Crown-of-thorns starfish (Acanthaster)

node · earth · Indo-Pacific

A large corallivorous sea star that everts its stomach over living coral and digests the tissue off the skeleton, leaving white scar. It is native and normally present at low densities; the problem is outbreaks, in which densities rise by orders of magnitude and a reef can lose most of its living coral within a few years. Genetic work published in 2008 showed that what had been treated as a single Indo-Pacific species, Acanthaster planci, is at least four species with distinct ranges, so accounts written before that date describe a composite. On the Great Barrier Reef, analysis of twenty-seven years of monitoring attributed forty-two per cent of the observed coral cover loss between 1985 and 2012 to crown-of-thorns predation — more than bleaching contributed over the same period, and second only to tropical cyclones. Why outbreaks happen is genuinely unresolved: the leading hypothesis is that nutrient runoff from agriculture boosts the phytoplankton that starfish larvae eat, raising larval survival; competing accounts invoke removal of predators, or natural population cycles that outbreak regardless.