Intermediate 9 minutes
Stinging organisms: what stings without looking like a dangerous animal
Fire coral, hydroids, fire worms, urchins, and sponges: the most common injury in diving
Almost nobody gets hurt by approaching a shark. People get hurt by resting a hand on what looked like a rock, grabbing onto a line covered in white feathers, or brushing a knee against something that looked exactly like a plant.
In this article you'll see the four types of stinging or piercing organism a diver encounters attached to the substrate (fixed cnidarians, bristle worms, spiny echinoderms, and sponges) how to recognize them, what mechanism each one uses, and how their injuries are treated.
It's by far the most common wildlife injury in diving, and the least anticipated one, because these organisms don't look like dangerous animals: they look like scenery. This article is educational material; the general handling of injuries is in the article on marine wildlife injuries.
Fixed cnidarians: fire coral and hydroids
The same mechanism as jellyfish, but stationary and disguised as something else.
Fire coral isn't a true coral but a hydrozoan, more closely related to jellyfish than to stony corals. It forms mustard-colored or light brown colonies, often with whitish tips, and grows in sheet, branch, or encrusting forms over other structures.
Its surface is covered in tiny polyps with nematocysts, the mechanism described in the jellyfish article. Contact produces an intense, immediate stinging sensation, redness, welts, and sometimes blisters, with discomfort that can last for days and reactivate with the hot water of a shower.
Feathery hydroids are the other major culprit, and are especially deceptive because they look like algae or small, delicate white ferns. They colonize lines, buoys, wreck remains, and artificial structures: exactly the things people grab onto.
Anemones round out the group. Most produce little effect on human skin, but some large species are clearly stinging, and contact with thin-skinned areas (face, inner arms, neck) can be uncomfortable.
Handling is the same as for any stinging injury: don't rub, rinse with seawater, remove visible remnants with tweezers, and apply heat for the pain. Never fresh water, for the reason explained in the jellyfish article.
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