A Growth Increment Analysis of Donax Variabilis: Comparison Between Diurnal and Semidiurnal Tidal Habitats

This paper will report the findings of an investigation into the growth increments of the variable coquina clam Donax variabilis. Donax variabilis is a short-lived bivalve mollusc that filter-feeds in the swash zone, moving in and out with waves and tides and burrowing itself in the sand. The species is commonly found along beaches from the Atlantic coast of New England to the Texas Gulf of Mexico coast, USA. Previous research indicates that the coupled light and dark bands observed in a thin section of the clam's shell correlate to either daily or tidal growth increments. However, this research was conducted on specimens collected from an area of the Gulf of Mexico coastline that experiences diurnal tides, thus it was difficult to differentiate between daily or tidal increment growth periodicity. In order to better interpret the incremental growth of Donax variabilis an increment count of specimens collected from the Gulf of Mexico will be compared to that of specimens of roughly the same age collected from the Atlantic coast of Florida. Sequential stable oxygen isotope analysis of the shells as compared to local water temperature data will provide an independent temporal control. Since the Florida Atlantic coast experiences semidiurnal tides it may be possible to distinguish between daily and tidal influences on the growth increments of Donax variabilis. Determining the periodicity of increment growth of this clam will aid in future investigations into paleoclimatic proxy applications of the species.