Coral competitor becomes ally in fight against starfish

Coral and algae don’t get along. On reefs, algae compete with coral, reducing coral growth and survival. Scientists suspect that the algae may also promote harmful bacteria or coral-eating species, causing further coral damage.

But coral have an even bigger worry: the crown-of-thorns starfish. These are large (up to about a third of a meter) seastars covered in venomous spikes that feed on coral polyps. Outbreaks of the starfish, which are native to the Indo-Pacific, can damage large sections of reef and even kill an entire coral colony.

So the combination of starfish and algae sounds bad for coral — but it may not be, a new study finds. The algae might protect the coral from the predatory seastars, Cody Clements and Mark Hay of Georgia Institute of Technology in Atlanta conclude August 26 in the Proceedings of the Royal Society B.

Clements and Hay performed a series of experiments in Fiji on Montipora coral, a favorite prey of crown-of-thorns starfish. They photographed reefs that had been attacked and those that hadn’t, quantifying algae cover on both. They planted tiny coral surrounded by various amounts of Sargassum algae to see how coral growth was affected. And they set up feeding trials for the starfish to see whether they would attack coral when algae was present.

The more algae that was around, the more coral growth declined, the researchers found. But the Sargassum seemed to have a protective effect when the starfish was present: There was less predation on reefs that had more algae. Attacked colonies had about 8 percent algal cover compared with 55 percent on reefs that were untouched. And in the feeding experiments, corals without any algae were always attacked while those with higher amounts of Sargassum were rarely consumed.

The algae could even force the seastars to change their feeding preferences. Crown-of-thorns prefer Montipora coral over Porites coral. But if the Monitpora is surrounded by algae, the starfish settle for the less-preferable Porites meal, the researchers found.

What is it about the algae that turns the starfish off? It is probably the physical structure of the Sargassum, the researchers suggest. When they put fake plastic Sargassum around the coral, seastars were much less likely to attack, similar to what happened with the real algae.