PHOTOGRAPH BY JOEL SARTORE

There is something fundamental about shells’ aesthetics that pleases the brain, that must be very powerful. This is not just symbolic thinking. It’s this very modern sense of what is beautiful.

Seashells are the work of marine mollusks, the most diverse group of animals in the oceans. They inhabit worlds tiny—​spiraled Ammoni-cera washing up on beaches around the globe with exquisite stripes too small to admire; and worlds vast—​Tridacna gigas, or giant clams, weighing hundreds of pounds and glowing with millions of microalgae.

Marine mollusks settle on reefs and rocks and seagrasses and sandy beaches and mudflats and countless places above and below. The violet sea snail Janthina janthina lives only on tropical surface waters, a molluscan Huck Finn floating on its own bubble raft. If anything happens to its homemade boat, the purple-​shelled Huck will sink and die. Thin-​spindled Tibia fusus hunkers deep in the sand thanks to its siphon that draws water for respiration through a long, thin shell canal like a hypodermic needle from a vial. The carrier snails, Xenophoridae, cement other shells, bits of coral, and even little pebbles onto their own shells in elaborate camouflage.

Marine mollusks are vegetarians and cannibals, fish hunters and filter feeders, algae distillers, and carrion eaters. They are sedentary blobs that leap and swim. Shy beings that create the showiest architecture of all time. Squishy invertebrates that make some of the hardest building materials known. Vulnerable species with the longest evolutionary history of any living today.

A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer usually created by an animal that lives in the sea. The shell is part of the body of the animal. Empty seashells are often found washed up on beaches by beachcombers. The shells are empty because the animal has died and the soft parts have been eaten by another animal or have decomposed.

A seashell is usually the exoskeleton of an invertebrate (an animal without a backbone), and is typically composed of calcium carbonate or chitin. Most shells that are found on beaches are the shells of marine mollusks, partly because these shells are usually made of calcium carbonate, and endure better than shells made of chitin.

As mollusks live their daily lives in the sea, they take in salts and chemicals from the water around them. As they process these materials, they secrete calcium carbonate, which hardens on the outside of their bodies and begins to form a hard outer shell.

Although its shell is attached to it, it’s not part of the living body of a mollusk. This is because the shell is formed from minerals, not mollusk cells. As mollusks continue to excrete calcium carbonate, their shells continue to grow. When a mollusk dies, it leaves its shell behind for you to find along the sea shore.

A mollusk’s shell material, called nacre, is mostly calcium. However, it also usually has an outer layer of hard scleroprotein, which is similar to human fingernails.

a bright orange and pink sea shell
Credit: Photo by Megan Davis, Florida Atlantic University Queen Conch Lab

The above sea shell is called The horse conch.

Horse conchs are a class of mollusk called gastropods. Gastropods are an extremely diverse group of animals. They are one of the few groups that can live in salt water, freshwater, and on land. They can be carnivores, such as the horse conch, herbivores, or omnivores. In some tropical waters, hundreds of different species of gastropods can be found in a coral reef.

Fun Facts

Seashells have played many roles throughout history, from money to jewelry. But they also hold secrets of the ocean’s health.

Seashells were money before coin, jewelry before gems, art before canvas.

Like a hand cupped to the ear, it picks up ambient noise in the environment—​amplifying exactly what’s going on around us.

Shells are the most-​collected naturalia along with rocks; easier to amass than butterflies and more affordable than gemstones. They are collected by children and by kings.

Use of gastropod shells, specifically cowries, in traditional dress of the Kikuyu people of Kenya, Africa .
Angela Sevin – originally posted to Flickr as Kikuyu woman traditional dress