Posted by: Rob Viens | March 27, 2012

Purple Snails and Blue Slugs

No diary entry on March 27th, but plenty of interesting things floating around in the ocean for Darwin to notice.  Although he described them in his Zoological Notebook yesterday, I want to spend a little time today investigating the little floating snail which belongs to the genus Janthina. Let’s start with Darwin’s description:

“The sea in Lat 18°6′ S & Long 36°6′ W. on the 26th contained numbers of specimens of Janthina.— Most of them were very small: the animal of rather a larger shell protruded itself & was of the same violet colour as the shell.— When touched emitted a fine purple colour.” (Zoological Notebook, Mar 26)

The most common species of Janthina, the type Darwin probably saw, are the species Janthina janthina. These snails have a violet color that gives them their common name – “purple snails”.  Purple snails live in the surface waters of the open ocean (called a pelagic life style).  Think about this for a minute – most of the snails we see live on the bottom of the ocean (or crawl along the rocks and glass walls of our aquariums).  So how can they spend their life floating in the surface waters of the ocean?

Janthina that washed up on the beach (from Wikipedia Commons):


Good question – and one with a fascinating answer!  These snails secrete a substance (chitin) that hardens around little air bubbles. Collectively this mass of clear, hard bubbles form a raft (though it almost sounds to me like the analogy should be a bunch of helium filled balloons).  The snails then attach their “foot” to this raft and spend their life floating around on it. If they lose hold of their “raft” the snail will sink to the bottom and die. They truly are perpetual castaways – their fate tied to the whims of the ocean’s current.

Purple snails are also carnivorous, feeding almost exclusively on their favorite food –  the medusa of cnidaria. (Cnidaria include jellyfish, anemones, and corals. Their larval stage, which looks like a small jellyfish, is called a medusa.)  So in a sense, these patches of little purple snails Darwin observed, are like “herds” of small hunters scouring the ocean surface for prey. To the snails, the ocean is so large that it must feel like floating in an infinite universe of blue. Though, unlike human hunters, they have no means of propulsion, so they must drift until their prey finds them.

There are other exotic types of pelagic gastropods that I discovered while researching Janthina.  One of them, found more commonly around the coasts of Africa is the sea swallow – Glaucus atlanticusGlaucus is a pelagic sea slug (a nudibranch) that floats because of special air sacs in its stomach. Sea slugs (and “land” slugs for that matter) are basically snails without a shell – both belonging to the class of mollusks known as gastropods.

Like Janthina, the sea swallow is carnivorous and even has an immunity to the venomous cysts of the Portuguese Man o’ War (another cnidaria) that it likes to eat. Ironically it is also eats purple snails. Another very cool thing about this swimming slug – when it eats jellyfish, it extracts their venom and concentrates it in little sacs in it the “fingers” of it’s “wings” and “tail”. Since it concentrates the venom, Glaucus can produce an extremely toxic sting of its own.

Sea Swallow/ Glaucus atlanticus (from Wikipedia Commons):

Glaucus atlanticus

Both Janthina and Glaucus exhibit counter shading – they have light-colored “bottoms” and dark colored “tops” (think penguins).  This is a form of camouflage.  Predators looking up from below see them against a light colored backdrop of the sky, while predators from above see them against the dark color of the sea.  If they were all light- or dark-colored, they would be vulnerable from one of the two directions. Our friend evolution at work…

I never cease to be amazed by the beauty and wonder of the natural world. These gastropods are truly amazing examples of life’s ability to adapt and take advantage of all kinds of living conditions. I would love to sea either of these floating gastropods in the wild – a sort of drifting work of art.

Tomorrow – Darwin finds more stuff floating in the sea… (RJV)


  1. Ok, maybe biology is cool after all.

  2. I’d never seen even a picture of this “sea swallow”. What an interesting and beautiful slug. I have seen many nudibranchs of extraordinary colors. They are common yet quite varied in Monterey Bay where I used to dive. Often the bright colors and various color patterns are there as an evolved protection for the creature and for others that may try to eat them since many are very toxic.
    The most common in Puget Sound are lemon nudibranchs and they are a beautiful lemon and white. The most interesting I’ve seen was a “Spanish Dancer”. It’s a free swimming nudibranch which is a very luminescent purple, blue and muave color. It swims along using two folds (one on either side) which stretch the length of its body (sometimes as much as a foot long). These “wings” look sort of like “Dumbo’s ears”. Pat

  3. Ah yes, the enormously diverse world of the invertebrates! Close your eyes and think of an animal. Is it a dog or cat or even lions, tigers and bears (oh my!)? While invertebrates account for some 95% of known animal species, they rarely make it to our radar screens and most likely do not have the support groups that rally around pandas, whales and other animals (Save the Cnidarians!).

    Rob writes of the beauty and wonder of the natural world and of the adaptations (e.g., counter shading) on display everywhere. I like to tell students that some adaptations are big turning points in evolution. Take the cnidarians, for example. These hydras, corals and jellies have diversified into both sessile and motile forms, including the medusae, the favorite food of the blue snails. Yet, the cnidarians have used the same simple body plan for over 500 million years, including a single opening to its gastrovascular cavity (“stomach”). Yes, this opening serves as both a mouth and anus (perhaps this is where the expression, “talking shit” came from…). And now imagine evolution working its wonders to the point where you GET TWO OPENINGS TO YOUR DIGESTIVE TRACT? Imagine the compartmentalization that can get started…You can be finishing your morning’s coffee while eliminating the end products of last night’s meal…

    • Thanks Jim – my morning coffee just doesn’t taste the same today!

      But I agree – there are so many interesting events and changes in the history of life on Earth that it makes our field of study full of wonder. When you take all of the history of life on Earth into account, Darwin’s phrase “endless forms most beautiful” takes on a whole new meaning.

  4. […] (See some of Darwin’s encounters with sea slugs in Darwin and the Sea Bunnies and Purple Snails and Blue Slugs.) They are all basically snails without  shells, though the land slugs have the ability to breath […]

  5. […] Penguins have another classic adaptation – counter shading.  Their black and white color is a form of camouflage.  A predator looking down on a swimming penguin would see its “dark” back side, which blends in with the surround ocean.  A predator from below would see its “white” front side, which blends in with the sky above.  (To read about gastropods that have counter shading see Purple Snails and Blue Slugs.) […]

  6. I have seen a small purple land slug in central arkansas in a small town call zent. It was discovered while on a nature walk with some kids, we decided that if it is an undiscovered genus it should be called the. ” purple kaylas slug” after kayla who saw it first.

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