Contaminants in the shells of clams, cockles and limpets indicate pollution levels in the seas and rivers in which they live, writes Iola Smith.

Traditionally, shellfish have not been used to monitor pollution because of the difficulty in obtaining crucial information from their shells. That situation could now change, thanks to an innovative shell analysis system developed by Aberystwyth University's Institute of Earth Studies.

Nick Pearce, a researcher at IES, said: "When the fish are exposed to pollution such as heavy metals, layers of these contaminants build up in their shells. We are able to cut through those layers by using a laser, which makes a 20 micron size hole in the shell. As each layer is evaporised by the laser beam, the particles are collected in a stream of argon gas and then passed through a mass spectrometer. As a result, the precise amount of each metallic contaminant can be measured."

Already, he has found high levels of lead in some clams picked up on the local seashore. Analysis reveals that these clams, which can live for between 40-80 years, picked up deposits from a nearby disused lead mine which exploded in 1969 and carried its toxic waste down-river to Cardigan Bay. Other shellfish found nearby contained a cocktail of contaminants including copper, zinc and cadmium.

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Cockles from the Mersey estuary were similarly affected and further traces of metallic pollution were discovered in shellfish from the Bristol Channel coast.

This summer, Aberystwyth students are surveying the shellfish in Anglesey to discover whether copper from the island's mine workings is seeping into the sea.

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Dr Pearce is sure that shellfish will have an important role to play in monitoring the marine environment: "You could put a cage of cockles in a river, leave them for a year and then return to find out exactly what has happened during the intervening 12 months."