Monterey, Calif.—Mental problems at sea? Fish and mollusks could begin to have them—thanks to rising CO2 levels. Some of the resulting behaviors are odd, some compromising, and they reveal just how fundamentally carbon emissions are affecting our increasingly fragile Earth.

As humans emit more carbon dioxide into the atmosphere, more of the gas is absorbed by the oceans, gradually making the water more acidic. Numerous studies in recent years have documented how lower pH (higher acidity) can make it harder for shellfish and tiny organisms to form shells or internal skeletons and to reproduce. The acidity often forces the organisms to expend extra energy to counteract ill effects on their metabolisms as well. But now scientists are finding that lower pH can also mess with ocean animals' minds.

Small clown fish (yes, Nemo), for example, normally stay extremely close to the coral in which they spend their entire lives. But as the water becomes increasingly acidic—as in various recent experiments—they tend to wander farther and farther from home. This uncharacteristic "boldness" is not necessarily a good trait because the farther they swim, the more likely they are to get eaten by predators. Greater acidity also "impairs their ability to discriminate between the smell of kin and not, and of predators and not," according Philip Munday, a professor and research fellow at the Coral Reef Studies center at James Cook University in Australia, who conducted the experiments and presented results at a symposium here this week called The Ocean in a High-CO2 World.

Other species exhibit equally unusual behaviors. A snail known as Chilean abalone, which adheres to rocks along wave-swept shores, quickly rights and reattaches itself when it is dislodged, an important skill for avoiding predators. But when CO2 levels were raised by about 50 percent, some snails were slow to right themselves and others did not do so at all. "Their decision-making is delayed," said Patricio Manriquez, a researcher at the Southern University of Chile. Some snails took wrong turns in trying to avoid crab predators, and some even turned into the crab's claws instead of away from them.

In experiments done at the Monterey Bay Aquarium Research Institute here, hermit crabs living in highly acidic conditions did not show the increased boldness of Munday's clown fish, but they took far longer to withdraw into their shells than normal when they came under attack from a potential predator (in this case, a toy octopus).

Researchers are not sure what is causing the peculiar behaviors but Munday suspects that elevated CO2 levels interfere with a neurotransmitter called GABA, which plays a key role in modulating activity in the brain and nervous system of virtually all animals, including humans. In one experiment, Munday exposed reef fish to high CO2, which interfered with their sense of smell. He then administered a compound that helps to facilitate activity by receptors that sit on nerve cells and direct the cells' responses to GABA, and the abnormality was reversed. Because GABA is so ubiquitous, Munday fears that ocean acidification could cause sensory and behavioral problems for many sea creatures if global CO2 levels continue to rise.