Paleoecological Implications of Cohort Survivorship for Mya arenaria in Massachusetts Estuarine Waters

A number of recent studies cast doubt on the reliability of using size-frequency distributions of fossil species for inferring ecological age-survivorship curves, fossil community structure, and paleoenvironments. Survivorship curves for fossil taxa are really post-larval size-survivorship curves. Data are lacking for the normally enormous mortalities that occur during larval stages. Such curves represent the effects of differential taphonomy upon various size classes. Before dismissing the paleoecological application of ecological survivorship curving, the patterns of sizefrequency distributions of very large and spatially disparate populations of extant species should be explored. Emphasis to date has been upon temporal, not spatial, averaging in the generation of fossil assemblages. We provide size-survivorship and reconstructed age-survivorship curves on over 2.5 million live individuals of the infaunal bivalve Mya arenaria from 1, 093 collecting sites at 15 locations within four estuaries from eastern Massachusetts. A significant difference was found among the cohort structures of post-larval M. arenaria from the four estuaries, even though environmental settings were similar. A summary cohort curve was constructed that may be analogous to very extensive (but perhaps unlikely) spatial averaging in the generation of a fossil assemblage. The size-survivorship curves constructed for the four estuaries and for the 15 locations exhibit considerable variation from the summary curve. This supports the contention that size-survivorship data gleaned from ecological contexts should only be applied to the paleoecological record with extreme caution.