Growth Lines in a Bivalve Mollusk: Subdaily Patterns and Dissolution of the Shell

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Abstract
Scanning electron micrographs of sections of the prismatic shell of the bivalve Mercenaria mercenaria reveal narrow subdaily growth striations. The width of these narrow lines, formed by concentrations of organic material, corresponds to the quantity of shell material that would be expected to dissolve during periods of anaerobic metabolism. The pH in the extrapallial fluid of the bivalve decreases when the valves are closed, and the amount of dissolution of shell is related to the duration of valve closure.