Seaweed often inhibits the growth of corals, but it can help them when they are faced with a coral-eating starfish.

Seaweed can suppress coral growth by shading it from sunlight and by releasing toxic chemicals. Cody Clements and Mark Hay at the Georgia Institute of Technology in Atlanta surrounded branches of a coral species (Montipora hispida) near Fiji with varying numbers of fronds of a common brown alga (Sargassum polycystum). After 4 months, they found that the growth rate of coral branches unencumbered by the seaweed was 2.7 times higher than corals with 8 surrounding fronds (the highest number tested).

However, coral branches surrounded by four or more seaweed fronds were rarely attacked by the crown-of-thorns starfish (Acanthaster planci), which devoured all the exposed corals.

Proc. R. Soc. B 282, 20150714 (2015)