Seashells have many stories to tell us about the sea, what lives there and how everything is connected. Every day, as the tide ebbs and flows, new shells are washed up on the beach. Next time you pick up a shell, take a moment to think about who made it and the life it might have had. All shells were once part of a living, breathing sea creature. How are they formed? What animals use them? And what will happen to them as our oceans warm ?
What are seashells?
An empty seashell is a mollusc’s abandoned exoskeleton. Soft-bodied animals use them as their home and protection, their place to hide, and an attachment point for their muscles to help them move. There are many different shells to find, made by different kinds of molluscs, all around our coasts.
Rocky shores are home to many sea snails such as limpets, dog whelks and periwinkles. Sea snails move by crawling along on a muscular foot. Limpets, like other gastropods have a toothed ribbon-like tongue called a radula, which they use to scrape away algae – leaving a zigzag pattern as they do so (see image above). As the tide goes out, limpets return to the exact same spot each time: their home scar. Their volcano-shaped shells are difficult to grasp, and they use their foot like a suction cup and glue themselves in place with specialised slime. These hardy creatures can withstand stormy seas and baking sun, and may live 20 years.
Sandy shores are home to many bivalves such as cockles and razor clams, each with a pair of roughly fan-shaped shells that clamp tightly together to keep their soft bodies safe inside. Sandy seabeds, with no hard surfaces for seaweeds to attach to, offer few hiding places, so many of these creatures live safely below the surface, extending a siphon (rather like a snorkel) to breathe and to feed (hastily withdrawn when danger is sensed).
How are seashells made?
Unlike crabs, lobsters and other crustaceans, which routinely shed their shells and grow new, bigger ones, molluscs keep their shells all their lives and make them bigger each season as they grow. If you look closely at the innermost whorl of a sea snail you may be able to see the tiny shell it had when it first hatched from an egg.
As it grows, a mollusc uses its soft body tissue, called the mantle, to lay down another layer of shell. Shells are mainly made of calcium carbonate. You can watch a short video here.
Like tree rings, seashells have seasonal growth lines that are sometimes visible from the outside. Counting the most prominent lines across a bivalve shell can give you an idea of how old it was when it died.
Many marine molluscs live for several years, and some for much longer. Ocean quahogs from the North Atlantic can live for centuries. One individual, nicknamed Hafrún, meaning ‘mystery of the ocean’ in Icelandic, lived for 507 years, making it one of the longest-lived animals to have its age accurately measured.
Scientists use seashells, especially long-lived ones, as climate archives that hold detailed information about the changing ocean. Shells can tell the past temperature and acidity of seawater the molluscs grew in; they can even indicate when volcanoes erupted or hurricanes struck.
Why do seashells have different shapes?
Shells seem to come in an endless variety of shapes, but they are generally versions of a spiral. That’s obvious in sea snails and less so in clams and other bivalves but their shells are spirals too, just ones that flare wide open. The precise shape of their shells are due to their different habitats and ways of living and moving.
Molluscs may also increase their defences by adding corrugations and spines to their shells, making them difficult for predators to handle. Spiny oysters in the Mediterranean are covered in prongs that encourage seaweeds and sponges to settle and grow, giving them camouflage on rocky reefs.
(Image: Wikimedia Commons)
How do seashells get their colours and patterns?
Lots of molluscs add a shiny layer of mother-of-pearl, or nacre, to the insides of their shells. This is designed not for beauty, but for strength, as its structure helps to make shells crack-proof. Other molluscs secrete pigments in their shells that help hide them in their habitat. Flat periwinkles have rounded shells that resemble the green and brown gas chambers of the bladderwrack seaweeds they inhabit along rocky shores. Janthina snails float below the sea surface, suspended from a raft of air bubbles and their shells are deep violet-blue, camouflaging them in open water.
Scientists have not yet worked out exactly why molluscs evolved their shell patterning. One idea is that they use their patterns as a marker to guide them as they lay down more shell material. Shell-making is not a continuous process, so a mollusc needs to remind itself where the previous growing season left off, so it can align its mantle and grow in the right places, or it could end up with a wonky shell!
Who else uses shells?
We are not the only shell-collecting animals. Hermit crabs have lost the ability to make their own shells and must instead borrow empty ones. When the time comes for them to find a bigger shell, they organise themselves into orderly queues, with the smallest individual at one end and the largest at the other. Two big crabs may tussle over the biggest, best shell. Once their contest is decided and the victor claims the prize, all the other smaller crabs move into the vacated shell of the crab next in line.
Down in the deep sea, carrier shells (Zenophoridae) pick up empty seashells, pebbles and coral fragments, gluing them onto the outside of their shells as extra defence from predators.
And the most intelligent molluscs of all, the octopus, has learned how to use seashells as tools. Like hermit crabs, octopuses’ ancestors gave up the ability to make shells long ago. But they have been observed using a pair of matching clam shells to make into a shelter if they feel threatened.
Based on an article by Helen Scales on BBC Discover Wildlife.
How will shellfish be affected by climate change?
Every year, the ocean is estimated to absorb more carbon dioxide (CO2) as than all the world’s forests do. That may seem like a good thing for humanity – the more the ocean can absorb, the less CO2 remains in the atmosphere to drive the greenhouse effect that’s behind climate change.
But scientists are concerned about all the CO2 the ocean absorbs. Absorbing CO2 makes the ocean more acidic – this is known as “ocean acidification” – and this makes it difficult for crabs, clams, and other shellfish to grow their shells as strong as they need to be, and they can die as a result of ocean acidity.
Our sea shell challenge:
can anyone beat Andy’s record of four wentletraps?
What are wentletraps?
Wentletraps are marine snails with unique shells that form a high, conical spiral and have deep sculptured ribbing. They sometimes have a porcelain-like appearance. Most are white and no more than 5 cm long. Their name in Dutch means spiral staircase.
Wentletraps are widely distributed, from the tropics to the Arctic and Antarctica. They prefer sandy or muddy sediments and are often found near anemones or coral, from which they get their nourishment. The common wentletrap (Epitonium clathrus) is found in European waters.
The precious wentletrap (Epitonium scalare) was a rare and desirable specimen, which exchanged hands for large sums of money in the 18th century. As they became more common, they were still valued for their beauty.
Further reading
There are many books about shells and the seashore, but here are a few enjoyed by Strandliners.
Shells:
Spirals in Time, The Secret Life and Curious Afterlife of Shells by Helen Scales
Shell Life on the Seashore by Philip Street
Shell ID Guide:
Wild ID Common Seashells of Britain and Ireland, Field Studies Council
Beachcombing:
The Essential Guide to Beachcombing and the Strandline by Steve Trewhella and Julie Hatcher
