Fish lose their fear on a denuded reef

Coral reefs are biological hotspots, valued for both their beauty and biodiversity. But there is no reef that has been left untouched by humans. Thirty percent of the world’s reefs have been completely destroyed or left severely degraded, and 60 percent are facing extinction by 2030. Just how that destruction filters through the food web, though, is bound to be complicated.

For example, you might expect that a juvenile fish that makes its home on a dead coral reef would be more vulnerable to predators simply because it would be more visible on the barren habitat. But for ambon damselfish (Pomacentrus amboinensis), it’s more complex than that: On a degraded reef, juveniles of this species become more fearless and more likely to be eaten, Oona M. Lönnstedt of James Cook University in Australia and colleagues report February 5 in the Journal of Animal Ecology.

Ambon damselfish spend their larval stage in the deep pelagic and then settle as juveniles on the Great Barrier Reef off Australia. When the young fish arrive, they depend on their sense of smell to detect chemicals released by injured members of their own species (being a young fish apparently carries a high likelihood of being attacked) and then couple that with a visual cue of a possible predator to figure out what is potentially dangerous.

Juvenile damselfish released by the scientists onto healthy patches of reef stuck to smaller areas and took shelter frequently when they encountered an injured fish. But young ambon damselfish placed on dead coral had very different behavior, blithely swimming out in the open when they got the danger cues.

That behavioral change had immediate effects on fish survival. About 45 percent of fish placed on live reef failed to survive the following 48 hours in the experiment. That’s a high rate of mortality, but fish on the dead reef fared far worse, with 79 percent failing to make it through the two days.

Dead coral is different from live coral in many ways, but the most important one for young ambon damselfish may be the chemicals emitted by the reef. Vision helps when you’re a fish, but it only works when there’s light; smell is a key sense that works regardless of light level and probably more important for them. And in an earlier experiment, Lönnstedt and her colleagues found that fish kept in tanks with dead coral wouldn’t display the same type of antipredator behavior that damselfish did when kept in tanks with live coral.

“If the presence of nearby coral masks local environmental cues, the fish may well try to sit further away from the coral and higher up in the water column to have access to non-contaminated information coming from local water currents,” the researchers write. “This behavioral adaptation may in turn put the fish in a risky position, leaving them further away from shelter and more susceptible to attack from surrounding ambush predators.”

That makes the fish appear to be fearless, but that’s probably not a completely accurate description of what’s going on. It’s not that they don’t feel fear — they just can’t sense what they should be fearing.