Janthina exigua Lamarck, 1816
Overview
This is a large, shelled pelagic snail, 1.7 cm high. It lives in the pleuston and does not possess swimming fins. The dextral shell is high, dark violet and has about 5 whorls. There is a small keel and an incision in the outer aperture rim. The operculum is absent in adults. It is a carnivore species which predates on relatively large floating prey. This species occurs in the warm waters of all oceans (Janthina exigua dorsal).
Taxonomic Description
The shell is dextral, conical, egg-shaped, the spire is rather tall with well defined apex, the edge of the aperture is passing in an even curve to the columella with a deep incision in the middle (Janthina exigua). The body whorl is large. The surface has regular deep furrows, and this makes it easy to separate this species from the others in the genus. The furrows slant from the suture backwards to the middle, where they form a sinus and where, in continuation with the bottom of the incision in the aperture there is a very inconspicuous keel. The aperture is wide. From there the furrows pass obliquely forwards to end at the umbilicus (Janthina exigua 1, Janthina exigua 2). The shell is very thin, translucent and dark violet. An operculum is absent in adults. The jaws are large and the radula has no median teeth.
Shell height up to 17 mm, aperture height up to 11 mm.
Juveniles
The juveniles have a dextrally coiled shell. A special description is not available.
Reproduction
This species is an viviparous, protandric hermaphrodite.
Ecology
This species is a carnivore. It is a surface dweller, feeding on Velella, Janthinidae Halobatidae and Siphonophora.
Distribution
This species lives in the warm waters of all oceans, see the Janthina exigua map.
Types
Not yet located.