Eats coral, spines venomous, and its population outbreaks are the real problem.
A large sea star to about 40 cm with up to twenty-one arms and the whole upper surface armed with long sharp spines. Colour varies from grey-green through purple to red. The spines carry venom and are brittle enough to break off in a wound; a puncture is painful, slow to settle and prone to infection, and hot-water immersion and medical attention are the standard response.
It feeds on hard coral, everting its stomach over a colony and digesting the tissue off the skeleton, leaving a clean white scar. At normal densities it is part of a working reef and does no lasting harm. The problem is outbreaks — episodes in which numbers rise by orders of magnitude and strip large areas — which are among the principal causes of coral loss across the Indo-Pacific. Nutrient runoff boosting larval survival and the removal of predators such as the giant triton are the leading explanations, and outbreaks are not thought to be a natural cycle in their present frequency. Mostly nocturnal; by day it is folded into a crevice.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.