Ontogenetic trends in aspartic acid racemization and amino acid composition within modern and fossil shells of the bivalve Arctica

Ontogenetic trends (umbo to growth edge of shell) in aspartic acid (Asp) ra cemization and amino acid composition and their evolution over time are exa mined in serial samples of annual growth bands from a time-series of three live-collected and two fossil (ca. 500 and 1000 y BP) shells of the long-li ved bivalve Arctica islandica. The rate of Asp racemization is shown to be higher in the umbonal portion of the shells (laid down when the clams are y oung) but constant from a biological age of 10 to 20 y to more than 100 y. Corresponding changes are also seen in amino acid composition and concentra tion: with increasing biological age of the clam: total amino acid concentr ation increases substantially, the acidic amino acids Asp, glutamic acid, a nd alanine decrease in relative concentration (mole-percent) and more basic amino acids including tyrosine, phenylalanine, and lysine increase in rela tive concentration. These ontogenetic trends are generarlly retained in the fossil shells. These trends may reflect changing protein composition relat ed to changes in growth rate. Clams grow considerably faster in their youth than when they are older, as indicated by changes in the annual growth inc rements. Production of more acidic proteins, which play a role in crystal g rowth, may be favored during the phase of faster growth, whereas more struc tural proteins, perhaps enhancing structural strength of the shell, may be favored during later growth. These ontogenetic differences in protein compo sition affect the observed rates of racemization of the protein pool. Some weak diagenetic trends in amino acid composition and abundance may be repre sented in the time series of shells. These results emphasize the importance of standardization of the location from which samples are taken from shell s for dating by amino acid racemization analysis. Copyrights (C) 2001 Elsev ier Science Ltd.