Sea butterfly

A sea snail that lives its whole life in open water. The foot, which in a snail on the bottom is a muscular sole for crawling, is here drawn out into two thin transparent wings that beat in a figure-of-eight — a rowing stroke closer to an insect’s than to anything else in the sea. The shell, where there is one, is glassy and thin, coiled in some groups and drawn into a slender cone or a squat pyramid in others.

Feeding is the surprising part. Many thecosomes hang motionless and secrete a mucus web far larger than themselves, sometimes several centimetres across, and wait for it to collect plankton and marine snow before eating the whole net. A disturbed animal abandons the web; a blackwater photograph showing the web intact is a photograph of an animal that has not noticed you.

The name covers two different things and they are worth keeping apart. The Thecosomata are the shelled sea butterflies described here. The Gymnosomata — sea angels — are their shell-less relatives, and they are predators that eat sea butterflies and almost nothing else, extending a set of tentacles or hooks from the head to pull the prey out of its shell. Finding the two together is uncommon and worth the whole dive.

They are also the animal most often cited in work on ocean acidification, because the shell is aragonite and thin enough to dissolve measurably in undersaturated water. That is a genuine finding and it is also frequently overstated in popular accounts; the useful version is that they are sensitive, well studied, and easy to monitor, not that they are about to vanish.

Small, fast for their size, and erratic. They are among the harder blackwater subjects to hold in focus.