Marine ecology: the incredible shrinking sea snail

For many marine biologists, concern over rising levels of carbon dioxide in the atmosphere focuses as much on ocean acidification as global warming. As seawater absorbs CO2 it becomes slightly more acidic, making life harder for creatures that need to produce shells from calcium carbonate.

Now an international study suggests that some species may survive acidification through miniaturisation. This phenomenon, the "Lilliput effect", has enabled some creatures with shells to survive huge rises in CO2 levels in the distant geological past. The researchers used places where CO2 seeps naturally from the Mediterranean seabed as test sites to find out how sea snails cope with acidification today. Their findings are published in the journal Nature Climate Change.

"Two species of snails growing at shallow water CO2 seeps were smaller than those found in normal [acidity] and adapted their metabolic rates to cope with the acidified seawater," says Vittorio Garilli of Paleosofia-APEMA in Sicily. "These physiological changes allowed the animals to maintain calcification and to partially repair shell dissolution."

The study showed that the snails living near the seeps were about a quarter of the size, measured by volume, of their counterparts living in normal Mediterranean conditions. They had adapted to the acidity over many generations, not only by dwarfing themselves but also by developing ways to lay down small new patches of calcium carbonate to repair places where acidity dissolved away some of the old shell.

"It is critical that we understand the mechanisms by which certain species survive chronic exposure to elevated CO2, since emissions of this gas are already having adverse effects on marine food webs and putting food security at risk," says co-author Jason Hall-Spencer of Plymouth University.

The fact that the two sea snail species in this study could withstand acidification should not give rise to any complacency. Though some shellfish and corals survived very high natural CO2 levels aeons ago, many more became extinct.

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