Damsels in distress: Coral reef decline denudes damselfish of defenses

A study shows that one of damselfish's key abilities, chemical cues to identify predators, is incapacitated when the coral at the core of its ecosystem perishes. 

First Look

The plight facing coral reefs in a warmer world is nothing new, with warnings peppering the headlines in recent decades about the decline of these critical species – and the subsequent degradation of the ecosystems they support.

But in a world first, scientists have uncovered one very specific effect of this worrying trend, considering the impact of dying coral on the ability of reef-dwelling fish to identify and avoid predators.

What the researchers found, as published Wednesday in Proceedings of the Royal Society B, was that the common damselfish, able to learn the tell-tale chemical signatures of dangerous predators in a healthy coral reef, are completely stripped of this capacity in a reef comprised only of dead coral.

“Baby fish use chemical alarm signals released from the skin of attacked individuals to learn the identity of new predators,” explained lead author Mark McCormick of James Cook University in a press release. “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.”

It turns out, however, that the olfactory landscape of the reef seems to change when the coral dies and becomes smothered in algae. Under such circumstances, the ambon damselfish (Pomacentrus amboinensis), a habitat generalist, is no longer able to learn any such predator cues.

In contrast, another species, the neon damselfish (Pomacentrus coelestis), which only makes its home among the skeletons of dead coral reefs, retains this ability to learn the chemical cues of predators whether in its preferred habitat of lifeless coral or indeed in a healthy coral ecosystem.

“Understanding how some species can cope with or acclimate to the detrimental impacts of habitat degradation on risk assessment abilities will be crucial to defining the scope of resilience in threatened communities,” note the authors.

In a report released last month, scientists estimated that 30 to 50 percent of the coral in the Great Barrier Reef is already dead. Latest estimates put that figure on a global scale at 19 percent.

The importance of coral reefs and the ecosystems they support goes beyond their intrinsic value as a part of the richness of the natural world. Indeed, humans need coral reefs – not just for their beauty, biodiversity, and tourism revenue, but also for their ability to slow down hurricanes and tropical storms, as the Monitor has previously reported.

“Coral reefs buffer adjacent shorelines from wave action and prevent erosion, property damage and loss of life,” says the National Oceanic and Atmospheric Administration. “Reefs also protect the highly productive wetlands along the coast, as well as ports and harbors and the economies they support. Globally, half a billion people are estimated to live within 100 kilometers of a coral reef and benefit from its production and protection.”

As global temperatures continue to rise from human-induced climate change, tropical storms and hurricanes are likely to increase simultaneously – and humans will have fewer of the world’s natural storm barriers.

Perhaps the plight of the damselfish can play a small part in promoting the preservation of these invaluable ecosystems.