A Thorny Issue

July 1, 2011
By Mandy Calkins

On the Great Barrier Reef lurks a creature straight out of science fiction, with many arms, stinging spines, and a monstrous appetite.


Meet the crown-of-thorns, a real Godzilla among sea stars. It can grow up to 3 feet in diameter, with up to 21 arms, and its upper surface is covered with venomous spines. The crown-of-thorns feeds on live coral, and in moderate numbers, is a normal part of the reef ecosystem.

Problems start when these sea star have a population boom, or outbreak. They devour coral faster than it can grow, even consuming slow-growing coral species they wouldn’t normally eat. This feeding frenzy can decimate
a reef’s coral, and though reefs can recuperate, it takes a decade or more for coral levels to be replenished.

Scientists aren’t sure if crown-of-thorns outbreaks are natural phenomena, or if human activities help sea star populations grow out of control.

One suggestion points to blooms of algae caused by pollution and agricultural runoff from land. Algae provide food for crown-of-thorns larvae, and when more larvae survive, the population explodes. Another possibility is that fishing cuts down the numbers of the few fish that prey on the crown-of-thorns.

Whether or not these outbreaks are a natural cycle, researchers worry that reefs under stress from human activities will have a tougher time recovering in the aftermath. Scientists are keeping tabs on water quality and other measures of ecosystem health to help ensure that, when the crown-of-thorns invade again, the reefs are ready for them.