Tuesday, August 11, 2009

Why are Seashells Different Colors?

Seashells are some of the most beautiful, mysterious and interesting products of ocean life (at least in my humble opinion). They have been incorporated into human societies for thousands of years as tools, art, jewelry, and money. Sometimes even as religious icons or symbols.

The colorful shells that we pick up in the sand on beaches are created by marine mollusks which are generally defined as soft-bodied invertebrates that create calcified shells for protection, camouflage and/or a place to live. Typically we're referring to snails, sea slugs and even squids and octopi (which do not have hard shells, but are still part of the mollusk family). The shells are composed mostly of CaCO3, or calcium carbonate, which is also found in rock, eggshells, and pearls and is the main cause of hard water. But, CaCO3 is white in color. The colors we see in seashells are often caused by impurities and waste from the organism captured in the shell when it is formed. Coloration is dictated mostly by diet and the water environment the creature lives in. For example, cowries that live and feed on coral, have shells that take on the same hues as the coral. This natural chemical reaction also helps to protect them from predators by allowing them to blend in with their environment.

What about all those iridescent shells that are multicolored and shiny? There's a different process that creates that effect. This iridescence is called mother-of-pearl and is caused by a coating of nacre on the shells. The nacre is secreted by oysters and some other mollusks to protect their bodies from parasites and disease. The nacre coating is very thin, in the range of several hundred nanometers thick, but is very strong and resilient. This nacre is also the same substance that composes pearls.


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