Purple Storm Snail

Purple Storm Snail
Janthina janthina with its bubble raft. Photo courtesy of Gerald McCormack and Cook Islands Natural Heritage Trust.

Janthinidae are a family of holoplanktonic sea snails (they live in the water column for their entire life cycle and cannot swim against a current). The family is comprised of two genera: Janthina, the violet snails, and Recluzia, the rare brown snails. The common names for Janthina janthina include violet sea-snail, common violet snail, large violet snail, and purple storm snail. These snails are protected from day one by a camouflage strategy known as countershading. The ventral side of their shell – which points towards the sky when they’re floating – is much darker in color than the top. The snail’s dependence on a raft of bubbles affects its posture on the water. They float upside down, so the darker purple ventral side faces up, helping them blend into the darker water below. When seen from beneath, their paler purple/gray coloration on top blends in with the light coming from above. Because of their upside-down position in the water column, they’re technically reverse countershaded. They have thin, smooth, fragile shells that are wider than they are tall and have a slightly depressed-globose shape (short, fat spiral). They reach about 1.5 inches in height. The snail has a large head on a very flexible neck with small eyes situated at the base of its tentacles. Purple snails are safe to pick up and get a closer look at, but they do secrete a purple ink that stains.

The storm snail is found floating at the surface of the water in the tropical and subtropical Atlantic, Indian, and Pacific oceans. The species is a member of the open-water, surface-dwelling community known as the neuston. While only about 1.5 inches long, it’s one of the largest organisms living in this thin but incredibly expansive habitat. These snails float on the ocean surface by attaching themselves to rafts of bubbles about 5 inches long. Some species have veliger, or free-swimming larvae, but adult snails are actually unable to swim and are completely dependent on their air bubble boats, which they can only create at the surface. If they lose hold of their “raft, the snail will sink to the bottom and die. As perpetual castaways, their fate is tied to the whims of the ocean’s current. They are often found in large groups and sometimes become stranded on beaches when they are blown ashore by strong winds. North of Bob Hall Pier is one pocket they sometimes wash up in.

Janthinids are grouped within the large benthic snail family Epitoniidae, in the superfamily Janthinoidea. Through DNA sequencing and molecular phylogenetics, it has been determined that janthinids are evolutionarily derived from epitoniids, which are specialized predators and ectoparasites of benthic cnidarians. There are a couple of hypotheses on how a benthic epitoniid came to live life on a floating bubble raft. The first is a modified juvenile drogue thread. The drogue is a mucus thread that many benthic snail species use for pelagic dispersal as juveniles; it helps them drift from place to place like a kite on a string. Janthina species juveniles build floats consistent with the drogue thread hypothesis, but to our knowledge, Recluzia species juveniles do not. The second hypothesis is that the float began as a modified egg mass, which typically has cells in various stages of development, from embryos to empty husks. The empty husks trap air and provide buoyancy; some species of both Recluzia and Janthina use this strategy. Both strategies initially produce temporary periods of passive rafting, but adding air-filled mucus bubbles to the drogue thread or the egg mass allows them to live a fully neustonic existence.

The physical characteristics of Recluzia and Janthina were compared with those of the ancestral epitoniids to determine if one might be a transitional form. Recluzia shares six characteristics with epitoniids; Janthina has none. This points to Recluzia as the transitional form, which lends support to the egg mass hypothesis. With this strategy, males would have to stick with females to have a raft. The evolution of autonomous float building by juvenile Janthina species removed the necessity of meeting a female before metamorphosis. They use their foot to agitate the water’s surface, creating bubbles, which they bind together with mucus. This mucous-bubble mixture hardens to form a floating raft, which the snail attaches itself to. If it becomes partially detached from its raft, the storm snail quickly creates new bubbles with its foot by extending it towards the surface, rapidly folding it to encase an air bubble, and coating the air bubble in mucus secreted from the propodium (anterior part of the foot). Creating a mucous-bubble takes roughly 10 seconds and the snail can make about 10 in a row before pausing.

Storm snails feed off a variety of floating creatures such as by-the-wind-sailors, blue buttons, and Portuguese man o’ wars. They are preyed upon by fish, birds, sea turtles, and other mollusks, including nudibranchs. The snails sometimes have pelagic barnacles attached to their shell as hitchhikers. Storm snails are protandric hermaphrodites, meaning that they begin life as a male and later change into female. In oviparous species, females collect eggs within the bubble raft until they are fertilized and eventually develop into larvae. In ovoviviparous species, such as Janthina janthina, fertilization is internal, but there is no direct mating. Instead, the males release their sperm into the water, hopefully in the general direction of a female, where the sperm fertilizes the eggs. The eggs develop internally and the young are born live, with the tiny purple snails immediately able to build their own rafts.

The purple dye produced by storm snails when they are irritated has been theorized by some to be a possible source of tekhelet, a dye used in Jewish tradition, such as in the clothing of the High Priest, the tapestries in the Tabernacle, and the tzitzit (ritual fringes) affixed to the corners of one's four-cornered garments. A different marine snail, Hexaplex trunculus (historically known as Murex trunculus), is generally accepted as the producer of this dye, but there is still some ongoing debate due, in part, to the accounts of Israeli Chief Rabbi Herzog in the late 1800s. Rabbi Herzog wrote a dissertation on the identity of the hillazon, the biblical creature responsible for tekhelet. While in his dissertation, he does seem to champion Murex as the genera to which the hillazon could belong, a letter he later wrote suggests that Rabbi Herzog believed Janthina to be the true candidate.

The dye from Janthina janthina is an astaxanthin-protein complex derived from one of the storm snail’s main sources of food: by-the-wind-sailors (Velella velella). Astaxanthin by itself is a carotenoid with a deep red hue, the same pigment that gives wild salmon their color. In by-the-wind sailors, it becomes a blue/purple carotenoprotein, depending on temperature and salinity. The processing of this blue carotenoprotein by the storm snail results in a dye that is practically identical to the pigment produced by Hexaplex trunculus. Interestingly, astaxanthin is sold commercially as a medical supplement rather than a pigment, specifically as a powerful antioxidant that can pass through the blood-brain barrier. The astaxanthin for this industry is harvested from microalgae grown in large outdoor bioreactors. In 2002, a researcher documented that he extracted the dye from living storm snails by irritating the snails to produce dye from their hypobranchial gland. Supposedly, a single snail can secrete a whole ounce – but perhaps algae are easier to farm than angry snails.

Where I learned about purple storm snails, and you can too!

World Register of Marine Species

Texas Marine Species

Current Biology

Science Direct

Science Daily

Nova Southeastern University

Florida Museum

Australian Geographic

Australian Museum

National Shell Museum


Zooplankton Guide

Project Noah

Atlantis Diving

Yeshiva Academic Institutional Repository

My Modern Met

Smithsonian Magazine

Caller Times