Vast outbreaks of crown-of-thorns seastars can consume and devastate corals on tropical reefs, as if corals didn't already have enough problems. Prior to their coral-eating adulthood, however, the seastars (sometimes known as starfish) go through an algae-eating juvenile phase. New research reveals the starfish can wait in this phase, building their numbers, until the time is right to transform and descend on a reef like locusts.

Unlike most of the threats coral faces, crown-of-thorns attacks are not human-induced. Nevertheless, their rampages appear to be becoming more frequent, which is probably a consequence of our actions. One popular theory blames extra nutrients from fertilizer runoff increasing the phytoplankton starfish feed upon during their larval stage. However, Professor Maria Byrne of the University of Sydney is skeptical.

“The connection is tenuous,” she told IFLScience, pointing to coral untouched by runoff that have suffered major crown-of-thorns outbreaks. Instead, “there is this whole juvenile stage in between that has been ignored.”

It used to be thought that all it took for algae-eating juveniles to transition to adulthood was for the starfish to become large enough to wrap itself around some vulnerable coral. In Biology Letters, however, Byrne and PhD student Dione Deaker show transition timing depends not just on size but on the availability of coral, at least in the lab. One set of seastars fed on algae for 10 months, growing to 16-18 millimeters (1.6-1.8 centimeters) before making the transition. Others, raised without tempting corals to consume, went for 6.5 years without becoming adults, yet grew no larger in size. It seems they were just waiting for the right moment, and when this came, they proved just as voracious predators of corals as their younger counterparts. It helps explain how such enormous outbreaks can start so suddenly.

Byrne told IFLScience no one knows how seastars move into predator mode, but she thinks it may involve the development of the capacity to consume the waxy esters that make corals inedible to many other animals.

"Another important implication of our findings is the possibility that the current adult starfish killing programs used to manage crown-of-thorns starfish might, in fact, trigger a feedback mechanism in the starfishes' transition to coral predator as juveniles are released from adult competition," Byrne noted in a statement.

Nevertheless, even if current reef protection methods are ineffective, it is not clear what should occur instead. Byrne sees little alternative to programs that remove all the adult crown-of-thorns from the reefs most frequented by tourism and she stresses that pollution and nutrients from the land damages coral in other ways.

It's possible, Byrne noted, that animals that prey on juvenile crown-of-thorns have been depleted by human activity. If so, the best thing we can do is bring them back, but first they would need to be identified.