

• Margaret Beautrais is the educator at Whanganui Regional Museum.