Butterflyfishes typically nibble away at corals, but according to a new Scientific Reports study, these reef fishes are avoiding corals that have been touched by seaweed, which is growing unchecked because of things like overfishing and fertilizer runoffs. This shift in butterflyfish preference is a possible early warning sign of reef troubles. 

Healthy reefs harbor many species living in complex communities. But over the last few decades, coral reef cover has decreased by as much as 90 percent in the Caribbean and 50 percent in the tropical Pacific because of factors such as climate change, overfishing, and pollution. Formerly coral-dominated communities are increasingly being converted to seaweed-dominated wastelands. 

While seaweed is natural and important for the ecosystem, it grows much faster than coral, and under normal circumstances, plant-eating fish keep this growth in check. Previous work revealed that seaweed placed near coral makes the macro-algae more palatable: Seaweeds expend more energy competing with corals for space on the seafloor instead of making onerous chemicals to fend off herbivorous fishes. 

Now, to study this seaweed-coral interaction from the point of view of corals and coralivores, a trio led by University of Delaware’s Danielle Dixson recorded 5,391 associations between corals and nine species of butterflyfishes at Fiji’s Votua Reef. Some of the corals were in contact with seaweeds that are chemically active (Galaxaura filamentosa) and chemically inactive (Sargassum polycystum). 

"It turns out that, unlike the herbivores that prefer seaweeds that have been in contact with corals, the coralivores avoided these corals," Dixson explains to IFLScience. "They are avoided when the visual cue is there, but they are also avoided when the visual cue has been removed, meaning that there is some kind of chemical signature that the fishes are detecting."

The seaweed appears to alter the biochemistry of corals, but it’s unclear if the corals are changing internally and becoming less nutritious or if they’re just less appealing because they've spent all their energy defending themselves against seaweed. Either way, it’s likely a bad sign since butterflyfishes are like canaries in a coal mine: They’re often on a more restricted diet (some only eat corals), and they’re strongly associated with reefs that have high coral cover, compared to those with more seaweed. 

"Their presence or absence on a reef can be used to assess what is happening behind the scenes or the state that the reef is in even after the blooming seaweeds have stopped blooming," Dixson adds. "They are sensitive to changes on the reefs, and from our study, have the ability to make pretty complex decisions on corals to associate with and those to pass up."