Mollusks are invertebrate animals with shells made of calcium carbonate (CaCO3). This phylum includes cephalopods (squid, octopus, cuttlefish), gastropods (snails, slugs), and bivalves (clams, oysters, scallops, and mussels). The subject of this week’s SEM Images of the Week is a mussel shell I saved from dinner a month ago. I bought it at the Whole Foods on Columbus Ave and 97th Street in Manhattan near my apartment. I purposely decided to eat mussels that day just so I would have a shell to place in the SEM. Mussel shells are hard and low in moisture, which makes them perfect for the high voltage, high vacuum environment inside a working SEM.
Calcium carbonate comes in one of two polymorphs — two different geometric arrangements of the calcium and carbonate ions — calcite and aragonite. All mollusk shells are made from aragonite. So are pearls, coral, and bird eggs. Followers of this website should expect to see other examples of aragonite appearing in the future (eggshells leftover from breakfast, snail shells from lunch in Paris, coral from my next trip to the Great Barrier Reef, pearls from Marge Simpson, etc.). The aragonite in this mussel shell formed crystals of varying shapes — prismatic rods; layered sheets; bristly mats; and soft, rounded hexagons.
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| The tip of a broken edge. | Zoom in on the broken edge. The shell is made of layers of aragonite | |
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| Around the lip of the shell, the aragonite is arranged in rods. | In some regions, the aragonite crystals are loosely arranged into short spikes that remind me of a rough doormat. | Inside where the mussel lives, the aragonite appears as stacks of soft, rounded hexagons. |
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| A single aragonite rod sitting on a bed of rod ends. | Shellfish Harvest ID Tag. These live blue mussels were cultivated on ropes in waters off Prince Edward Island, delivered to a distribution center in Pigeon Cove, then purchased, steamed and eaten in New York City. | |
Image credit: Glenn Elert