Larvacean

The animal is a tadpole-shaped tunicate a few millimetres long — a rounded trunk and a flat tail held bent under it — and it is not usually what a diver notices. What shows in a beam is the house: an oval or lobed cloud of transparent mucus many times the animal’s length, visible only when the light crosses it at an angle, with the animal itself as a small dark speck somewhere inside. Salps and doliolids, both of which have their own entries, are the common confusion and both have a firm test and visible muscle bands; drifting egg masses and clumps of marine snow are the other. This entry is pitched at the class, because the animals are millimetric and the house is at best a guide to the genus. Oikopleura and Fritillaria are the usual ones here; the giant Bathochordaeus, whose house can approach a metre, lives deeper than a blackwater dive reaches.

The house is a secreted filter, and it is the point of the animal. The beating tail drives water in through coarse screens that keep large particles out, then through fine internal filters that concentrate bacteria and nanoplankton — food too small for anything else of that size to catch — and on to the mouth. The filters clog within hours, at which point the animal abandons the house and secretes a replacement from cells on the trunk, several times a day. The discarded houses sink, and they make up a substantial part of the marine snow and of the carbon that leaves the surface ocean.

Photographically they are among the harder subjects, because the house is nearly invisible head on and appears only as the beam passes across it. A good many are recorded by accident, as a faint oval structure behind something else in the frame.