Polyaspartate is a biopolymer in oysters that stops growth of shells by inhibiting growth of excess calcium carbonate.
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Polyaspartate is a biopolymer produced by oysters to help mold their shells into their characteristic shape. Polyaspartate stops excess growth of calcium carbonate, the principal constituent of oyster shells. (Ashley 2002:32-33)
Journal article It's Not Easy Being GreenSci AmJune 28, 2017 Steven Ashley
Journal article Control of Calcium Carbonate Nucleation and Crystal Growth by Soluble Matrx of Oyster ShellScienceMay 10, 2006 A. P. WHEELER, J. W. GEORGE, C. A. EVANS