|A live Janthina janthina (with bubble raft) that has been swept up onto a beach in Maui, Hawaii. This is the normal view from above: the spire of the shell is held pointing down like this when the animal is floating on the surface of the ocean water.|
Janthina janthina, common name the violet sea-snail or common violet snail, is a species of holoplanktonic sea snail, a marine gastropod mollusk in the family Epitoniidae, the violet snails or purple storm snails. 
This species is found worldwide in the warm waters of tropical and temperate seas, floating at the surface. More specifically, this species is located in the tropical and subtropical Atlantic, Indian and Pacific oceans. This information was obtained through an academic study of the differences between the different species within this genus.They are often found in large groups and sometimes become stranded on beaches when they are blown ashore by strong winds.These snails are a unique part of the pleuston, organisms living on or at the very surface of the water, because of their relatively large size. They have veliger or free swimming larvae, but the adults do not swim and cannot create their rafts except at the surface where air bubbles are available.
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.
J. janthina is a member of the family Janthinidae, 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.
- Gofas, S. (2009). Janthina janthina (Linnaeus, 1758). In: Bouchet, P.; Gofas, S.; Rosenberg, G. (2009) World Marine Mollusca database. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=140155 on 2010-05-16
- MolluscaBase (2019). MolluscaBase. Janthina janthina (Linnaeus, 1758). Accessed through: World Register of Marine Species at: http://marinespecies.org/aphia.php?p=taxdetails&id=140155 on 2019-10-09
- Morrison, Sue; Storrie, Ann (1999). Wonders of Western Waters: The Marine Life of South-Western Australia. CALM. p. 68. ISBN 0-7309-6894-4.
- Churchill, Celia; Valdes, Angel; Foighil, Diarmaid (2014). "Molecular and Morphological Systematics of Neustonic Nudibranchs (Mollusca: Gastropoda: Glaucidae: Glaucus), with Descriptions of Three New Cryptic Species". p. 174.
- Lalli, Carol M.; Ronald W. Gilmer (1989). Pelagic Snails: The Biology of Holoplanktonic Gastropod Mollusks. Stanford University Press. ISBN 978-0-8047-1490-7.
- Powell A. W. B., New Zealand Mollusca, William Collins Publishers Ltd, Auckland, New Zealand 1979 ISBN 0-00-216906-1.
- Rothschild, Susan B.; Nick Fotheringham (2004). Beachcomber's Guide to Gulf Coast Marine Life: Texas, Louisiana, Mississippi, Alabama and Florida. Taylor Trade Publishing. ISBN 978-1-58979-061-2.