Corals, fish know bad reefs by their whiff

Compounds drifting off certain overgrown seaweeds discourage youngsters from settling in failing habitats

NEXT GEN REEF  Little pink bulges indicate this coral is about to release gametes for open-water fertilization. Where the resulting babies will settle down to live could depend on how badly reefs stink. 

Danielle Dixson

Editor’s note: The journal Science retracted this paper August 9, 2022, after the University of Delaware, where coauthor Danielle Dixson works, investigated accusations of data manipulation. “The University of Delaware informed us they no longer have confidence in the validity of the data” in several of the figures, said Science’s retraction notice. The paper claimed that free-swimming coral youngsters at the age to choose a home reef use something in the water to avoid places where seaweed is choking out corals. One of the disputed data figures shows the young corals spending at least five times longer swimming in seawater collected on coral-dominated reefs instead of seaweed-choked ones. The reported results were dramatically clear (yielding the very strong P value of 0.001). Some of Dixson’s gloomiest findings, on how ocean acidification impairs fish, have been challenged by later research results (SN: 1/8/20).

Young corals and fish can tell bad neighborhoods from good ones in part by the stink of overgrown seaweeds.

This distaste for compounds leaching from seaweeds is the first evidence of a “bad habitat” chemical cue that could discourage youngsters from even trying to make their homes on degraded reefs, says Mark E. Hay of the Georgia Institute of Technology in Atlanta. The discovery reveals a conundrum for reef conservation, because the places biologists are trying to attract young corals and fish are usually disintegrating reefs where seaweeds are taking over.

Seaweed thickets can outcompete tiny corals and offer few rocky hidey-holes for little reef fishes. Exactly how the youngsters sense the taint of seaweeds isn’t clear. But in tests with paired water samples, young corals and fishes normally liked seawater dipped from coral-rich protected reefs from Fiji — but not when certain fast-spreading seaweeds had been sitting in those water samples for just an hour. Surveys on the reefs themselves revealed more young corals settling down in seaweed-sparse areas than in overgrown ones. And when researchers set out tiles on poles above seaweeds, more coral larvae settled there than on tiles in the thickest growth at the bottom, Hay and his colleagues report in the Aug. 22 Science.

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