HydrozoansHydrozoa

The white feathery tuft growing on the wreck is not one animal. It is a few thousand of them, joined at the stomach.

A hydrozoan colony buds new polyps that stay attached and share one continuous gut, so food caught at one end of a branch feeds the whole structure. The strange part is what the polyps do with that freedom: they take up trades. Some feed and do nothing else. Some are armed and stand guard along the stem. Some exist only to reproduce and cannot eat at all. Each is a complete individual reduced to a single job, and the colony runs them the way a body runs its organs. The Portuguese man o' war is the far end of that idea, a float, a set of tentacles and a gut that are separate animals, none of which can survive alone.

Half the life cycle then swims away. Many hydrozoans bud small medusae off the colony, transparent bells that let go, drift into open water and spawn there, and the larva settles on rock and starts the colony again. The same species is a fixed, plant-like growth one month and plankton the next, which is why the hydroid and the jellyfish of a single species were often named separately before anyone connected them.

Fire coral is a hydrozoan in the wrong costume. Millepora lays down a hard limestone skeleton, branches into plates and antlers, and takes the mustard color and pale tips that make people file it with the stony corals. The surface gives it away: a stony coral is covered in cups where the polyps sit, and fire coral is smooth, pricked with pinholes of two different sizes, with hair-fine polyps standing out of them. It is common on shallow reef crests here, and the burn it is named for usually arrives several minutes after you have stopped touching it.

The rest of the group is easier to miss. White stinging plumes stand on wrecks and under overhangs, and the lace corals are small pink or violet crusts that also build in limestone, tucked into shaded cracks where nobody is looking.