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Public Radio's Environmental News Magazine (follow us on Google News)

Sex & the Single Snail

Air Date: Week of

In recent decades, ways have been found to combine tin with organic molecules to create new compounds. One such chemical used to keep the bottoms of ships clean saves the world-wide shipping industry billions of dollars annually. But as producer Bob Carty reports from Halifax, Nova Scotia, there's evidence that such chemicals are killing dolphins and changing the sex of harbor snails.


CURWOOD: It's Living on Earth. I'm Steve Curwood.
For the past half century, industrialized societies have been conducting an experiment without thinking it through first and without controls. The experiment involves the creation and use of synthetic chemicals. The natural environment has since been permeated by them, and they will not go away. They persist and accumulate. One family of synthetic chemicals is made with tin. Ironically, tin was one of the first metals humans smelted and one of the first to cause toxic poisoning. But in recent decades, scientists have found ways to combine tin with organic molecules to create compounds that are highly profitable. One such chemical saves the worldwide shipping industry billions of dollars each year. But as producer Bob Carty reports, it's also having a bizarre and deadly effect on creatures that live in the sea.

(Fog horns and calling gulls)

CARTY: Point Pleasant Park at the entrance of Halifax Harbor is where bikers come to pedal between ocean surf and pine trees. Where kids come to throw around a frisbee, and where Nick Prouse comes to collect snails.

PROUSE: They're great. I love walking along the shore and seeing all these snails crawling around. It's just great. I'm the Snail Man.

CARTY: Nick Prouse is a biologist with Canada's Department of Oceans and Fisheries. He collects dog whelk snails: small, plentiful snails about the size of a silver dollar.

PROUSE: Dog whelks inhabit areas all around the coastline, so they're found in harbors, near boating activity, away from boating activity. They're everywhere, so you can use them as a good bio-indicator organism.

CARTY: And what Nick Prouse has found from collecting snails in dozens of harbors in the Maritimes is a testament to what humans have done to the oceans. He remembers the first day several years ago when he first took a very close look at the inside workings of dog whelk snails from Halifax waters.

PROUSE: And I went out to a site, a rocky shore site in Halifax Harbor and grabbed some snails and brought them back to the old lab. And saw that the females had small penises and I went, "Uh oh." That's not supposed to happen. Basically, I crack open the shell and then examine the soft tissues inside, and you can sex them very rapidly by looking at different factors. Looking at the gonad. And if it's female you look at the site behind a tentacle, a right tentacle. And if there's a little bud there it's possibly a penis, or in some cases it's a full-blown penis. Now you can measure, imposex by measuring the penis length and its relative size to males.

MACGUIRE: The phenomenon is called imposex: the imposition of male sexual characteristics on females.

CARTY: Jim MacGuire is a senior research scientist with Canada's National Water Research Institute. He has seen imposex not only in the oceans, but also in the freshwaters of the Great Lakes. The culprit, MacGuire explains, is a powerful hormone-changing chemical called tributyltin.

MACGUIRE: It causes those kinds of changes at concentrations as low as or less than 1 nanogram per liter. And that makes it in my book one of the most toxic chemicals that we've ever deliberately released to the environment.

CARTY: Tributyltin, or TBT, was created back in the 60s by mixing the metal tin with carbon molecules. It was designed for one specific purpose: to protect the hulls of ships from the algae and slime and barnacles that grow on them and that slow the ships down. TBT paint is a biocide: it kills everything it touches. To apply it, workers have be dressed like astronauts with air-fed respirators, full face masks, chemical-resistant gloves and clothes and footwear. Ironically, they call TBT an anti-fouling paint.

MACGUIRE: Historically, over the ages, people have tried different kinds of things to prevent fouling. Because, of course, they want to be able to keep up the top speed of their vessel and minimize the fuel consumption. So, various things have been tried like lead cladding, or pitch, or what have you. When TBT, as we call it, tributyltin, TBT compounds were developed as anti-fouling agents, what people didn't bank on was that such an effective and extremely toxic anti-foulant would likely be toxic to non-target organisms as well.

CARTY: TBT paint is formulated so that it leaches off the hulls of ships, slowly releasing toxic tributyltin into the water. Biologist Nick Prouse says it works so well that ships don't have to come into the dock yard to scrape off the barnacles for up to 7 years, instead of as little as 2 years.

PROUSE: Worldwide, use of tributyltin saves probably about $2 billion annually. That's a saving in ships' fuels and in ship maintenance. It's very, very effective.

