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Will shellfish shrink as oceans turn acidic?

U. MELBOURNE (AUS) / U. SOUTHAMPTON (UK) — Increasing ocean acidity is affecting the size and weight of shellfish—a trend that’s widespread across species.

These animals are an important food source for marine predators such as tropical seabirds and seals as well as being a valuable ingredient in human food production. Consequently, these changes are likely to affect humans and the ocean’s large animals.

UK scientists from the British Antarctic Survey (BAS) and the National Oceanography Centre (NOC) at the University of Southampton, together with colleagues from Australia’s James Cook University and Melbourne University and the National University of Singapore, investigated the natural variation in shell thickness and skeletal size in four types of marine creatures living in 12 different environments from the tropics to the polar regions.

Their aim was to get a clearer understanding of similarities and differences between species, and to make better predictions of how these animals might respond to increasing acidity in the oceans. Their findings are detailed in the journal Global Change Biology.

“This effect is strongest at low temperatures and the results showed polar species to have the smallest and lightest skeleton, suggesting that they may be more at risk in the coming decades as the oceans change,” says Lloyd Peck of the British Antarctic Survey.

“Interestingly, where ecology requires animals to have strong skeletons—for instance to protect them from impacts from floating ice in Antarctica—skeletons are made thicker and stronger,” Peck adds. “However, they still form a smaller part of the animal’s body mass, because the shape of the species changes to enclose much more body for a given amount of skeleton.

“Thus life finds a way, but still follows the overall trends of decreasing skeleton size in areas where the ocean chemistry makes it more difficult to obtain the necessary building blocks. If there is time for species to evolve in temperate and tropical regions it is one way they may be able to overcome some of the future effects of ocean acidification.”

The effort required by clams, sea snails, and other shellfish to extract calcium carbonate from seawater to build their shells and skeletons varies from place to place in the world’s oceans. A number of factors, including temperature and pressure, affect the availability of calcium carbonate for species that produce carbonate skeletons.

There is already evidence that ocean acidification is affecting the ability of some marine species to grow, especially during their early life stages, and there is mounting concern about whether or not these species can evolve or adapt to cope with increases in acidity in the coming decades.

This study shows, over evolutionary time, animals have adapted to living in environments where calcium carbonate is relatively difficult to obtain by forming lighter skeletons.

Carbon dioxide from fossil fuel combustion is altering seawater chemistry in the same way, in a process called ocean acidification, and this is making it harder for marine animals to make shells and skeletons.

“In areas of the world’s oceans where it is hardest for marine creatures to make their limestone shell or skeleton, shellfish and other animals have adapted to natural environments where seawater chemistry makes shell-building materials difficult to obtain,” says Sue-Ann Watson, formerly of the University of Southampton and British Antarctic Survey (now at James Cook University).

“Evolution has allowed shellfish to exist in these areas and, given enough time and a slow enough rate of change, evolution may again help these animals survive in our acidifying oceans,” adds Watson.

The four different types of marine animals examined were clams, sea snails, lampshells, and sea urchins. Scientists found that as the availability of calcium carbonate decreases skeletons get lighter and account for a smaller part of the animal’s weight.

The fact that same effect occurs consistently in all four types suggests the effect is widespread across marine species, and that increasing ocean acidification will progressively reduce the availability of calcium carbonate.

The Natural Environment Research Council funded the research.

More news from the University of Melbourne: http://newsroom.melbourne.edu/news
More news from the University of Southampton: www.southampton.ac.uk/mediacentre/

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