Not a coral, and the commonest sting on a reef because it looks like one.
A hydrozoan rather than a true coral, growing as flat encrusting sheets, upright blades or branching antler shapes, in a fairly consistent mustard to tan-yellow with pale, almost white tips. The surface is smooth and porcelain-like at a distance and carries fine hair-like polyps that are visible only close up. The pale tips and the smooth surface are the practical marks: a hard coral of similar shape shows individual cups.
It stings on contact, and it is the most frequent sting a diver picks up because it is shallow, common, often on the exact piece of reef or wreck someone reaches for, and does not look dangerous. The sting is a burning welt that comes up within minutes and can itch for days. It also fouls mooring lines and wreck structure, so a hand placed on a line at a safety stop is a common way to get it. Recorded from the shallows to about 30 m.
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.
