What Is A Shell?
Shells are lovely natural objects, equals in beauty to any flower or butterfly, they are more than just pretty baubles found on beaches. They are the exterior skeletons (exoskeletons) of a group of animals called mollusks. The word "mollusk" means "soft-bodied;" an exterior skeleton is very important to these creatures, providing them with shape and rigidity, and also with protection, and sometimes camouflage, from predators.
Mollusks are classified into major groupings according to the characteristics of their shells. Snails (Gastropoda) have a single shell which spirals outward and to one side as it grows. Most Cephalopoda (octopi and squid) have no shell, but the Chambered Nautilus of that group has a shell. This shell does coil, but it coils flatly, in a single plane. Tusk shells (Scaphopoda) also have a single shell, but it does not coil at all; it grows in a narrow and very slightly curved cone shape. Bivalves (Bivalvia), including oysters, clams, scallops and mussels, have two parts to their shells that enclose their tender bodies like the two halves of a hinged box. Chitons (Polyplacophora) are little armored tanks, with a row of eight overlapping plates protecting them. The Neopilina (Monoplacophora), are deep-sea "living fossils;" they have a single shell which hardly coils at all, but fits over their bodies like a protective cup. (Some gastropods (the limpets) have shells like this too, but their body structure is very different.) Last are the deepsea worm-like Aplacophora, with no shell at all, but little calcareous spines on their bodies.
Mollusks make their shells from calcium they derive from their environment, either the food they eat or the water they dwell in. When a tiny mollusk hatches from its egg, it comes into the world equipped with a tiny shell. This shell is actually a part of the animal, growing as it grows, accommodating its needs. Each different species of mollusk makes a shell that is, in most cases, unique to it alone. Indeed, this uniqueness of form is partly what allows amateur shell collectors (conchologists) and professional scientists who study mollusks (malacologists) to determine a mollusk's species. Each species is destined genetically to develop the same type of shell its progenitors did. But, just as with humans, there are many distinct differences. Food, climate, environment, accident, and the mollusk's particular heredity all play their parts in making each shell an individual.
Just as important as protection and rigidity, is the assistance a shell renders its maker in pursuit of the necessities of its life. Some shell shapes are streamlined for ease of burrowing through mud or sand. Some bivalves are heavily ridged in ways that help them stay anchored in the bottom where they live. Others are assisted in burrowing by special shell sculpture. There are shells that grow long spines to entrap and encourage growth of camouflaging seaweed and corals. Others, like cowry shells, are smooth and polished and highly glossy with bright interesting patterns on their shells. Often the animals themselves are patterned as well. When a predator disturbs the animal, it withdraws inside its shell, revealing a surface that is very different in color and pattern than the one the hungry predator had its eye on. It is speculated that quick change artistry has a startle effect on the predator. (And still other mollusks, like the slugs, squid and octopi, and those lovely creatures, the nudibranchs, have given up building a shell altogether, relying wholly on other defenses.)
Indeed, a great deal may be learned about a mollusk's individual life by examining its shell. Many bear old healed-over breaks and chips that speak of a battle won with some predator. Others show color change which testifies to a change in diet or water chemicals. Mollusks that are very old in snail-years may become greatly thickened and dulled in color. Rough water conditions discourage the growth of long spines on normally spiny species, while quiet habitats allow extravagant spiny extensions. (It is now believed that this effect may have to do with a mollusk's different diet in rough water.) Sometimes a shell may have so great an encrustation of marine organisms on its shell that locomotion becomes impossible and the animal starves to death.
And shells aren't found only on beaches. They can be found anywhere mollusks live. Both gastropods and bivalves inhabit rivers and streams and lakes, where they have made adaptations to freshwater living. Many species have also adapted to life on land, and can be observed in leaf litter, in trees, on plants, under rocks and buried in loose dirt. In the ocean, mudflats and mangrove areas are homes to hundreds of mollusk species. Some live on the tree roots and branches just at water line. Others bury themselves in the mud or sand bottom, emerging to feed, often at night or at low tide. There are mollusks which live only on coral reefs, and those which inhabit the ocean depths. Some, like the slit shell and the Neopilina, were thought to be extinct until they were rediscovered in these depths. Shells live on seaweed, on coral holdfasts, under rocks, buried in the roots of undersea grass beds. Some shells even float throughout their entire lives on a raft of bubbles. Each has a specialized habitat and an ecological niche.