A day at the beach isn't complete without a sandcastle decorated with shells or some in your pocket to bring home. Adorning bathroom shelves and windowsills, seashells are not only reminiscent of a visit to the sea, but have many stories to tell about the animals that made them and the wonders of their aquatic world.
Shells have so many stories to tell us about the sea, what lives there and how it's all connected – often in unexpected ways. Every day, as the tide rises and falls, new shells arrive on the beach, waiting for us to come and find them. Next time youget a beautiful shellon the shore, take a moment, as you admire its shape and form, to think about who made it and the life it might have had.
Any shell, after all, was once part of a living, breathing sea creature. But what exactly is it? How are they formed? And what animals use them? Our expert guide has all the answers...
What are shells?
Every time you pick up an empty shell, you're holding the abandoned exoskeleton of a mollusk, which these soft-bodied animals use as a multipurpose tool. This is their home, the place where they hide and the attachment point of the muscles that help them move. There is a wealth of seashells to be found, made from different types of molluscs, in habitats all over the UK coast.
Rocky shores and tide poolsthey host many marine snails (gastropods) with elegant spiral shells, including dogfish and snails. When alive, a sea snail pokes its tentacled head out of the open hole in its shell and crawls on one muscular leg.
At low tide, coat-of-mail butterfly-like tunicates crawl under the rocks, their shells in eight plates on their backs. Sandy beaches are the domain of cockles, clams and other types of bivalves, each of which bears a pair of wrinkled and fan-shaped shells that squeeze tightly together to keep their soft bodies tucked inside.
Many bivalves burrow deep into the sand and reach the water with a breathing tube, called a siphon, to breathe and filter out tiny particles. You will often find single, bivalve shells or pairs still attached to each other. Sometimes, you'll stumble upon bean clam shells scattered on sandy beaches like thousands of purple, orange and yellow butterflies.
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How are shells made?
Unlike crabs, lobsters and other crustaceans, which usually shed their shells and grow new, larger ones, molluscs keep their shells throughout their lives and gradually expand them. Look closely at the innermost whorl of a sea snail and you should be able to make out the tiny shell it had when it first hatched from an egg.
As it grows, a mollusk uses its soft body tissue (the mantle) to lay down more layers of shell composed mostly of calcium carbonate, the same chalky material as the hen's egg shell. Like tree rings, shells have seasonal growth lines that are sometimes visible from the outside. Counting the most prominent lines on a bivalve shell can give you an idea of how old it was when it died.
Many marine molluscs live for several years. some for much longer. Oceanic quahogs from the North Atlantic can live for centuries. One person, nicknamed Hafrún, which means "mystery of the ocean" in Icelandic, lived for 507 years, making him one of thethe longest living animalsthat his age had ever been accurately measured.
Scientists use shells, especially long-lived ones, as climate records that contain a wealth of detailed information about the changing ocean. The shells can tell the previous temperature and acidity of the seawater in which the molluscs grew. They can even indicate when volcanoes have erupted or hurricanes have hit.
Why do shells have different shapes?
Shells come in a seemingly endless variety of shapes, but in reality they are all versions of the same basic design: a spiral. This is obvious in snail shells and less so in clams and other bivalves, but their shells are indeed spirals, only the ones that open wide.
The precise shapes of the molluscs' spiral shells are a nod to their different habitats and ways of living and moving.
Scallops rest their flattened, fan-shaped shells on the sandy bottom. Occasionally, they mix in the water column and swim short distances by banging their shells together like castanets and shooting propulsive jets of water through their siphon.
Turret or auger shells are long and thin, resembling small unicorn horns, enabling them to burrow down into sand and mud.
Butterflies have volcano-shaped shells that clamp tightly to rocks so they don't get swept away by waves or pecked. Their conical shells are difficult to grasp and they use their foot as a suction cup and stick in place with specialized slime.
Molluscs often enhance their defenses by adding grooves and spines to their shells, making them difficult for predators to handle. Spiny oysters in the Mediterranean are covered in teeth that encourage algae and sponges to settle and grow, giving them camouflage on rocky reefs.
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How do living things use shells?
Humans are not the only animals that collect shells. Keep a close eye on the shells in a rock pool and you might spot some that aren't sliding slowly around, but falling.Hermit crabsthey have lost the ability to make their own shells and must borrow permission.
When it comes time for the hermits to find a larger shell, they organize themselves into neat queues, with the smallest individual at one end and the largest at the other. Two large crabs may fight over the bigger, better shell. Once their contest is decided and the winner claims the prize, all the other smaller crabs move into the empty shell of the next crab in line.
Down in the deep sea, transport clams (Zenophoridae) pick up empty shells, as well as pebbles and coral fragments. They stick these pieces on the outside of their shell as an added defense against predators.
And the smartest molluscs of all, theoctopuses, have learned how to use shells as tools. Like hermit crabs, the ancestors of octopuses gave up the ability to make shells long ago. But today, they can sometimes be seen with a pair of matching clam shells in their possession. When the octopus feels threatened, it quickly turns it into a shelter.
How do shells get their colors and patterns?
Many molluscs add a shiny layer of mother-of-pearl, or pearl, to the inside of their shell. Abalone are some of the most lustrous shells, their insides are coated in oily greens and blues, and people often use them to make jewelry, buttons, and decorative inlays for furniture.
However, the glistening interiors of these shells evolved not for beauty, but for strength. The engineers discovered that the pearl's nanostructure helps make the shells resistant to cracking. Nacre, made of essentially the same calcium carbonate as the outer shell, is laid out in a series of tiny brick-like layers held together in an elastic matrix of chitin—the main building material of insect exoskeletons. This can stop cracks from spreading when a crab grabs a shell in its claws or when a fish bites. If you find a seashell that is shiny on the outside, it has worn away in the crashing waves, revealing the trinket underneath.
Other molluscs secrete pigments into their shells that help conceal them in their habitat. Flat myrtles have rounded shells that resemble the green and brown gas chambers of algae that live along rocky shores.JanthinaThe snails float beneath the surface of the sea, hanging from a raft of air bubbles, and their shells are a deep purple-blue, camouflaging them in open water.
Meanwhile, the colors and patterns on many shells remain far more mysterious and serve no obvious purpose. The radiated Artemisia, for example, is a bivalve common along European coasts with a zigzag-covered shell. It spends its time crouched deep in sand, mud and gravel, so these patterns are not seen until empty shells wash up on a beach.
Why do the shells have different designs?
No one has yet figured out exactly why molluscs evolved their shell design. One idea is that they use their designs as a marker to guide them as they lay out more shell material.
Shell making is not a continuous process. A mollusk needs to remind itself where the previous growing season left off in order to align its mantle and continue growing in the right places, otherwise it could easily develop a useless, irregular shell.
Can shells tell us what molluscs eat?
There are signs to look for on shells that will tell you what kind of food their molluscs ate when they were alive. Herbivorous algae-eating snails, such as snails, tend to have smooth, rounded openings in their shells. You can spot predatory snails by the notch in their shell. This is where their elongated nose (or proboscis) sticks out and sniffs the water for chemical traces coming from their prey. A sea snail follows its nose and crawls onto its target.
Main image: Cocklesell on the beach. © Getty