Fish that dine on corals may pay it forward with feces. Marine biologists found high concentrations of living symbiotic algae in the feces of coral predators on reefs in Mo’orea, French Polynesia.

It’s an unexpected twist on coral reef symbiosis, said Rice University marine biologist Adrienne Correa, whose lab discovered coral predator feces are jam-packed with living symbiotic algae that corals depend on for survival. The discovery confirms that feces from coral-eating fish is an important environmental source of symbiotic dinoflagellate algae on coral reefs.

Correa said coral-eating predators are typically thought of as biting and weakening reef structures, thereby 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,’» she said.

Rice doctoral student Carsten Grupstra, lead author of the study in Animal Microbiome, said, «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.»

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.

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