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 atlanticus. Glaucus 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):
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)