THEY have formed a succulent and nutritious part of the human diet for thousands of years. But a new report is warning that plentiful supplies of oysters and mussels could disappear over the next century because the oceans are becoming increasingly acidic.
• Evidence: a heritage official checks native oysters in the sea loch at Linne Mhuirich north of Lochgilphead in the Taynish National Nature Reserve. Photograph: Donald MacLeod
Dr John Baxter, the co-editor of an international report into the acidification of the world's seas, said increasing levels of carbon dioxide being released into the atmosphere by industrialised countries was gradually changing the acid level of waters across the world.
If the trend continued, the shells of thousands of species would be eroded and the creatures eventually wiped out - creating a huge knock-on effect on other fish and marine life.
As lobsters and crabs have shells with a different chemical composition, it is not clear how they will be affected by increasingly acidic sea water, the report says.
But as well as shellfish eaten by humans, certain types of plankton at the bottom of the marine food chain and coral reefs would also face serious ecological damage.
The first major marine areas to be affected, says the report, are the northern oceans, which are home to a wide variety of important marine life.
The Arctic Ocean is expected to be the first to reach a dangerous level of acidification with 10 per cent of its area hitting the threshold at which damage will occur by the end of this decade.
Dr Baxter, the principal adviser in marine ecology for Scottish Natural Heritage, presented the report, Ocean Acidification: Questions Answered, at a scientific meeting last week. SNH co-funded the report, alongside Natural England, the UK Ocean Acidification Research Programme and the Prince Albert II of Monaco Foundation.
Baxter said: "The only way around this is for the amount of CO2 being released into the atmosphere to be reduced.
"To a certain extent, this can be done by carbon capture - when carbon released from power stations is trapped underneath the seabed and stopped from being released into the atmosphere - but, other than that, we just have to start using less fossil fuel. The alternative is pretty worrying."
He said that while some animals and plants may adapt to the new environment, many would be wiped out.
"We just don't know what species adapt, if any," Baxter said, adding that the last time there had been serious ocean acidification, 55 million years ago, around 80 per cent of sea species had become extinct.
While ocean acidification occurs naturally, studies show it has increased far more rapidly in the past 150 years.
More than 440 billion metric tons of CO2 has been released into the atmosphere since the end of the first industrial revolution in the 1830s - with half of this generated in the past 30 years.
As atmospheric CO2 increases, more of it is dissolved in the oceans, causing chemical changes which make sea water more acidic. While it is unlikely the world's seas will ever become positively acidic - like lemon juice or vinegar - even a fairly mild level of acidification would have a significant effect on the world's ecosystem.
"Carbonate levels in sea water, which are presently high enough to allow calcium carbonate structures like shells and skeletons to stay intact, can drop to levels that will mean these hard structures will begin to dissolve," the reports says.
"If levels of atmospheric and oceanic CO2 continue to rise at current rates… by 2100 it is likely that the entire Arctic Ocean will be in a state that can dissolve unprotected calcium carbonate structures."
Evidence had already been found that the shells of one species - the planktonic foraminifera - are already 30-35 per cent lighter than their pre-industrial-era counterparts, demonstrating that the effect has already begun.
Environmental groups said acidification was a growing danger to marine life. Richard Dixon, director of WWF Scotland, said: "Climate change would already be much worse if the oceans weren't absorbing huge amounts of the carbon dioxide we are spewing into the atmosphere, but the consequence is that the planet's seas are becoming more and more acidic. This is potentially disastrous for marine life around the globe, devastating much of the base of the food chain and further endangering fish-stocks that many human populations rely on.
"We spend a lot of time worrying about droughts, floods and ice caps, but what's happening to the oceans may actually be the most dangerous impact of climate change."
Duncan McLaren, chief executive of Friends of the Earth Scotland, added: "Ocean acidification threatens not only coral reefs and marine biodiversity, but also the commercial fisheries that depend on them."