Ancient creatures keeping water pure

Phreatoicids depend on the survival of their wetland habitats. Photo / NIWA

Ancient slater-like creatures that hitched a ride when New Zealand split from Gondwana have been found alive and well, and living in Christchurch's water supply.

Scientists feared ancient phreatoicids (pronounced free-at-o-ik-ids), described as like a cross between a slater and a sand flea, were extinct after they went missing from the scientific record for 60 years.

But a painstaking search has revealed all nine known species - and four new species - were living unnoticed in South Island pools, swamps and drains.

The two centimetre creatures played a major role in cleansing Canterbury's groundwater and keeping Christchurch's drinking water naturally pure, said Graham Fenwick, Niwa's assistant regional manager in Christchurch.

In fact, three species of the tiny, blind crustaceans - including the first phreatoicids ever discovered, 120 years ago - lived only in the groundwater 20m below the Canterbury topsoil.

Christchurch gets its drinking water from the ground, a source so pure it goes straight to homes without chlorination. "We like to think of the Canterbury plains as a big bio-filter, so any contaminants that go into the ground water from rivers or from the land's surface [are filtered by] a whole ecosystem down there," said Dr Fenwick.

He said phreatoicids were part of that cleansing system. "They feed on clay-sized particles and digest bacteria and other little organisms that might otherwise be harmful," he said.

"They are very useful to have around."

Phreatoicids came with New Zealand when it split from Australia about 18 million years ago.

Their future survival depends on New Zealand protecting its wetlands, as they could not live in rivers, said Dr Fenwick. "They need to have constant water but they don't like big flows. They like to be in a place where there is always going to be some water but they are not going to be washed away."

The animals have a very small natural range - one species lives only on tiny Ruapuke island in the middle of Foveaux Strait - and seem to like eating leaf mulch.

Department of Conservation workers and scientists from the National Institute of Water and Atmospheric Research started looking for them after noticing they had not been seen much since the 1940s.

The oldest record of a phreatoicid is a fossil 350 million years old, but Dr Fenwick said it was not surprising ordinary people did not often see them.

"They are pretty hard to spot because they live in really mucky environments right amongst mud and dead leaves, stones and dead wood."

Dissections will be carried out with microscopes and needles and the creatures compared to phreatoicids from Australia.

"If people find them we would love to see specimens," Dr Fenwick said.