Parasagitta setosa

Parasagitta setosa is a small arrow worm in the family Sagittidae, previously referred to as Sagitta setosa. It is native to the northeastern Atlantic Ocean, the North Sea and the Mediterranean Sea, and also occurs in the Baltic Sea and the Black Sea.

Description

P. setosa grows to a maximum length of 14 mm (0.55 in), its tail being up to a quarter of this length. It is a thin, transparent, flexible worm with two pairs of rounded lateral fins composed completely of ray-like structures. There are eight to nine curved grasping hooks on the head. The teeth are in two rows, with six to eight in the front row and ten to sixteen at the back. The eyes have a star-shaped segment spot and the collarette is small or not present at all. The seminal vesicles are very near the posterior fins which are separated from the caudal fin.

Distribution

P. setosa is a common neritic species forming part of the plankton in waters less than about 200 metres deep. It occurs in the northeastern Atlantic Ocean, the North Sea, the Mediterranean Sea and the Black Sea. It is the most common arrow worm in the North Sea and Black Sea, occurring in great drifts in July and August in the Black Sea and from September to November in the North Sea. It is also found in the Baltic Sea when influxes of saline water flow in from the Kattegat.