Violet snails are holopelagic in their way of life and circumequatorial in their distribution. They are normally found in lower latitudes than those of north-west Europe but are occasionally washed ashore on southern and western coasts of Britain and Ireland after prolonged southwesterly gales. The colour of the shell (pale on the spire, dark on the base) relates to the fact that they normally float upside down, the shell borne on the epipodial lobes, maintaining their position at the surface of the water by a mucous float containing many bubbles of air. Its formation has been described by Fraenkel (1927). The lobed anterior margin extends out of the water so as to catch a bubble of air in the funnel where glands surround it with mucus. It is passed to the back end of the foot and applied to the base of the float, the mucus hardening on contact with the water. The float is attached to the grooved posterior part of the pedal sole and is essential for the maintenance of the animal's position in the water: if lost, the snail sinks and, unable to make contact with the air, cannot rise again.
The snails are predators, using primarily the coelenterates Porpita and Velella as food. During feeding (Graham, 1965) the snout is extended and the buccal cavity everted to form a proboscis armed at the tip with the radular teeth and jaws. Pieces are bitten off the prey by the jaws and pulled into the gut by inversion of the buccal region. The coelenterates have been said to be anaesthetized by the purple secretion of the hypobranchial gland.
There is still much to be learned of the sexual and reproductive activities of Janthina. The snails have been said to be protandrous hermaphrodites as Graham (1954) found that all his small specimens were male, and all larger ones female, but Wilson & Wilson (1956) found some large males and small females, perhaps indicating a balanced hermaphroditism (Montalenti & Bacci, 1951). How spermatozeugmata behave is not clear. The animals seem unable to control their movements in the water, so that a close pairing appears to depend on chance. Where the spermatozeugmata shed their load of sperm in oviparous species is not known, but disintegrating stages were found by Graham (1954) in all parts of the female tract of J. janthina. This may explain the supposition of Pruvot-Fol (1915) that the animals arc simultaneous hermaphrodites. The statement by Laursen (1953) that the female duct of J. pallida opens to the sole of the foot is erroneous; in all species it opens on the right side of the mantle cavity. Since the larva! shell has about 4 whorls, larval life would seem to be long, but it is not known where the larvae live, what they feed on, nor, as the animals do not settle, what gives the signal for metamorphosis. As noted by Marcus (1956) the protoconch of J. janthina shows growth lines and spiral lines like the protoconchs of other species, not the pustular surface figured by Laursen (1953). In some species the larval shell has its axis inclined at c. 45" to that of the teleconch: what significance this has is not known.
Separation of the species recorded from British and Irish shores is relatively straightforward. The shell of J. exigua is small and distinct from the others in the growth lines, which give it an almost file-like surface, and in the deep sinus on the outer lip. J. pallida may also have a distinct sinus but this is often not pronounced; it is, however, more globose and, in particular, the aperture is rounded where outer lip and columella join. In J. janthina (the commonest species stranded) the shell is very variable in shape and may be globose or flattened, but it is relatively smooth, has hardly any notch on the outer lip and this meets the columella

These gastropods are pelagic, floating on the surface of the sea with the aperture of their fragile, lavender shells held upward . They are usually associated with the coelenterates Velella and Physalia, the Portuguese-man-of-war, on which they feed, and with which they are cast up on shorelines after a storm at sea.
The shells of the Janthinidae are among the thinnest of gastropod shells. They are globose, with a low spire, the last whorl is inflated, and there is a large aperture but no operculum. The shells are of varying shades of blue to violet.
The sexes are separate. J. janthina is viviparous, producing living young. In J. globosa and J. exigua the egg capsules are attached to the float. The veliger larvae are bilobed, and, as in the epitonids, larval janthinids have a hypobranchial gland which produces violet secretions (J. B. Taylor, 1975). The protoconchs are ovate.
The three species recorded here are found throughout the Pacific, Indian, and Atlantic oceans.