Habitat threatened by bleaching (4)

I believe the answer is:

reef

(Other definitions for reef that I've seen before include "Ridge near the surface of water" , "Bank of jagged rock near the surface of the sea" , "Kind of knot (avoided by sailors?)" , "Ridge of rock underwater" , "Reduce a sail's surface - shipping hazard" .)

I've seen this clue in The New York Times.

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