CARTY: Too effective, it turns out. By the mid-80s, data started to come in about TBT wiping out oyster beds and causing deformities in clams. At the end of the 80s, the main shipping countries agreed to ban the use of TBT paints on pleasure craft and on ships less than 82 feet long. But big cargo ships and tankers can still use TBT. So can aluminum boats. And there's little control on illegal use. Now, it appears that TBT may be affecting more than the lowly snail.

(Newscast music. Male newscaster: "Scientists in the United States are trying to come up with the answer to a mystery: the death in the last 2 months of more than 200 dolphins all found on mid-Atlantic beaches. The scientists say that while bacteria or chemicals are doing the damage, what they can't understand yet is why the dolphins' immune systems have been so terribly weakened. Research is now going on to find out why...")

CARTY: Since the late-1980s, upwards of 800 dolphins a year have appeared dead on American beaches. At first, experts speculated that substances like dioxins, PCBs, or DDT were weakening the dolphins' immune systems. But one scientist decided he'd look for traces of tributyltin in the dead coastal dolphins. That scientist was Dr. Kurunthachalam Kannan of the National Food Safety and Toxicology Center at Michigan State University.

KANNAN: You know, we expected high concentrations in blubber, but we were really surprised to know that these compounds are accumulating in liver or kidney. So anything that is happening to these organs, it is going to directly affect animal. There's a good possibility for TBT to be one of the responsible contaminants for immune suppression in these dolphins.

CARTY: The cause and effect link between TBT and dolphin deaths is still not definitive. But Dr. Kannan is also finding tributyltin in dead sea otters on the California coast, and in birds living around the Great Lakes. In his lab, he has watched just one part per million of TBT cause cell death in human tissue. For dolphins, the problem seems to be that their digestive systems turn TBT into other forms of the compound: dibutyltin and monobutyltin. Those kinds of organo-tins are known to weaken immune systems, and those kinds of organo-tins are now turning up in human environments.

(A latch clicks; whooshing sounds)

FORSYTHE: This is a gas chromatograph. This is the kind of machine that we would use to detect organo-tins.

CARTY: Dr. Don Forsythe is an analytical chemist with Canada's Health Department. He's the government scientist who looks into how much humans are exposed to butyl-tins. Dr. Forsythe points out that butyltins are used in a number of products: in fungicides, disinfectants, wood preservatives, and in some polyvinylchlorinated plastics, or PVCs. And PVC plastic is a common material used in food containers.

FORSYTHE: We found that there were organo-tin compounds being leached into wines which were being transported in PVC transport containers. They took quick action to remove those particular types of transport containers from their well, from their transport fleets.

CARTY: Did you find any leaching in food?

FORSYTHE: We did in fact find levels of tributyl-, dibutyl- and monobutyl-tin present in some of the products that we sampled, primarily in oysters and clams.

CARTY: Health officials say that low levels of butyl-tins in human foods are not a known health risk. But there's not a lot of data to begin with. This is largely an unexplored area of scientific research, and the people working on it tend to be government scientists with shrinking budgets. Scientists like Don Forsythe, who worries that each year, billions of pounds of PVC plastics containing butyl-tins are used in piping for water supplies. Dr. Forsythe ran tests to see if any of the chemicals were leaching into drinking water.

FORSYTHE: It appeared that even after repetitive use of the pipe, you could still expect to get a certain amount of leaching of these materials from the pipe surface into the drinking water.

(Foghorns and calling gulls)

CARTY: Back in Halifax Harbor, biologist Nick Prouse would also like to see more research on butyltins. The partial restrictions on TBT paints have helped some recreational harbors. But in commercial harbors like Halifax, 100% of the female snails are still affected by imposex. As the females become sterile, the species is disappearing. Nick Prouse says the problem is not restricted to seaports.

PROUSE: I'm also finding in shipping lanes outside harbors in deep water, worldwide there's been about, over 100 species documented that are affected by tributyltin.

(Gulls call)

CARTY: The governments of the United States and Canada are working toward a worldwide ban on tributyltin anti-fouling paint by the year 2006. Japan has already banned the substance. And the US Navy has also turned to alternatives, more of which are being developed but none of which are as cost-effective as TBT. But then, the question is what are the real long-term costs of using TBT and its derivatives? We really don't know. The experiment continues. For Living on Earth, I'm Bob Carty in Halifax Harbor.



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