BarnaclesCirripedia

A barnacle looks like a small mollusc and it is nothing of the kind. It is a crustacean lying on its back inside a box of its own limestone plates, glued to the rock by the front of its head, kicking food into its mouth with its feet.

It starts as a swimming larva that spends days walking over surfaces on its antennules, testing texture, current and the presence of other barnacles, because this is a decision it makes once. When it commits it cements itself down head-first, builds the plates up around itself and never moves again. The feathery fan that sweeps out of the opening and curls closed is its legs. The old name for the group, Cirripedia, means feather-footed, and it is a literal description.

The glue is the reason people study them. Barnacle cement sets underwater, bonds to rock, steel, plastic, whale skin and turtle shell, and holds through years of surge; it is among the strongest adhesives any animal makes. On a ship's hull it costs real fuel, and surgeons would like something that behaves the same way on wet tissue.

Being cemented down creates one problem that armor cannot solve, which is finding a mate. The answer is reach. Most barnacles are hermaphrodites, and a barnacle extends a penis several times its own body length to feel around for neighbors, the longest in the animal kingdom relative to body size, and it rebuilds it to suit the site: short and thick on rock that takes a beating, long and thin in shelter. It is also why they settle in crowds, because a barnacle alone on a clean rock is a barnacle with no future.

One branch gave up the shell entirely. Sacculina and its relatives enter a crab as a larva, spread through it as a root system, stop it breeding and take over its behavior so completely that the crab tends the parasite's egg mass as if it were its own brood. Nothing about the adult looks like an animal, let alone a barnacle, until you look at the larva.