Crown-of-thorns Starfish

Because at high densities they eat coral faster than it can grow back. A reef can absorb a background population that removes a small amount of coral each year, and some scientists think low numbers even help diversity by clearing space from dominant fast-growing corals. But during an outbreak, when thousands of starfish are present, they can strip most of the living coral from long stretches of reef within months to a couple of years. On the Great Barrier Reef, repeated outbreaks since the 1960s are estimated to have caused a large share of the total coral lost, and because outbreaks now come more often than the reef can fully recover between them, the long-term trend is downward.

This is still debated, but the leading idea concerns the survival of the larvae. A crown-of-thorns female produces tens of millions of eggs, and normally almost all of the resulting larvae starve or are eaten before settling. If conditions briefly favour them, a much larger fraction survives, and a few years later that cohort appears on the reef as an outbreak. The favouring condition is thought to be extra food for the larvae in the form of plankton blooms fed by nutrients washing off farmland after heavy rain. Reduced numbers of predators that eat adult starfish, such as giant triton snails, may then let an outbreak build and persist.

A jab from the spines drives venom and often a piece of the brittle spine into the wound. The immediate effect is intense pain out of proportion to the puncture, with bleeding, redness, and swelling that can last for days, and sometimes nausea, vomiting, and weakness. Embedded spine fragments can cause prolonged pain, granulomas, and infection, and occasionally need removal. First aid is to immerse the area in water as hot as can be tolerated, which helps break down the venom, and to seek medical care for anything more than a minor jab, for spines that may be retained, or for a joint injury.

On reefs of high value, such as tourism sites on the Great Barrier Reef, trained divers cull starfish during outbreaks. The current method is a single injection of ox bile salts or a household vinegar solution into each animal, which kills it within a day without harming other wildlife and without the risk of the older approach of cutting them up, since fragments with part of the central disc can regenerate. Culling can protect individual reefs but cannot stop outbreaks across an area the size of the Great Barrier Reef, so it is paired with efforts to improve water quality by reducing farm runoff.