It is a thin, transparent, flexible worm with two pairs of rounded lateral fins composed completely of ray-like structures.

[2] Parasagitta setosa is a common neritic species forming part of the plankton in waters less than about 200 metres deep.

[3] Parasagitta setosa is one of a number of marine animals that make large vertical migrations on a daily basis.

[4] The abundance in any area of P. setosa and other mezo-zooplankton, can be easily disturbed by the introduction of an alien species.

In the Black Sea, the population of the once abundant P. setosa plummeted, and the anchovy stocks collapsed.