FirewormsAmphinomidae

Yes, it hurts, and it goes on hurting long after the animal has crawled away.

The white tufts standing out along both sides of a fireworm are not hair. They are hollow needles of calcium carbonate, barbed along their length and held so loosely that brushing the worm is enough to transfer them. They snap off in the skin, stay there, and release an irritant as they go. The burn starts within a minute or so, runs for hours, and leaves a swollen patch that itches and prickles for days after the pain has gone.

The important part is what not to do. Rubbing the spot breaks the bristles shorter and works them deeper, which is the usual reason a sting lasts a week instead of an afternoon. Adhesive tape pressed onto the skin and pulled off lifts most of them out, tweezers get the stubborn ones, and it is worth clearing them before doing anything about the pain. A reaction that spreads beyond the contact area is a reason to get medical help rather than tough it out.

For all that, nothing here is aggressive. Fireworms are polychaetes, ordinary segmented worms that put their whole defense budget into not being swallowed, and they cannot chase, bite or aim. Essentially every injury is a hand put down blind on a ledge, a rope of something soft picked up out of the rubble, or a night diver kneeling in the sand. They are most active after dark, and a dead fish on the bottom will collect a knot of them.

By day most are wedged out of sight in a crevice. The bearded fireworm, the largest of the ones here, is the exception worth knowing: it works slowly over stony corals, gorgonians and anemones in the open, grazing the tips and leaving bleached stubs behind it. It looks slow, soft and entirely harmless, and it is neither of the last two.