A sand anemone shaped like a small bush, with a sting that welts and lasts.
A sand-dwelling anemone with its column buried and only the crown showing: eight to a dozen thick arms, each branched and rebranched into short blunt side-branches, so that the animal reads as a small grey, brown or dull green bush 10 to 25 cm across rather than as an anemone at all. Touched, or given the wash of a fin, it withdraws completely into the sand in a second or two and leaves a hole. That behaviour is the reliable separation from the soft corals it is taken for — Dendronephthya, a small Xenia colony — none of which vanish. Identification is at the genus or the family Actinodendridae: Actinodendron and Megalactis are told apart on characters not visible in a photograph, and this entry is pitched accordingly.
The sting is the reason it has a name in English. The nematocysts are unusually powerful for an anemone and the result of contact is not a nettle rash: burning pain, raised welts, blistering in some accounts, and marks that can persist for weeks. Reports from divers and from aquarium keepers are consistent on this. It sits on open sand, at the height of a settled knee or a trailing hose, and it is among the few animals on a muck site that will cause a real injury from a moment’s inattention rather than from handling.
It is a carnivore, taking small fish and crustaceans that blunder into the arms, and the repeated branching is capture surface. Documented commensals are thin compared with the carpet anemones Stichodactyla and Heteractis, which host anemonefish and several shrimps; small shrimps are photographed among Actinodendron arms from time to time, but the associations have not been properly described and are not something to expect.
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.
