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Janthina janthina is a member of the Janthinidae family, snails that trap air bubbles with a layer of clear chitin to maintain their positions at the surface of the ocean where they are predatious on the hydrozoans. In addition to the bubble raft, only the veliger, or larval stage, has an operculum, and the shell is paper-thin to allow the animal to float upside down at the surface. The snail's shell is reverse countershaded, because of its upside-down position in the water column. There is a light purple shade on the spire of the shell, and a darker purple on the ventral side. The animal has a large head on a very flexible neck. The eyes are small and are situated at the base of its tentacles. The snail begins life as a male and later changes to the female of the species. The eggs are held by the female until they develop into a larval form. The shell is almost smooth with a slightly depressed-globose shape. It is thin and delicate, and is without an operculum. The colour of the shell is violet, with a paler upper surface. The height of the species shell is up to 38 mm, the width to 40 mm.
These snails are pelagic, drifting on the surface of the ocean, where they feed upon pelagic hydrozoans, especially the by-the-wind sailor Velella velella and the Portuguese man o' war Physalia physalis.
This specimen was found washed up on the beach on Padre Island, Texas. It's bubble raft was still intact, the animal was alive, and there are clusters of pink eggs hanging from the bubble raft. The shell is about a half inch across, and the bubble raft is about an inch long.