Janthina janthina (Linnaeus, 1758)
Overview
This is a large, shelled pelagic snail, 3.1 cm high. It lives in the pleuston and does not possess swimming fins. The dextral shell is depressed, triangular in profile, has about 4 whorls and is blue to violet. There is a small keel and a faint incision in the outer aperture rim. The operculum is absent in adults (Janthina janthina 1). It is a carnivorous species which predates on relatively large floating prey animals. This species occurs in the warm waters of all oceans.
Taxonomic Description
The shell is dextral, conical but quite variable between trochoid to flattened, with flat apex. The 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 (Janthina janthina). The shell surface is striated and blue to violet (Janthina janthina living dorsal, Janthina janthina living oral). 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 3, Janthina janthina 2). The operculum is absent in adults. The jaws are large, the radula is well developed with numerous teeth and no central teeth.
Shell height up to 31.4 mm, aperture width 21.8 mm, breadth up to 20.0 mm.
Juveniles
The juveniles have a dextrally coiled shell. A special description is not available.
Reproduction
This species is viviparous, protandric hermaphrodite.
Ecology
This species is a carnivore. It is a surface dweller (Janthina janthina floating), feeding on Velella, Janthinidae, Haliobatidae, and Siphonophora.
Distribution
This species lives in the warmer waters of all oceans and occurs in the Mediterranean, see the Janthina janthina map.
Types
Helix Janthina Linnaeus, 1758: 772.
Type locality: Mediterranean.
Original type description
Testa subimperforata subrotunda diaphana fragilissima, apertura postice dilatata, labro emarginato. Habitat in Europa, Asia, Africa; in Mediterraneo frequentior; etiam pelagica.
Translated: Shell, umbilicus slightly open, slightly rounded, transparent, very fragile, aperture broader anteriorly, lip with sinus.