A sea snail, grazing on algae among the corals, smells a predatory cone shell. Wary of the cone shell’s venomous dart, the snail flicks its foot against the sand and jumps—yes, jumps—away. The snail’s quick escape works, but it may fail by the end of this century as oceans acidify.
Ocean acidification harms sea animals
Since the Industrial Revolution, oceans have been absorbing 25-30% of the CO2 released by fossil fuel burning. When gaseous CO2 dissolves into the oceans, it reacts with water to form carbonic acid (H2CO3) which acidifies the oceans. Ocean acidification harms marine animals like corals and molluscs—snails, oysters, mussels etc.—as it deprives them of calcium carbonate to build skeletons and shells: these animals stop growing, and may even slowly dissolve.
Animals behave weirdly in acidified waters too. When placed in elevated-CO2 seawater, coral reef fishes approach predator scents, hide less, lose their way home and cannot recognize food; hermit crabs move less and delay swapping small shells for bigger ones; sea snails avoid predator more. However, these observations used seawater of extremely high CO2 levels. What about CO2 levels expected in the near future—would animals be affected too?
Dr. Sue-Ann Watson of James Cook University and her team addressed that question. They placed jumping snails—Gibberulus gibberulus gibberulus, found in Australia and other tropical seas—for five days in either normal seawater or seawater treated with elevated CO2 levels expected by 2100, and then compared their responses to predators; elevated-CO2 snails performed badly.
Confronting a cone shell 2cm away, only half as many snails in elevated-CO2 seawater as those in normal seawater jumped away from a cone shell. Elevated-CO2 snails also reacted slower than normal snails and often jumped towards the predator–not a smart move.
Ocean acidification impairs the nervous system
Watson’s team tested three hypotheses to explain the snail’s retarded escape: elevated CO2 increases metabolism in the snails, distorts the smells from the predator, and disrupts the nervous systems in snails. After some experiments rejected the first two hypotheses, Watson probed deeper into the snail’s nervous system.
“Our previous studies of behavioral effects [in fishes] showed that the GABA neurotransmitter system was involved,” says Dr. Göran Nilsson of University of Oslo, co-author of this study. High CO2 might affect snails similarly too.
GABA (gamma-amino butyric acid) is a type of neurotransmitter—chemicals that transmit signals between neurons and other cells—found in all vertebrates and likely all invertebrates. GABA works like a key that opens specific locks—the GABA receptors, of which GABA-A is the most common—on the cell surface and allows chloride (Cl–) ions to enter the cell. Acidic environments however, may reverse the flow of chloride ions and disrupt GABA functions.
To test if the GABA-A receptors were implicated, Watson’s team used a synthetic chemical compound called gabazine that fits into the GABA-A receptors but does not open the locks. This would effectively reduce the number of available GABA-A receptors and flow of chloride ions from the cells.
Remarkably, adding gabazine to elevated-CO2 seawater restores the snail’s normal response to predators. Previously, gabazine has also restored normal behaviors in elevated-CO2 fishes. These results suggest that CO2 affects animal behavior by interfering with GABA-A receptors.
Why should we care?
Molluscs “are ecosystem engineers”, says Dr. Frédéric Gazeau who studies ocean acidification in the Mediterranean Sea. Mollusc shells provide habitats and protection for many aquatic organisms. Many molluscs are “filter-feeders” that “filter the particulates from the water column,” says Gazeau. “They decrease the amount of organic matter in the water and basically ‘clear’ it.” Alternations in mollusk behaviors would surely change marine ecosystems.
“The GABA-A receptor is the ‘brake’ in most neuronal circuits,” Nilsson emphasizes. “It is very important for every aspect of brain function.” Ocean acidification, by interfering with GABA-A receptors, could wreak havoc far beyond jumping snails and molluscs—every marine animal is susceptible.
Scientists get busy going forward
Although elevated-CO2 affects the behaviors of many marine animals and “may potentially alter marine ecosystems,” Watson adds it is too early to conclude on the future of jumping snails. Scientists must first examine the actual impact of elevated CO2 for predator-prey interactions in nature. Furthermore, individual snails vary in their responses to elevated CO2—some jumped faster than others, some farther away. Other studies reported similar variation among individuals which prompts Dr. Patricio Manriquez, who studies sea snails in Chile, to note that some individuals may already be well-adapted to future climate scenarios.
Ocean acidification progresses slowly, and its “effects we see now and in the coming decades will be there for centuries,” says Gazeau. As we learn to live with and mitigate ocean acidification, some animals—but not all—may adapt in time. No marine species however, will escape ocean acidification.
Watson, S., Lefevre, S., McCormick, M., Domenici, P., Nilsson, G., & Munday, P. (2013). Marine mollusc predator-escape behaviour altered by near-future carbon dioxide levels Proceedings of the Royal Society B: Biological Sciences, 281 (1774), 20132377-20132377 DOI: 10.1098/rspb.2013.2377
If you are feeling academic and interested in ocean acidification and its impact on our environment, you can’t do better than to read Philosophical Transaction B’s Ocean Acidification issue.