Sex And The Changing Female Snail

The lingering remnants of a banned chemical are causing female marine snails to grow penises in Puget Sound and the waters of British Columbia.

The startling hormonal change caused by tributyltin (TBT) at a concentration of a few parts per billion illustrates how drastically tiny amounts of powerful man-made chemicals can affect the environment, scientists say.

"It has this effect in very low concentrations," said University of Washington zoologist Alan Kohn.

TBT was banned as a bottom paint for boats in 1987 by the Environmental Protection Agency in the United States, but Kohn found that half the female snails his classes recently inspected at Alki Point were still developing male organs.

In a 1987 study by University of Victoria biologist Derek Ellis, all the female marine snails inspected in Elliott and Commencement bays and the Port Angeles breakwater had developed male organs, recalled Vallan Piccolo, environmental supervisor of the Puget Sound Water Quality Authority.

The change is called imposex.

Ellis subsequently found the same effect in southern British Columbia harbors up to the tip of Vancouver Island. He is urging Canada to fund research to see if the TBT ban that country adopted in 1989 is reducing the number of snail sex changes.

Canada allows TBT to be used on boats more than 80 feet in length, outboard motor shafts and aluminum boats.

The boat industry likes TBT for the same reason marine

biologists abhor it: The power of the chemical is extremely effective in killing organisms that try to attach to boat bottoms.

Ellis pointed out that in the worst instances, female ovaries can turn into testicles or the penis can block release of snail eggs, ending reproduction.

But Kohn said the Alki female snails with penises still appeared to be reproducing.

The TBT "acts by increasing the level of (the male hormone) testosterone," Kohn explained. "It has its effect only on females - it causes a masculinization of their hormones."

Harbor areas of both Washington and British Columbia have tested positive for the compound.

Why the tin compound prompts the change is not understood, he said, nor is it clear how the snails absorb the chemical. The snails, which grow to about 2 inches, feed on barnacles and mussels.

Other marine life does not appear to have the same bizarre reaction, Kohn said.

Sex changes are not unknown in the animal world. For example, land and sea slugs mate but both have the organs of both sexes and can play either the male or female role.

These snails normally do not have that ability, however.

Lee Harding of Environment Canada told The Associated Press that monitoring the snails has not been a high priority but said he will recommend the situation get more attention.

"From an ecological point of view, to have the gastropod (snail) population diminishing could be quite profound," he said. The snails help keep other species in check.