All About Seashells

The following is an article from the book Uncle John’s Perpetually Pleasing Bathroom Reader.

If you put a large seashell to your ear, you can hear the ocean! Not really. What you hear is the shell amplifying the ambient noise around you. But it’s a wonderful thing to believe when you’re a kid. Here are some other fascinating facts about nature’s most curious— and beautiful—“ living houses.”

REMAINS OF THE DAY

Seashells come in a vast array of shapes, colors, and sizes, but they all have one basic (and creepy) thing in common: They’re the partial remains of dead animals. Finding a seashell is the equivalent, in a way, of finding a human skeleton on the beach. But seashells are the outer skeletons (technically, exo-skeletons) of their deceased inhabitants. Their soft remains have either been eaten or rotted away.

There are literally hundreds of thousands of animal species that grow and leave behind seashells, ranging in size from microscopic to sofa size. Most of what we think of as classic seashells were made by marine mollusks. That’s because they make the sturdiest, longest-lasting shells. Marine mollusks include gastropods— which include an enormous variety of sea snails; bivalves— such as clams, oysters, and scallops; scaphopods— that make tusk-shaped shells; and some cephalopods— such as the nautilus and spirula. (There are many types of mollusks that make no shells at all, including sea slugs and octopuses.)

BUILDING BLOCKS

All mollusks have the same basic body form: They have a head, which holds the sense organs; a visceral mass, or the internal organs; and a foot. And all mollusks— even ones that don’t create shells— have an organ known as a mantle, which comes from the Latin mantellum, meaning “cloak” or “cape,” so named because it sort of looks like a cape draped over the animal’s back. The mantle has the crucial job of containing the mollusk’s visceral mass. On mollusks that produce shells, it has another job: Build and maintain the shell.

Seashells are made up almost completely of the calcium-based mineral calcium carbonate. Animals that create these shells acquire the ingredients needed to make it— calcium, carbon, and oxygen— from their food sources and even from the water around them. Those ingredients are collected from the mollusk’s bloodstream by specialized cells in the mantle. They are then combined with different proteins made just for the job and secreted out of the mantle surface. The resulting material quickly hardens into shell.

CONEHEADS

You’ve probably seen seashells that have a coiled, spiral end (sort of like a soft-serve ice-cream cone) with an opening at the other end. These are the shells of gastropods— the snails and slugs of the world. (That also includes the snails in your garden, but we’re talking about the ocean variety.) The pointy tip of this seashell is the oldest part of the shell. Gastropods begin building them while they are still in a microscopic larval stage. That tiny shell is known as a protoconch (pronounced “proto-kahnk”), and it already has a spiral configuration— simply because gastropod DNA instructs the creature to create its shell in this way, the same way human DNA instructs your body to grow your bones as they do.

As the tiny gastropod grows, it adds to its protoconch by secreting shell material from its mantle onto the shell’s opening, causing the shell to grow in successively larger spiraling coils— to accommodate the creature’s growing body— down and away from the shell’s tip. (Picture a spiral staircase— one that grows from the top down, and gets larger as it goes down.)

Pictured on the right is the seashell from a type of sea snail known as a whelk. (It was drawn in 1878 by Dutch-Belgian macacologist— someone who studies mollusks— Pierre-Henri Nyst.)

• The top of the shell is a protoconch. It’s also known as the apex of the shell.

• The coils growing down from the shell’s tip are known as whorls. The top whorls make up the shell’s spire.

• The lower whorl is the body whorl. While alive, it’s where the bulk of the animal’s soft body resides.

• The opening is the aperture. It’s from here the animal is able to reach out of the shell to move, eat, and breathe.

• If you were to cut a gastropod shell open, you would see its columella— or “little column”— a column of shell material running from the apex down along the central axis of the shell, like the supporting pole of a spiral staircase.

THE OCEAN’S ARCHITECTS

Shell building is more than simply adding material to the lip of the opening. Mollusk shells are complex structures, made up of three distinct layers, each created in a different manner:

• The periostracum, meaning “around the shell,” is the thin hard outer layer. It’s made of a protein-based substance called conchiolin, and acts as a kind of varnish that protects the shell.

• The prismatic layer consists of prism-shaped (many-sided) crystals of calcium carbonate that are stacked perpendicular to the direction of the periostracum. This the thickest, strongest layer of the shell.

• The nacreous layer, or nacre, is the smooth inner layer that comes in contact with the creature inside. It’s made up of microscopic plates (another crystal form of calcium carbonate, called aragonite) laid like stonework and held together with gluelike proteins. The first two layers are created by cells at the leading edge of the mantle— at the outer lip of the shell’s aperture. The nacre layer, on the other hand, is secreted by the entire outer surface of the mantle. The mantle adds nacre to the inner surface of the shell throughout the creature’s life, making it thicker and thicker as it grows.



