We all know the current widespread bleaching of corals around the world is a tragedy for the health of the oceans. But how will the death of corals impact the plethora of life that call the reefs home? Many will lose their food source, others will lose their shelter, but some fish might be impacted in a rather unexpected way. Researchers have found that it could limit the ability of certain prey to learn how to avoid predators. The study has been published in Proceedings of the Royal Society B.
Using damselfish living on the Great Barrier Reef as parts of it become bleached, researchers were able to study how the fish respond to the chemicals released by the coral as it dies, as well as those emitted by the algae and sponges that take over. They found that dead and dying reefs become too “smelly,” masking the scents that the fish use to pick up on predators, and thus impacting their ability to learn who to avoid.
“Baby fish use chemical alarm signals released from the skin of attacked individuals to learn the identity of new predators,” explains co-author Mark McCormick of James Cook University in a statement. “By pairing the alarm cue from their wounded buddy with the smell or sight of the responsible predator, fish are able to learn which individuals are dangerous and should be avoided in the future.”
Damselfish, as seen here, rely on olfactory cues from distressed mates in order to identify predators. Professor Mark McCormick
On dead coral, however, the fish did not respond to the cues. To investigate further, they then looked at the responses of fish who naturally live on dead coral anyway. Because they have evolved to live in this environment, they can compensate for the smell of dying coral, and indeed, the researchers found that they were able to learn to avoid new predators using smell as a cue. This, claim the researchers, shows how the death of the reefs do alter the olfactory landscape, inhibiting the ability to learn in some species of fish.
Currently, the Great Barrier Reef, the world’s largest single structure made from living organisms, is facing a grave threat. Rising surface ocean temperatures and acidification have led to massive bleaching of the reef, with almost 93 percent of reefs off the Australian east coast having been hit. The damage is part of only the third global coral bleaching event ever to have been declared – all of which have happened since 1998 – and the longer it continues, the worse the outlook for the corals. Already, regions in the north of the reef have experienced up to 50 percent mortality.
“If the process of cataloguing and avoiding predators is hindered in some species by coral degradation and loss, then much of the diversity of reef fish could be lost too,” says co-author Dr. Oona Lönnstedt from Uppsala University. The deaths of the reefs then, may not simply impact how the thousands of species depend on the corals themselves, but also how they interact with each other.