A large, shelled pelagic snail. It lives in the pleuston and does not possess swimming fins. Shell is dextral, conical but quite variable between trochoid to flattened, with flat apex. Up to four whorls are slightly rounded triangular resulting in a triangular to rounded triangular aperture. The apex is well defined, the body whorl is large, the aperture is wide [J.janthina-ventral]. The shell surface is striated and blue to violet [J.janthina-dorsal]. The first 1.5 whorls may have sharp, regular striae which change over into a fainter irregular striation. The various striae pass from above obliquely backwards towards the middle of the whorl, from where they slant forwards and downwards to the columella. Faint irregular longitudinal striation is found. The columella is straight frequently with slight torsion [Janthina janthina]. The operculum is absent in adults.
Radula well developed with numerous teeth and no central teeth. The jaws are large.
Juveniles have a dextrally coiled shell; a special description is not available.
The dextral shell is depressed, triangular in profile, has about four whorls and is blue to violet.
Shell height up to 31.4 mm, aperture width 21.8 mm, breadth up to 20.0 mm.
The species lives in the pleuston and it is adapted to surface dwelling [J.janthina-floating].
Janthina janthina is a carnivore, feeding on relatively large floating prey animals such as Velella velella (Cnidaria: Hydrozoa), other Janthinidae, Halobatidae (Insecta) and Siphonophora (Cnidaria).
Distribution in the North Sea
Unlikely to be found in the North Sea.
Cosmopolitan species, present in all oceans between about 50°S and 55°N.
[After Van der Spoel et al., 1997]