Cool Nacre Fact: Nacre is also known as “mother-of-pearl”— and is, in fact, what the outer layers of pearls are made of. Why is it so colorful, and why does it seem to change color if you move it? Because the microscopic plates of aragonite that make up nacre naturally cause the light that strikes it to diffract— giving the inside of seashells (and pearls) their natural iridescence.

CLAM UP

The other classes of mollusks all grow their shells in a way that’s similar to the gastropods. Bivalves such as clams and oysters, for example, begin to grow their shells while still microscopic larvae. Of course, with bivalves it’s not just one shell, but two nearly identical shells, or valves. A flexible band of ligament connects the two shells and serves as a hinge, allowing the creatures that inhabit them to open and close the shells against each another. Bivalves grow those shells by secreting shell material onto the outer edge of each shell— which means the shell closest to the hinge is the oldest part of bivalve shells, and the shell at the opening is the newest.

If you ever find horn- or tusk-shaped seashells— they actually look like little hollow elephant tusks with an opening at each end— they’re from mollusks known as scaphopods. They grow from the skinny end out to the larger end, and range from about one to five inches long. If you happen to see one alive, you’d probably just see its rear end— the skinny end of the shell. You can sometimes see them sticking up out of the seafloor. Scaphopods spend their lives with their heads— at the thicker end of the shell— buried in the seafloor, foraging for food and pooping their waste out of their rear ends into the water.

MORE MOLLUSKS

Conch: These are a family of about 60 species of large sea snails, some of which grow to nearly two feet long. They’re characterized by having rows of spikes on their shells and, when fully adult, a wide flaring outer lip. Conchs are among the world’s best known seashells, and you can play them like musical instruments: Cut the pointy end off the conch and blow into it like a trumpet.

Periwinkle: Periwinkle shells look a lot like garden-snail shells, but with quite pronounced and pointy spires and usually six to seven whorls. They come in a variety of colors and patterns.

Cowry: Cowry shells are egg-shaped and have a slit-like aperture than runs along the lower portion of the shell. The lips of the aperture are ribbed— making the opening look almost like a toothed mouth. The amazing thing about cowries, though, is that they are incredibly smooth and shiny and often spectacularly colored and patterned. In fact, the term porcelain comes from the word porcellana— the Italian name for the cowry shell.

Auger: You’ll recognize these shells, as they have a long, multi-whorled pointy spire above a quite small aperture. (They are so named because they are thought to resemble augers, or drills.)

Nautilus: These free-swimming cephalopods look like their cousins— octopuses and squids— except that nautiluses have shells. There are only six species; they make shells that range from about six to eleven inches in diameter. If you’re ever lucky enough to find a nautilus shell, you’ll know it: It’s cream-colored with reddish-brown tigerlike stripes. It has a spiral configuration, like the gastropods, but the spiraling coils grow out and around the shell’s apex, growing larger as they do.

SEASHELL MISCELLANY

• A few other animals that make seashells: tube worms— gastropods that make squiggly worm-shaped shells; and echinoderms— such as starfish and sea urchins. (Sea-urchin shells are called tests. A well known type of test: the sand dollar.)

• Next time you see a slug, look at it closely: That leathery patch on its back, near its head, is its mantle. (Slugs don’t use their mantles to make shells— but you can at least say you’ve seen one.)

• Cuttlebone is the elongated disc-shaped seashell of the cuttlefish, a squidlike cephalopod. What’s different from other seashells is that cuttlebone grows inside the cuttlefish’s body. Cuttlefish use their multi-chambered gas-filled cuttlebones for buoyancy control.

• Many gastropods have what’s known as an operculum (“little lid”). It’s a flat round piece of shell attached to the gastropod’s foot that can be retracted into the shell’s aperture, sealing the creature tightly inside. These beautiful seashells are also known as “cats’ eyes.”

• Scallop shells are ribbed and almost flat, resembling an open-handed fan. Botticelli’s famous Renaissance painting The Birth of Venus depicts the goddess Venus rising from the sea on top of a large shell. It’s a scallop shell. (So is the logo of Shell Oil.)

• Shell Beach, Western Australia, is entirely made up of the fragmented shells of clams known as cockles— the same “cockle shells” in the nursery rhyme “Mary, Mary, Quite Contrary.”

• Most gastropod shells are “right-handed”: The whorls grow to the right— clockwise— away from the protoconch. About 10 percent are “left-handed,” meaning they grow counterclockwise. Another species that has a 9-to-1 ratio of righties to lefties: human beings.

_______________________________

The article above is reprinted with permission from Uncle John’s Perpetually Pleasing Bathroom Reader. The 26th annual edition of Uncle John’s wildly successful series is all-new and jam-packed with the BRI’s patented mix of fun and information.

Since 1988, the Bathroom Reader Institute had published a series of popular books containing irresistible bits of trivia and obscure yet fascinating facts. If you like Neatorama, you'll love the Bathroom Reader Institute's books - go ahead and check 'em out!


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