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Snails are members of a class of mollusks known as Gastropoda.  The word is derived from the latin words for stomach and foot.  So, these are "stomach-footed" animals, and I can't think of another species that typifies this description as much as the moon snail does.  

Nearly all marine snails move using a muscular foot that oozes over the ocean bottom, lubricated by a continuously laid down layer of slime. In the moon snail's case, this foot can be expanded to a ridiculous size by engorging it with water. The foot expands to the point where it actually engulfs the entire shell.

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The moon snail's foot not only helps it to glide along the surface but also to dig underground, a place where this animal spends most of its daylight hours. The shot above is spectacular in the way that it shows how this snail digs. Basically, the tip of the foot is inserted into the sand. This underground part is then filled with seawater and, acting like an anchor, is contracted.  Instead of pulling the foot tip out of the sand it actually pulls the rest of the body down under with it. As more of the foot is dug in more of the foot is filled with water, and thus more of the animal is pulled underground. It's like willfull quicksanding.

Once the animal is buried it can move horizontally underground by selectively filling different parts of the foot with water to pull it through the sand. But why travel this way? One reason is for protection, and the other is to find its favorite food.

Snails protect themselves by being able to withdraw the entire body into the hard spiral shell. Looking at the two photos above you may wonder how it could possibly fit that foot into its shell. Well, it isn't easy, and living mainly underground this snail normally doesn't have the need to do so. But it can be done. The expanded foot is mostly water and the muscular foot is, compared to most species, very porous.  When retracting into the shell, it simply squeezes the water out of the tissues, much like ringing out a sponge. Once back inside the shell the opening is sealed shut with a chitinous "trap-door" structure called an operculum.

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Operculum of a moon snail

Moon snails are predators on bivalve shellfish. They travel just below the surface in a relentless search for clams to feed on. Once they have captured their prey, they surround the bivalve with the foot and position the clam so that the snail's mouth comes in contact with the outer shell. Then the radula goes to work.

Nearly all snails possess a radula.  It is the toothed structure that allows herbivorous snails to scrape algae off a rock and a scavenging snail to tear off chunks of flesh from a dead fish. It is also the tool used by snails like this species to drill a hole right through the hard shell of a clam.

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Clam shells with perfectly round holes in them, like the one above, are common sights on ocean beaches. This is a moon snail victim. The radula is often described as a tooth that acts like a drill. A better analogy is to picture a chain saw with alternating "forward" and "reverse" settings. The structure is basically a band of flesh studded with dozens of teeth. Whether feeding on algae or drilling shells, this organ works by scraping the surface back and forth repeatedly. With each forward and reverse pass the teeth wear away at the substrate (the teeth erode as well, but these are quickly regenerated).

In drilling snails like this one, the scraping action is aided by the addition of an acidic enzyme which dissolves calcium, making the process of boring through a quarter-inch of mineral shell protection that much easier. Once the hole is made the tubular mouth is inserted into the clam and sucks out the insides. The radula deserves a diary all its own, but here is a microscopic view to give you an idea of how it might work.  Credit embedded in the photo.

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There are two common species of moon snails here in New England: The Atlantic moon snail (Neverita duplicata ) and the northern moon snail (Lunatia heros ). Their structure and behavior are nearly identical, so to distinguish the two I'll have to go off on a tangent and discuss bellybuttons for a second.

All snail shells, as they grow, revolve around a central columella, or axis.  In young snails this growth results in a hollow tube running through the center of the shell called an umbilicus.  Normally this "navel" is filled in with calcium as the animal grows. In the northern moon snail this umbilicus is never filled in.  In the Atlantic moon snail it is, as you can see below:

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Now, on to the sand collars. These delicate structures are the egg cases of this snail. When a female is ready to lay her eggs, and she lays several thousand at a time, she rises above the sand and encases her entire shell with the foot.  Cilia on the foot pick up individual grains of sand and spread them over her body until she is covered with a grain-thick layer. These sand grains are then cemented together with mucous. So she is now encased in a single layer of cemented sand grains.

Next the eggs are produced and, again using the cilia, are evenly distributed between her flesh and the sand layer.  Once she's done laying the eggs she creates another layer of sand, again cemented together with mucous, between her foot and the eggs. The end result is a paper-thin sand sculpture molded into the shape of her foot-covered shell and made up of two layers of sand with a thin layer of eggs in between them, completely surrounding the mother's body. Now she needs to escape. Using the foot action described above, she digs straight down beneath the sand and ducks underneath her creation, leaving this "sand collar" behind:

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When wet the collar feels rubbery. Once it
dries it becomes very brittle and will crumble
at the slightest touch.

After a few weeks the eggs begin to hatch and the thousands of larvae emerging break through the collar and disintegrate it.  The larvae are planktonic for a brief period before the shell begins to form. They then settle to the bottom and start the cycle all over again. And unlike some of the animals described in previous diaries, who die after spawning, the mother now goes on a clam-killing rampage to recover all the energy lost during this ordeal.

Other diaries in this series can be found here.

Originally posted to Mark H on Fri Sep 15, 2006 at 05:55 PM PDT.


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