The power of sclerochronology as a tool for interpreting the life history of aquatic organisms and the marine environment they inhabited lies in the occurrence of periodic and repeating structures within the skeletal parts of these organisms. Previous work examining the internal growth patterns of northern quahog (=hard clams), Mercenaria mercenaria (L.), along the Atlantic Coast has established their periodicity; however, the two existing descriptions of the annual growth pattern for hard clams from Narragansett Bay, RI are not consistent with one another. One study identified a translucent slow growth band in the summer and the winter, whereas the other study identified a translucent slow growth band only in the winter. To facilitate future sclerochronological analyses using Narragansett Bay hard clams, variations in the existing descriptions of the annual growth pattern were clarified. We found high variability in the annual growth pattern of M. mercenaria collected during the summers of 2005 and 2006. In approximately half the mature individuals observed, the winter growth breaks were separated by three bands, an opaque band of rapid growth in the spring, a translucent slow growth band in the summer, and a second opaque band of rapid growth in the fall. As these individuals approached senility, the pattern became compressed into two seasonal bands, a single opaque band of rapid growth in the spring and a single translucent band of slow growth encompassing both summer and fall. Growth in the remainder of the samples exhibited only two seasonal bands throughout the mature and senile period, an opaque band of rapid spring growth and a translucent band of slow growth throughout the summer and fall. To confirm the patterns observed, we analyzed δ18O from a subsample of shells. After placing the earlier descriptions and the current description of growth in Narragansett Bay hard clams in chronological order, it appears as though the changing environment of Narragansett Bay was reflected in the growth pattern of these clams. This study emphasizes the importance of validating annual growth patterns even in a region where the periodicity of the species has been confirmed. Changes in the marine environment may alter the patterns in the skeletal structures of aquatic organisms as well as their growth rates.
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Vol. 26 • No. 4