Looks like a fern, is a colony of animals, and it is the commonest sting on a muck dive.
A hydroid colony to about 50 cm, growing in stiff feather-shaped fronds of olive to brown that stand out from rock, rubble and wreckage. What looks like a plant is a colony of individual polyps sharing a common stem, some for feeding, some for defence, some for reproduction, all connected. The stinging cells are concentrated on the fine side branches.
It earns a page as the animal a diver is most likely to be hurt by, and as habitat. Hydroid colonies are the food of the aeolid nudibranchs — the blue dragon among them — which store the colony’s own stinging cells in their cerata and reuse them; they carry skeleton shrimps, Bornella and small commensal crabs, and juvenile fish shelter in the fronds. Recorded from the shallows to about 40 m. A colony brushed by a wrist between glove and suit is the standard mechanism, and it is a good argument for covering the gap.
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.
