It is relatively well known that most of the fully functioning corals found on colorful coral reefs are a symbiosis between a coral (the animal itself) and the microscopic algae that inhabit it. This duo forms the physical basis of coral reefs, where quarter of Earth’s marine species reside. But what is much less well known is how corals obtain their mates from algae.
Spawning corals begin their lives by flying alone as free-living larvae without algae partners. They will 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 spent her career studying corals and their symbionts, and she has an idea about the origin of at least some of the coral symbionts: fish droppings.
In recent research, Correa and his team showed that the feces of coral-eating fish are loaded with algae species that can establish symbiotic relationships with corals. However, scientists have yet to fully connect the dots and show that adult or larval corals pick up symbionts from fish feces. But the fact that sea anemonesa closely related organism gets its algae this way gives the idea a boost.
Correa and his team hope to test the connection in experiments that will begin later this year at the Moorea Coral Reef Long-Term Ecological Research site in French Polynesia. For Correa, answering the question of where coral symbionts come from is urgent given climate change.
Warming oceans are stressing corals. Under heat stress, corals bleach and expel their algae. Bleaching can be fatal to corals. With mass bleaching events becoming more common and threatening entire reefs, Correa wonders if coral-eating fish hold the key to coral resilience.
During bleaching events, individual corals react differently. Some, even within the same species, recover faster, in part because their symbiotic algae differ from those of the most affected corals. Although many corals only form relationships with one type of algae, others, such as the main reef-building species of the genus acropora, can join with several different species of algae. Some algal symbionts do acropora corals more heat tolerant than others. Because of this variability, the role of fish as symbiotic dispersers could be important in determining how well a coral reef recovers.
While some species of coral-eating fish eat other things as well, some fish, such as oval butterfly fish, ornate butterfly fishY broom fishThey only eat corals, and these so-called obligate coral eaters prefer to eat healthy, unbleached corals. By eating the tougher algae on unbleached corals, those fish could help spread heat-resistant symbionts around the reef. In other cases, fish may benefit corals harboring only one species of symbiotic algae by dispersing their particular partner around the reef.
If Correa’s forthcoming experiments support his hypothesis, some fish populations could prove essential in helping to disperse the right kinds of algae so reefs remain resilient in warmer waters. “We can think about whether there are particular fish that we would like to raise and release onto the reefs,” says Correa.
The researchers are working to show that coral-feeding fish spread the corals’ symbiotic algae in their feces. If they’re right, it could open up new opportunities to help struggling reefs cope.
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 to the role fish feces could play. The fish tend to stick to their reef, says Goulet, and as such would probably only disperse the coral symbionts in a limited area.
“A coral reef is, in a way, like an island, even though it’s in the sea,” says Goulet. “Many coral reef fish and other organisms are confined to the reef they are found on.” If there’s a physical barrier between the reefs, like a sandbar, Goulet says, the fish won’t get through that barrier. The fish that leave “are at risk of being preyed upon because the reef provides protection.”
Ultimately, fish shedding algae through their feces alone won’t be enough to combat the most severe bleaching events, Correa says. “Fish feeding on corals on the reef alone can’t 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.