Cuthona viridis

(Forbes, 1840)

This species reaches a length of 20 mm in British waters (30 mm in Arctic waters). The body is either white or pale yellow. Opaque white pigment granules cluster on the tips of the oral and rhinophoral tentacles. The cerata are arranged in up to nine rows, with up to seven cerata per half-row. The cerata each contain a dark green hepatic lobe mingled with black specks. The pale, swollen cnidosacs are a reliable recognition feature.

It feeds upon the hydroids Abietinaria abietina and Sertularella spp.

It has been reported from the Arctic Ocean to Bear Island, as well as the seas around east and west Greenland and Iceland, reaching southward only as far as Roscoff on the Brittany coast of France (Distr. C. viridis).