Full circle: How corals survive on the waste of their predators

Coral predator poop is jam-packed with living symbiotic algae that coral reefs depend on for survival, a new study shows.

It’s an unexpected twist on coral reef symbiosis, says Adrienne Correa, a marine biologist at Rice University. The discovery confirms that poop from coral-eating fish is an important environmental source of symbiotic dinoflagellate algae on coral reefs.

Correa says coral-eating predators are typically thought of as biting and weakening reef structures, generating hiding spaces for other organisms and, ultimately, beach sand. In contrast, grazing fish that crop down bushy algae get the limelight for helping reefs maintain healthy coral cover.

“The message is, ‘Move over grazers, it’s not just you helping maintain coral dominance. These coral-eating fishes are probably helping too by spreading beneficial coral symbionts,'” Correa says.

“This tells us we don’t really know all of the interactions that are happening on coral reefs, and some species may be important for coral reef conservation in ways that we haven’t imagined,” says doctoral student Carsten Grupstra, lead author of the study in Animal Microbiome.

Coral reef food sharing

In exchange for a sheltered life, dinoflagellates nourish their hosts by sharing the food they photosynthesize. Millions of symbionts live in each coral, but some corals aren’t born with dinoflagellates. They acquire them as babies.

“When many baby corals settle on the reef bottom, they have to get their symbionts from the environment,” Correa says. “We’ve seen symbionts in the water and sediments and on big bushy algae on reefs, but we haven’t really looked at how those microorganisms get to all those places.”

In thinking about ways symbionts might be distributed on reefs, researchers were inspired by studies of pollinating insects and birds that pass beneficial bacteria between flowering plants.

Like pollinating bees that visit many flowers in a single flight, coral-eating fish constantly crisscross reefs and interact with many corals each day.

“Most of them take small bites of adult corals and don’t kill the colonies they’re eating,” Grupstra says.

100 million live symbionts

During an expedition to the Mo’orea Coral Reef Long-term Ecological Research station in French Polynesia, the team followed fish that ate different amounts of coral and algae. Using underwater clipboards, they made note of what the fish ate and where and how often they defecated. Grupstra also gathered samples of predator and grazer poop to examine in the lab.

“I left some samples in the window sill for a couple of weeks in Mo’orea,” he says. “Later, when I started looking at them (under a microscope), I found tons of symbionts. A lot of them were swimming around and some were dividing.”

The sheer number of live symbionts was both unexpected and potentially important in the larger picture of reef ecology, Correa says. While symbionts had previously been reported in feces from a limited number of coral predators, it was unclear how many of them were alive and potentially useful to corals.

Her group found high concentrations of live symbionts in the feces of a diverse group of coral predators. For example, the team estimated that two species at the Mo’orea research station, Chaetodon ornatissimus and Chaetodon reticulatus, each spread about 100 million live symbionts per day over an area of reef approximately the size of six parking spaces.

“This expands how we can think about the roles of coral-eating fishes on reefs,” Correa says. “They don’t just break up reef frameworks. They also disperse symbionts that corals and other organisms need.”

Predator poop 'hotspots'

The knowledge that predator poop creates “hotspots” of live symbiotic dinoflagellates on reefs raises questions about whether and how coral use them. Correa’s team has planned a series of experiments on both juvenile and stressed adult corals to determine how readily they absorb beneficial symbionts from the poop.

She says many questions remain about how and how frequently corals take up symbionts from the environment. For example, marine biologists widely agree that many coral babies get their symbionts from the environment, but it is unclear how often adults do this and under what conditions. Better understanding symbiont uptake could lead to new methods to help reefs recover from stress-induced “bleaching.”

Bleaching occurs when stressed corals expel their symbionts en masse, frequently leaving corals colorless, as the name implies. Bleaching events are increasingly common due to climate change. While some corals never recover from bleaching, others do, which raises the question of how bleached corals repopulate their symbiont communities. Grupstra and Correa are conducting research to find out whether contact with coral predator feces can improve bleaching recovery rates and long-term coral health.

The National Science Foundation, the Wagoner Foreign Study Scholarship Program, the Lewis and Clark Fund for Exploration, the Garden Club of America Clara Carter Higgins Fund, the Explorer’s Club Youth Activity Fund, and an Early-Career Research Fellowship from the Gulf Research Program of the National Academies of Sciences, Engineering and Medicine funded the work.

Source: Rice University