Could fish poop help corals recover from bleaching?

This story was originally published by Hakai Magazine and appears here as part of the Climate Desk collaboration.

It’s relatively well known that most fully functioning corals one finds dotting colourful coral reefs are a symbiosis between a coral (the animal itself) and the microscopic algae that dwell within it. This duo forms the physical foundation of coral reefs, where one-fourth of Earth’s marine species reside. But what is much less well known is how corals get their algal partners.

Spawning corals begin their lives flying solo as free-living larvae without algal partners. They’ll eventually acquire their algae from the environment. But where do those algae come from? Scientists aren’t really sure.

Adrienne Correa, a marine biologist at Rice University in Texas, has devoted her career to studying corals and their symbionts, and she has an idea about the source of at least some of corals’ symbionts: fish poop.

In recent research, Correa and her team showed that the feces of coral-eating fish are loaded with algae species that can establish symbiotic relationships with corals. The scientists have yet to fully connect the dots, however, and show that either adult or larval corals pick up symbionts from fish feces. But the fact that sea anemones, a closely related organism, get their algae this way gives the idea a boost.

Correa and her team hope to prove the connection in experiments they’ll begin later this year at the Moorea Coral Reef Long Term Ecological Research site in French Polynesia. For Correa, answering the question of where corals’ symbionts come from is urgent given climate change.

Warming oceans are stressing corals. Under heat stress, corals bleach, expelling their algae. Bleaching can be fatal for corals. With mass bleaching events growing more common and threatening entire reefs, Correa wonders whether coral-eating fish hold the key to coral resilience.

During bleaching events, individual corals react differently. Some, even within the same species, recover faster — partly because their algal symbionts differ from those of hard-hit corals. Though many corals only establish relationships with one type of algae, others, like the major reef-building species in the genus Acropora, can bond with several different algae species. Some algal symbionts make Acropora corals more heat tolerant than others. Because of this variability, the role of fish as symbiont dispersers could be important in dictating how a coral reef recovers.

While some coral-eating fish species also eat other things, some fishes, such as oval butterflyfish, ornate butterflyfish, and broom filefish, only eat corals — and these so-called obligate coral eaters prefer to eat healthy, unbleached corals. By eating the more resilient algae from unbleached corals, those fishes might help spread heat-resistant symbionts around the reef. In other cases, fishes might benefit corals that harbour only one species of symbiont algae by dispersing their particular partner around the reef.

If Correa’s upcoming experiments support her hypothesis, some fish populations might prove essential for helping to disperse the right kinds of algae for reefs to remain resilient in warming waters. “We can think about whether there are particular fish that we would want to farm and release on reefs,” Correa says.

Tamar Liberman Goulet, a coral biologist at the University of Mississippi who was not involved in the research, thinks Correa’s idea has merit. But she cautions that there are limitations on how big of a role fish feces could play. Fish tend to stick to their reef, Goulet says, and as such, would likely only disperse corals’ symbionts over a limited area.

“A coral reef is in some ways like an island, even though it is in the sea,” Goulet says. “Many coral reef fish and other organisms are confined to the reef that they are at.” If there is a physical barrier between reefs, such as a sandbar, Goulet says, fish will not swim past that barrier. Fish that do leave “risk getting preyed upon because the reef provides protection.”

Ultimately, fish spreading algae through their feces will not alone be enough to combat the most severe bleaching events, says Correa. “Coral-eating fish out on the reef on their own cannot fix it. There are too many stressors and the stressors are too severe.” Ultimately, environmental degradation and climate change will need to be addressed directly to fully protect coral reefs.