Walking along the high-tide line and looking for shells is a quintessential beach holiday pastime. There is something very special, very memorable, about strolling across the sand and searching for miniature treasures from the sea. Some days you’ll be lucky and gather more exquisite shells than your pockets can hold, while on other days the fun is more in the searching than the collecting.
But have you ever given a thought to what exactly it is that you’re picking up? What are shells, and how did they form? It’s something one writer looked into after her hammock chill-time was interrupted by the noise of hermit crabs’ shells clanking together.
I didn’t know it then, but these little hermit crabs will change shells between five and 10 times in their lifetime, finding a suitable home each time they outgrow their old one. The criteria: a shell that’s large enough for them to withdraw into, but light enough to carry around. The hermit crabs need the protection of shells in order to survive because, unlike the crabs you see scuttling along the edge of the tide, their abdomens are soft.
A seashell becoming home to a hermit crab is a fine marine example of “recycle, reduce, reuse”. The shell’s first incarnation is as the exoskeleton of a mollusc, which is an invertebrate – an animal with no backbone – that has a soft body protected by a hard case (like snails, clams and oysters). But the mollusc didn’t begin its life with a shell on its back. Depending on the species, most of these little organisms hatch as larvae from eggs, then feed on floating food particles until they’re ready to sink to the sea floor and begin the process of metamorphosis.
How the mollusc grows its shell in this phase and through the rest of its life is a complex chemical process that takes place via the mantle, where proteins are secreted. The proteins form a structure onto which calcium ions (collected from the mollusc’s environment through its gills, gut and epithelium) bind, forming a complex matrix of calcium carbonate crystals. The shell grows in layers from the outer edge, and folds in the mantle affect the sculptural appearance of the shell, forming ridges, spines or scales.
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