Divers waiting to decompress during extreme dives into the one of the world's deepest freshwater caves near Nelson unexpectedly discovered three new freshwater species living in the dark depths.
During the 2010-11 expedition of the Pearce Resurgence by a group of Australian divers, a number of stygofauna – which literally means animals from the River Styx – were brought back to the surface and given to National Institute of Water and Atmospheric Research expert Graham Fenwick.
The divers captured the transparent amphipod, a worm, and a small snail during staged decompression stops on the way up from exploratory deep dives.
Dr Fenwick said the discoveries led to another dive the following year, with National Geographic sponsorship for further research.
No other examples of biodiversity were found but the recent confirmation of the new species showed that animals did live in the dark underwater desert beneath Mt Arthur, he said.
"Their habitat is a lot like a desert in that there are no plants growing there and plants provide food. They survive on dissolved and fine particles of matter which are carried into the resurgence."
Dr Fenwick said the animals helped clean the water of organic matter.
"Here in this extreme environment natural cleanup processes are in place."
He said the find was quite unexpected and an important marker of the water's health.
"The first thing I knew about the Pearse was when these guys turned up with the animals."
However, while the discoveries were of new species to New Zealand, the existence of similar animals living at extreme freshwater depths was not uncommon globally.
There are 16 diveable caves in the world deeper than the Pearse Resurgence. Only 10 of these have been dived to depths greater than the current depth of 221 metres explored in the Pearse last summer. Such aquifer studies are yielding rich troves of biodiversity worldwide.
"It's important to do an inventory of life in New Zealand, and in this case it's a pretty special type of environment, and we don't have many limestone karst systems that are readily explored," Dr Fenwick said.
One undescribed (new to science) species of amphipod crustacean, completely colourless, dominated the stygofauna collected from the Pearse Resurgence. "It is six to eight millimetres long. The divers could see it crawling over rocks; it really is a beautiful animal," Dr Fenwick said.
The two other invertebrates discovered were a minute gastropod snail (about 1.5mm in diameter) and an oligochaete worm (about 8mm long). Both were taken from rare deposits of fine sandy sediments within the main shaft at depths of 15 to 34 metres.
A dye experiment indicated the cave system was more than 1000m long.