by Lynn Scheu
When we think of shells, it is seashells that probably come to mind first, in a multiplicity of shapes, colors and patterns. Scallops, cowries, conchs and cones in rainbow hues. Lacy frills, elongate spikes, gleaming egg shapes, huge bowls and tiny rice grains, all characterize marine mollusks.
Mollusks first evolved in the sea and have been adapting to its changing ecological niches for nearly 600 million years. So it is not surprising that marine species exhibit the phylum's greatest diversity. Mollusca in general exhibit more morphological diversity than any other phylum, including arthropods. The vast difference between the giant squid and microscopic nuculid clams is unmatched in the animal kingdom.
All seven of the existing classes of the Mollusca are present in the ocean; only the Bivalvia (clams) and Gastropoda (snails) have moved into freshwater; Gastropoda is the sole class to adapt to life on dry land.
Invertebrates, animals without backbones, are commonly presented with a special survival problem -- soft bodies, easily snapped up by a hungry predator. The shells which mollusks have evolved function to help them with this difficulty by presenting a tough exterior to predators in search of a meal. It bears mention here that many marine mollusks get along fine without a shell, having adapted in other ways which help them escape being eaten, at least long enough to reproduce their kind. Squid are fast swimmers and many species travel in schools. The octopus jets out a blob of "ink" to distract and fool a predator. Many sea slugs are able to consume stinging cells of other animals like coral. They transfer these natural deterrents to their own body cells where they function to instruct predators to avoid their kind. But in general, the phylum has flourished through epochs of geologic time wearing hard shells that afford excellent protection against all but the most determined foe.
Mollusks have also adopted an amazing array of life styles and habitats. Some groups are carnivores, some are strict vegetarians, others are scavengers or parasites or commensals; the bivalves, for the most part are sedentary filter feeders, but some are predacious. Marine mollusks burrow, creep, tunnel, float or swim. They make their homes in mud, sand, silt, coral grit, rock, shell, tidal pool or grass.
With such a multitude of lifestyles, it is not surprising that any number of evolutionary experiments and accidents have proved successful, explaining in part the fabulous diversity of pattern, size, and form among marine mollusks. Pattern often functions as camouflage. Wide flaring lips on conchs can help protect extended, grazing soft parts. Elongate anterior canals can likewise protect a mollusk's vulnerable siphonal "nose," stretched out to detect food or foe. Extended teeth on some muricids function as pry bars to pop open the protective plates of their food, barnacles. Low, flattened shells on limpets can withstand wave pressure. The high spires on many species aid in streamlining. Convoluted apertures function to keep out would-be predators. Thickened ribs add strength to shells of many species without adding commensurate weight and "construction costs." Specialized slits on abalones and others send waste products away from their water intake. In some cases mollusks from many different lineages have produced similar shells, probably because of a similarity of lifestyles, and similar constraints on how to build shells using only a logarithmic spiral, using only what their ancestors gave them.
It seems to most of us that we'll never become familiar with all the types of mollusks in the sea. And then, just when we begin to feel comfortable, we discover the entire array of microscopic mollusks waiting for us to discover them, or the hundreds of millions of years worth of extinct shells, preserved as fossils, that occur in our rocks and wash out in streams. Or we find that we'd like to narrow our scope somewhat by specializing in a particular group of mollusks. There's always something new to hold our interest, some fresh discovery, or new scientific advances leading to changes in the taxonomy of mollusks. Some new family of bivalves catches our attention, or the intricacy of chiton plates (Class Polyplacophora) fascinates us, or we decide we'd like to specialize in the tusk shells, Class Scaphopoda, for a while. And we're off again, on a new pursuit of the magnificent diversity that is marine mollusks.