Dog Whelks good indicators of TBT in seawater

By Jim Hurley - Nature Trail

Published 11/06/2016 | 00:00

Tributyl tin (TBT) has been known to cause infertility in female Dog Whelks.
Tributyl tin (TBT) has been known to cause infertility in female Dog Whelks.

The Dog Whelk is a very common sea snail found in tide pools on rocky seashores all around the coast of Ireland.

It is a meat-eater and feeds on barnacles, mussels and other marine life that it finds attached to the rocky seashores it lives on.

Since its prey is attached and can't escape, a hungry Dog Whelk leisurely bores into its victim's shell, injects digestive enzymes and, over time, sucks up the resultant soup of broken-down meat. Its diet is said to determine its shell colour.

Shell colour varies from white to black through a spectrum of greys, yellows, oranges and browns. And instead of being all one colour, shells are often decorated with bands of any combination of all of these colours.

Dog Whelks are not edible so they are of little interest to most people other than to children collecting their empty shells for their attractive colours.

However, during the 1970s and 80s scientists noted a decline in Dog Whelk populations. The cause of the decline was not immediately obvious. These sea snails breed in spring and those who are familiar with rocky seashores will know their yellow eggs about the size of grains of rice attached to rocks in groups.

Investigations revealed that females were not laying eggs. The reason for their inability to lay turned out to be caused by they each growing a penis that blocked their oviducts. Though mainly seen in Dog Whelks, the disorder, called imposex, was also found in other sea snails.

The cause of the disorder was believed to be due to marine pollutants. The culprit was discovered to be mainly a chemical called tributyl tin. Tributyl tin, popularly known as TBT, is found in anti-fouling paints used on boats and port structures. The discovery, together with fears for possible unknown impacts it might have on people, led to calls for the banning of that chemical.

A ban on the use of TBT in structures and vessels less than 25m long was introduced in Ireland in 1987. That resulted in an overall improvement except in the vicinity of fishing and shipping ports.

The most recent (2015) survey reported 'ongoing improvement' with levels of TBT falling significantly. Further improvement is expected as it is planned to eventually phase out TBT altogether.

Female Dog Whelks remain the most sensitive indicators of TBT in seawater and as such act as whistle blowers and guardians in the quest for clean seas around our shores.

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