40 — Violet snails, Janthina exigua

Violet snail, Janthina exigua, Papamoa Beach,
2/10/16. This is the underside of the
shell and clearly shows the bubble raft.

Violet snail, Janthina exigua, a dead bee, and
other flotsam, Papamoa Beach, 2/10/16. 

Violet snail, Janthina exigua, some stalked
barnacles (Lepas sp.), and to the left a by-the-wind
sailor (Velella velella), Papamoa Beach, 2/10/16.

Violet snails are found worldwide in warm seas and there a few species of these small attractive violet/purple snails found here in New Zealand. They wash up along the high tide line during prolonged periods of onshore winds, so they're often seen during or just after storms.

Violet snails live on the surface of the sea supported by a raft of bubbles, which the snail secretes. They float with the spire pointing downwards, so in the water they look upside-down compared with the way snails usually orient themselves. The violet snail pictured here is Janthina exigua Lamarck, 1822 and these were washed on on Papamoa Beach (2/10/16). This species is commonly washed up on Auckland's west coast beaches, but isn't all that common in the Bay of Plenty; usually it's Janthina janthina (Linnaeus, 1758), which is washed ashore here.

Violet snails are carnivores and are thought to feed on by-the-wind sailors (Velella velella (Linnaeus, 1758)), plus other (presumably smaller) violet snails, water striders (Halobatidae), and blue bottles (Physalia physalis (Linnaeus, 1758)). All these surface animals (neuston) tend to wash up together, along with whatever else was floating around with them (e.g., seaweed), so if you see one, you're likely to see several different species.

For the most part violet snails aren't very big and J. exigua is the smallest of the three species (to ~15mm) commonly found in New Zealand waters. Although, occasionally J. janthina can reach sizes of ~35mm. I was once told that the dunes are a good place to look for the largest specimens, as a strong onshore wind will blow them up there.


More info:

http://www.mollusca.co.nz/speciesdetail.php?speciesid=945&species=Janthina%20exigua