A marine shellfish introduced to Tasmania from New Zealand about 70 years ago has now made its way to Sydney Harbour.

"It's a millenium marathon with uncertain environmental consequences," says CSIRO's Dr Nic Bax, who estimates the introduced species now covers an area of seafloor about the size of Tasmania.

The introduced New Zealand screw shell (Maoricolpus roseus) is a filter-feeding mollusc. It has become established in vast beds in northern Bass Strait and off the coasts of eastern Tasmania, Victoria and New South Wales.

Scientists are concerned at its impact on other mollusc species, including scallops and the native screw shells. Shellfish-eating fish species, and the food chain that depends on them, are also affected.

"There are very few predators that can break through the hard shell of the screw shell and eat it," says Dr Bax, an ecologist at CSIRO Marine Research.

Commonly found on beaches and shorelines, with an attractive spiral shell similar to native screw shells, it is one of an estimated 200 marine species intentionally or accidentally introduced to Australian waters since the start of trade and shipping.

Dr Bax said the New Zealand screw shell is a threat to other seafloor species because it breeds so prolifically, smothering the sea floor, down to eighty metres depth along the Continental Shelf.

It has not been reported, to date, on Tasmania's west coast or west of Bass Strait.

"We are only just starting to understand the habits of this species," says Dr Bax. "It is highly competitive with other species, and builds substantial beds to the detriment of other animals on the sea floor.

"The concern is not simply some economic impact down the track," he says.

"We don't know yet how far north it may travel, nor do we know how much it will alter the existing natural habitat and shut down the habitat of other species.

"We're not sure how it may have arrived here but shipping trade at that time revolved around apples and timber, and ships came loaded with dry ballast that consisted of stones gathered from New Zealand's shore."

Another likely source is the live oyster trade from NZ, which was very common at the time, Dr Bax says.

Dr Winston Ponder of the Australian Museum believes the mollusc is likely to displace similar related species of screw shells, several of which occupy the same depth range and sediment profile as the Maoricolpus.

Australia, he says, has the richest fauna of screw shells or turritellids in the world.

The place of introduced molluscs will be discussed during an international conference Molluscs 2000, in Sydney from December 4 - 8.

Researchers are obtaining anecdotal evidence from Tasmanian, Victorian and NSW fishers to build a profile of Maoricolpus roseus and its progress in Australian waters.

Contributing to the profile are researchers at CSIRO's Centre for Research on Introduced Marine Pests, the Tasmanian Museum and Art Gallery, Hobart, and the Australian Museum in Sydney.

High density of New Zealand screw shell, on the continental shelf east of Tasmania.

(The Malacological Society of Australasia web site, http://www.austmus.gov.au/malsoc/)