Even small changes in ocean acidity dramatically reduce the sea bass’s ability to smell, a new study shows, making it much harder to sniff out both food and danger.
Rising levels of carbon dioxide lead to more acidic oceans—the extra dissolved CO2 in the water creates carbonic acid. A team led by Cosima Porteus at the University of Exeter studied the behavior of European sea bass (Dicentrarchus labrax) in water with two-and-a-half times the present-day level of CO2—the level experts predict for the end of this century. They also tracked the fish’s nerve responses. Even though the rise in CO2 is predicted to decrease the ocean pH by just 0.3 to 0.4 pH units, the effects are clear: In the more acidic water, the sea bass had to be 42% closer to a smelly object to detect it (Nat. Clim. Change 2018, DOI: 10.1038/s41558-018-0224-8).
The fish use smell to find food and to detect predators, so losing the ability to smell, Porteus says, “has the potential to affect their survival in the wild.”
The fish in the more acidic water swam less than fish in water with present-day levels of CO2.They also were more likely to freeze—stop swimming and stay still—a sign of anxiety.
The team then measured signaling changes in the fish’s olfactory neurons—the nerves responsible for responses to smell. They exposed the fish to 10 different odors in both present-day ocean water and more acidic water. These included amino acids that signal different food sources, and bile acids that would indicate to the fish that a shark or other predatory species was present. The magnitude of the signal from the neurons was lower in six of the 10 cases, and the threshold for detecting odors increased for four of the 10 cases.
Genetic sequencing showed that genes responsible for sending olfactory information to the brain were not expressed as much when fish were exposed to CO2, and genes that slow down learning were expressed more. This could be bad news for evolution of fish to cope with changes, although evolutionary impacts are hard to predict from this study, Porteus says. It is something she hopes to address in future.
The sudden exposure of the fish to such high levels of CO2 is an artificial situation, cautions Gabriele Gerlach, an expert on bioevolution at the Carl von Ossietzky University of Oldenburg. Gerlach admires the work otherwise, but says that other studies have shown that the stresses of the extra acidity diminish for the second generation of fish exposed to CO2.
Despite any limitations to the experiment, Porteus was surprised by the stark nature of her team’s results. “We didn’t expect a relatively small decrease in seawater pH to have such a large effect on the sensitivity of the sense of smell in fish,” Porteus says. And the sea bass isn’t the only fish that would be affected, Porteus explains. “We think the ability to smell odors is similar in most if not all fish species, so what we have found for sea bass will almost certainly apply to all fish species.”