However, closer inspection reveals that all is far from well for some inhabitants of our rocky shores.
During one extra-low tide last summer, I clambered around the rocks of the Waitemata Harbour’s North Head looking for some of the less conspicuous residents: whelks. Whelks are rather sinister little marine snails which get their food by drilling holes through the shells of rock oysters, mussels and barnacles, and sucking the flesh out.
In the normal scheme of things they are a common sight on our rocky shores. For instance, at Leigh Marine Reserve on Northland’s east coast, hordes of oyster-borers (one of our commonest whelks) can be seen at low tide, clustering together in damp crevices and on the shady side of boulders to avoid the moisture-sapping heat of the sun.
Yet during my hour-long search at North Head, I found only a dozen or so oyster-borers, despite the plentiful supply of barnacles and succulent-looking mussels. Sharp-eyed shoreline watchers have noticed that all around our coastline, whelks seem to be disappearing in droves. The cause? A highly poisonous chemical called tributyltin, or TBT, which we introduce to our seas through marine antifouling paints.
TBT paints were first made in the mid-1960s, and immediately found favour with boat owners as powerful weapons in the relentless battle against barnacles. However, the very strength of the chemical has also led to a bizarre and alarming assortment of unpredicted side-effects on a variety of sea-dwellers. Creatures that inhabit the narrow margin of the sea between the high and low tide marks seem to be especially vulnerable.
TBT paints work by slowly dissolving in seawater and maintaining a poisonous layer around the hull of the boat. This protective envelope discourages barnacles from taking up residence. However, as TBT is only very slightly soluble in seawater, some of it will float up to the surface where it spreads out in a layer rather akin to an oil slick. As the tide goes out, o a thin film of TBT is left stranded, draped over the exposed surface and all its inhabitants.
In 1970, in the eastern United States, biologists noted that some American varieties of whelk were becoming masculinised that is, that females of the species were developing male sexual organs. This phenomenon, termed “imposex”, seemed to be most severe near marinas, and it was realised that some chemical agent originating from boats was the guilty party. Some chemical sleuthing showed TBT paints to be responsible.
Recently, detailed studies by chemists and zoologists at the University of Auckland have filled in many of the missing steps between the “masculinising” effect of TBT and the disappearance of whelks from our coastline. When TBT is absorbed by a female whelk, extra production of the male sex hormone testosterone is stimulated, and soon she begins to sprout an embryonic penis.
The next step is an infolding of a flap of her tissue to form a sperm duct, which eventually grows so large that it blocks the entrance to her capsule gland: the gland where fertilised eggs are encased with an impervious layer of calcium carbonate before they are ejected into the outside world. The female has become sterile.
At this point, her capsule gland may become so clogged with accumulated egg capsules that it bursts, the stress of which may kill her. Even if she avoids this fate, an equally dire one awaits her: her ovaries will switch from producing eggs to producing sperm, and she will become, in effect, a he.
These trans-sexual snails are a common feature of whelk populations around Auckland, City of Sails, home to an estimated 70,000 pleasure boats. A reef at West Tamaki Head, at the mouth of the Tamaki Estuary, has proved a useful study site, as a routine ecological description made there in 1962-63 provides a pre-TBT baseline for comparison. In those days, “numerous” oyster-borers-meaning they were too numerous to count-preyed on the rock oysters, barnacles and little black mussels of the reef. Furthermore, egg capsules were deposited every month or so over the two year study period.
A different scene awaited Michael Miller, of Auckland University’s Zoology Department, when he re-visited the reef in late 1991. Mussels, barnacles and rock oysters still cover the rock platform, but only a handful of oyster-borers could be found. These remaining individuals were all fairly old, judging from their thick shells and worn spires, and no newly hatched or juvenile animals or egg capsules were to be found. Evidently, breeding activity in this species had ground to a complete halt.
Miller’s suspicion that TBT-induced reproductive deformities were responsible was confirmed when he dissected some of these oyster-borers and observed that many of the females had regressed so far towards maleness that the only signs of their original sex were remnants of capsule gland trapped with in the sperm duct.
Scientists are astonished by the acute sensitivity of whelks to TBT. Concentrations of only one part per trillion are sufficient to induce masculinisation. This incredibly low concentration is about the same as one aspirin tablet dissolved in enough water to fill 300 Olympic-sized swimming pools! Whelks will respond to TBT concentrations that chemists are struggling to detect with a battery of expensive instruments.
Because of this sensitivity, our whelks are under siege on a big scale. Ministry of Agriculture and Fisheries scientists have studied whelks at 135 coastal North Island sites, and report that masculini-sation pervades populations in all areas of boating activity, including bays with only a handful of permanent moorings. However, at least our whelks are normal when we get to remote sites such as Leigh Marine reserve or Auckland’s West Coast beaches.
The prognosis is much more grim for the Northern hemisphere counterpart of the oyster-borer, the Atlantic dog-whelk, as every single dog-whelk population studied throughout Europe has been masculinised.
Government-led restrictions on TBT paints have gone some way towards stemming the flow of TBT into our coastal waters. However, TBT will continue to enter the sea from boats antifouled before the 1989 ban, ships antifouled in foreign ports, and, perhaps most significantly, from boat owners who have flouted the ban by laying in stockpiles of TBT paint, either before the ban or from overseas. One way or another, TBT is likely to be with us for sometime yet.
Perhaps the reluctance of boat owners to abandon TBT paints stems from a belief that the unlucky whelks may not have much importance in the overall scheme of things, and therefore be an acceptable price to pay. The problem with this argument is that we understand so little of the seas which surround us, and of the myriad lifeforms they sustain. We certainly didn’t manage to predict the side-effects of TBT; more likely, we stumbled across the more obvious ones on our doorstep. The question should haunt us: what other, unseen damage has been done?