80 — Violet snail, Janthina janthina

I've previously posted regarding the other two common violet snails which occur in New Zealand waters, but I would be remiss if didn't mention the most well-known species. Janthina janthina (Linnaeus, 1758), is one of the most commonly encountered of the violet snails (Family: Janthinidae). They are the largest of the violet snails, reaching ~4cm in size, and have a global distribution in warm seas. Violet snails float by means of a raft of bubbles (held together with what is essentially snot), which they secrete.

Ecologically, violet snails are what is known as holoplanktonic, meaning that they spend their entire lives in surface waters and basically float around at the mercy of wind and current. This is why they wash up on beaches after sustained periods of onshore winds.

Typically J. janthina feed on by-the-wind sailors (Velella velella Linnaeus, 1758) and Portuguese man-o-war (Physalia physalis Linnaeus, 1758). They are in turn preyed on by birds, fishes, and other molluscs. Once they die their shells can sink to sea bed, I have seen one which was collected in a deep-sea sample from ~1800m on the Challenger Plateau, west of New Zealand.

On Auckland's west coast beaches they are commonly seen washed up along the high tide line, but they are less common than J. exigua. On the east coast of the North Island this is reversed with J. janthina being the most common, plus J. globosa is occasionally also seen.

Janthina janthina, South Piha, 10/11/16.

The same animal, transported to a mid-tidal
rock pool. Note the bubble raft and the
orientation of the shell.

Janthina janthina, Papamoa Beach, 2/10/16.