A thinning of the protective cases of mussels, oysters, lobsters and crabs is likely to disrupt marine food chains by making the creatures more vulnerable to predators, which could reduce human sources of seafood.

"The results suggest that increased acidity is affecting the size and weight of shells and skeletons, and the trend is widespread across marine species," the British Antarctic Survey said in a statement of the findings.

Human emissions of greenhouse gases include carbon dioxide from burning fossil fuels, and some of that carbon dioxide ends up in the oceans, where it dissolves to form acid.

The ocean acidification makes it harder for creatures to extract calcium carbonate - vital to grow skeletons and shells - especially from chill waters in the Arctic Ocean and around Antarctica, according to the study in the journal Global Change Biology.

"Where it gets colder and the calcium carbonate is harder to get out of seawater, animals have thinner skeletons," said Professor Lloyd Peck of the Antarctic Survey.

So a shift towards acidification in the ocean was likely to force animals to have smaller skeletons, he said of the study by scientists in Britain, Australia and Singapore.

"We think that the polar regions, and especially Antarctica, are likely to be the first places where animals reach these critical problems for making skeletons."

Changes already happening in the chill waters were likely to be a sign of what to expect in future in temperate zones and the tropics, Peck said.

The experts studied four types of creatures - clams, sea snails, lamp shells and sea urchins - at 12 sites across the globe from the Arctic to the Antarctic.

"The fact that the same effect occurs consistently in all four types suggests the effect is widespread across marine species, and that increasing ocean acidification will progressively reduce availability of calcium carbonate," it said. In the past, animals had evolved to be able to live in places where calcium carbonate is relatively difficult to obtain - such as off Antarctica - by forming lighter skeletons, it said.

So there was hope they might be able to evolve again to adapt.

"Given enough time and a slow enough rate of change, evolution may again help these animals survive in our acidifying oceans," said Sue-Ann Watson, of James Cook University in Australia. - Reuters