If you were swimming or diving off the coast of Votua, Fiji a few years ago, you might have seen something strange: Two guys screwing plastic bottles into the sand and putting hunks of coral on a machine balanced above the water on a pole. This wasn’t performance art, but scientists illustrating that the old adage “the enemies of my enemies are my friends” holds true even on a coral reef.

Coral reefs are in trouble. They’re degrading world-wide, and the tropical Pacific has lost around half of its coral in just the last few decades. On these Pacific reefs, one of the major causes of coral loss is predation by the crown-of-thorns sea star (Acanthaster planci), which eats away at coral and opens the door for seaweeds and other algae to come in and compete for resources with whatever coral is left standing. That’s bad not only for the corals, but other plants and animals that depend on the reefs for habitat and food. A sea star outbreak, says biologist Mark Hay, can be devastating and have cascading effects on many other species. 

However, Hay has recently found that seaweed isn’t always trouble for corals. In fact, in the right circumstances, the corals’ competitors can become their accomplices against the sea stars, turning from “pure enemies into body guards.”

Hay and marine biologist Cody Clements studied corals, sea stars, and algae in a few spots around Votua on Fiji’s “Coral Coast” to see how their interactions affected coral growth and survival. This is where the bottles come in. They collected coral branches from 20 different colonies and glued them to the cut-off necks of plastic soda bottles. The bottle caps were embedded in the seabed and the necks were screwed back in. These experimental corals were then surrounded with different amounts of brown seaweed fronds. The researchers returned to the corals monthly to weigh them, using a waterproofed electronic scale that they held above the water on a tripod.

The pair found that coral growth decreased as seaweed density increased, which wasn’t surprising—but they also discovered that more seaweed cover cut the corals’ chances of being attacked by sea stars. Once the seaweed covered about 40 to 60 percent of the coral’s surface, its probability of being attacked dropped to zero. Even seaweed-covered corals that were attacked suffered less damage than corals that had none on them.

A seaweed cloak, Hay and Clements say, appears to keep the sea stars from detecting, getting to and feeding on the coral. Sargassum polycystum, which is the type of seaweed the scientists worked with, as well as many other seaweeds in the region, are tough and abrasive, and form thick canopies around coral colonies. That has costs for the corals, but also provides a sea star-proof covering that can give them a chance to survive attacks and recover. The corals' competitors wind up providing a net benefit. 

Fiji’s corals have another ally. During their research Hay and Clements met a local man who collects crown-of-thorns sea stars from the reefs and uses them as fuel for bonfires on the beach, an aggressive strategy that they say might help corals elsewhere.