Acidic oceans make it hard for snails to escape predators

A black turban snail grazing on algae. (Credit: Brittany Jellison/UC Davis)

Ocean acidification makes it harder for sea snails to escape from their sea star predators.

The findings suggest that by disturbing predator-prey interactions, ocean acidification could have ongoing consequences for food web systems in shoreline ecosystems.

sea star and snail
A sea star and its food, the black turban snail, in a tidepool near the Bodega Marine Lab. (Credit: Brittany Jellison/UC Davis)

For instance, black turban snails graze on algae. If more snails are eaten by predators, algae densities could increase.

“Ocean acidification can affect individual marine organisms along the Pacific coast, by changing the chemistry of the seawater,” says lead author Brittany Jellison, a PhD student studying marine ecology at the University of California, Davis, Bodega Marine Laboratory. “But it can also alter how species interact, such as by impairing the ability of prey to avoid predators.”

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For the study, published in the Proceedings of the Royal Society B, scientists collected ochre sea stars and black turban snails—two common species along the Pacific coastline—from tide pools on the Bodega Marine Reserve. In lab tanks, they explored interactions between the sea stars and snails under 16 different levels of seawater pH, or acidity, ranging from present levels to those expected for rocky intertidal pools by the year 2100.

The scientists found that lower pH levels, which indicate higher acidity, did not slow the snails’ movements or reduce their ability to sense the predatory sea stars. However, the more acidic waters did impair the snails’ escape response.

Usually, when a black turban snail senses an ochre sea star, it quickly crawls up and out of the tide pool to avoid it, as sea stars rarely leave the water to eat. But when pH levels fell to 7.1 or below, the snails failed to fully implement their escape response. Neither did the snails recover their escape response when the water’s acidity fluctuated between normal and more acidic levels.

Pacific Ocean is an early warning of what’s to come

The pH levels that spur these behavioral changes already occur in tide pools and are expected to become more frequent in coming decades, the scientists say, adding more research is needed to understand why the snails show a degraded escape response, or if they may adapt to more acidic ocean conditions in the future.

One-third of carbon dioxide emitted by humans enters the oceans, making seawater more acidic. Rocky tide pools, that experience pH levels predicted for the open ocean in the ocean, could be used as an indicator for future ocean conditions. Current models project a 0.3-0.4 drop in the global average of ocean pH by 2100.

“Dozens of West Coast species display escape responses to sea stars,” says senior author Brian Gaylord, professor of evolution and ecology. “We don’t yet know the extent to which ocean acidification could alter these additional predator-prey interactions, but there is clear potential for broader disruption of links within shoreline food webs.”

The National Science Foundation, a California Sea Grant, and UC Davis Bodega Marine Laboratory funded the work.

Source: UC Davis