Bad Acid: Ocean’s pH drop threatens snail defense

A predicted worldwide fall in ocean alkalinity could have subtle effects on a small shoreline snail, shutting down one of its best defenses against crab predators, researchers say.

CRUSH THIS. Common periwinkles grow extrathick shells when crabs are nearby, unless the water becomes too acidic. Rundle

The surface waters of the world’s oceans are slightly alkaline. As human activity continues to add carbon dioxide to the atmosphere, however, increasing amounts of the greenhouse gas dissolve in the oceans, pushing seawater toward acidity.

The common periwinkle (Littorina littorea) normally grows a thicker shell when living among predators, says Simon Rundle of the University of Plymouth in England. In lab tests, he and his Plymouth colleagues found that a big increase in seawater acidity had little effect on periwinkles’ shells—except when a predatory crab was lurking. In acidified seawater, shells of threatened periwinkles failed to thicken, the scientists report online and in an upcoming Biology Letters.

“The big take-home message is that effects of ocean acidification can extend beyond the direct effects,” says Rundle.

The oceans’ surface-water pH now averages 8.2, says Scott Doney of the Woods Hole (Mass.) Oceanographic Institution. (A pH of 7 marks the line between acidity and alkalinity.) A variety of evidence suggests that ocean water has become more acid since the start of the Industrial Revolution, sinking in pH by about 0.1 unit. If atmospheric carbon dioxide continues to build up at current rates, ocean pH could drop another 0.3 to 0.4 unit by 2100, modelers predict.

As the acidity increases, sea creatures have greater difficulty producing calcium carbonate. This mineral is the stuff of coral reefs, seashells, shields on plankton, and even the teeth of sea urchins.

So far, most research on changes in ocean chemistry has focused on individual species, according to Doney. Rundle says the periwinkle work aims to explore interactions among species.

The common periwinkle creeps along much of Europe’s coastlines. They often fall prey to common shore crabs (Carcinus maenas), which grab them “like ice cream cones,” says Rundle. The frailest periwinkles get crushed and eaten.

Rundle and his colleagues grew more than 100 periwinkles in tanks. For half the snails, the team bubbled extra carbon dioxide through the water to mimic an acidic ocean with a pH of 6.45. That’s much lower than predicted levels for this century, but Rundle says it provided a starting point for a series of experiments.

In tanks with normal seawater and a crab at the bottom, the periwinkle shells thickened by an average of 0.05 millimeter—a substantial fraction of their total thickness. In the acidified tanks, however, shells didn’t thicken when a crab was present.

Victoria Fabry, a biological oceanographer at California State University, San Marcos, says that she welcomes the new research because it goes beyond earlier studies of calcification and “shows that there’s a link most likely with survival of the organism.”

One of Rundle’s collaborators, John Spicer, says that studies of crabs are already under way. Will lower ocean pH bring them periwinkle feasts? Not necessarily, he cautions, since calcium dynamics could affect predators as well. “Maybe the crabs will have weaker claws.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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