PearlfishCarapidae

A pearlfish gets shorter when it grows up.

It begins as a vexillifer, a plankton larva that carries a flag: the first ray of the dorsal fin drawn out into a streamer that rises just behind the head, sometimes longer than the fish itself and often hung with tabs and blotches of pigment. What the flag is for has never been settled. The flag then goes, and the animal enters a second larval stage, the tenuis, a transparent thread that keeps stretching until it is longer than the adult it will become. The tenuis finds a host, goes inside, and loses well over half its length in the transformation that follows before it starts growing again as a juvenile.

Where it goes and how it gets in are on the species pages below, together with the difference between the two ways of living in there. What belongs here is that the host is not silent. A pearlfish has sonic muscles running from the skull onto the swim bladder; they pull the front of the bladder tight and release it, setting a thin window in its wall vibrating. The result carries out through the body wall of the animal the fish is sitting inside, and it sounds like knocking, or a drum roll. In recordings it happens when more than one pearlfish is in the same host. Two lodgers in one sea cucumber have something to say to each other.

The name came from a different host. Pearlfish turn up inside pearl oysters, occasionally dead and sealed over in nacre, which is how a group of fish that almost nobody sees ended up named after a gemstone.

Two are listed here, and both are met by accident: a small head sticking out of a sea cucumber at night, and gone before the light settles on it.