(Gegenbaur, 1855)
Description
A relatively large, naked pelagic snail, up to 1.5 cm long, with a slender, mainly transparent body which shows the reddish brown of the visceral mass shining through the body wall. The skin has purple and black chromatophores.
Head and body clearly separated. There is a median sucker arm with 5 suckers of which the anterior two are large, oval and pointed. Lateral arms with 7 to 8 sessile suckers [P.ciliata-a], rather short; median arm with 5 suckers of which the posterior two and the top sucker are small and the anterior large pair are typical. Though the suckers in the median arm are variable, the present species always differs from other species due to the two large anterior suckers. The lateral gill is large and it hangs free from the lateral side in the middle of the body; a posterior gill is absent. The posterior footlobe is long. When sexually active the genital organs may become so large that the body and the position of the alimentary organs are completely transformed. In this species, specimens are found with a gonad externally of the body, hanging at the right side under the wings. This was observed in specimens of 3 mm and larger.
Radula of older specimens (5.0 mm long) have the formula 7-1-7, up to 8-1-8, with 30 to 40 hooks [P.ciliata-r1, P.ciliata-r2].
Juveniles: the protoconch, which is lost after a few days, is nearly globular. The colour is faint brown especially in the incision between protoconch I and II [P.ciliata-prc]. Protoconch II is small, forming just a rim on protoconch I. The aperture of the protoconch is slightly oval.
Size
Maximum body length 15.0 mm.
Ecology
P. ciliata is a quick swimmer that hunts shelled pteropods (Thecosomata) and cladocerans (Crustracea) as food. Eurythermic species, living in the N Atlantic and N Pacific Oceans in shallow and deep water layers where it can occur in mass blooms.
Distribution in the North Sea
Northern North Sea, entered from the NE Atlantic.
World distribution
Extremely discontinuously distributed; only found in the N Atlantic, Atlanto-Mediterranean, Indian and N Pacific Oceans.
[After Van der Spoel et al., 1997]