Is that scallop shell broken?

By Alex Scott

Scallop shells are beloved by beachcombers for their classic shape and beautiful colors, but often they are discarded because their auricles, or “wings,” look asymmetrical. However, the scallop shell has evolved over more than 400 million years to have two different sized auricles for a very specific purpose. 

The scallop family, known to biologists as Pectinidae, are mollusks with two “valves” or shells connected by a hinge, and with two auricles that protrude out from the hinge. Scallops rest offshore on the sea floor on what scientists call their “right” valve. This shell is usually a little larger than the left valve so that more of the scallop is buried in the sand when it is at rest. The auricles are labeled anterior and posterior, based on where they are in relation to the right and left shells. 

When they are young, some scallops attach themselves to rocks and sand with a string of filaments, called the byssus. This string of filaments sticks out from inside the shell through the anterior auricle in an area known as the basal notch. This auricle is typically larger to make room for the byssus, and in some species, you can see a little comblike structure inside the shell that helps to separate the byssus strings from each other.

So the next time you see a scallop shell on the beach and lament that it doesn’t look symmetrical, remember that most scallop shells are asymmetrical, and marvel at this evolutionary trait that makes every shell truly unique!

This article appeared in the Beachcombing Magazine January/February 2020 issue.

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