The Rate of Growth of the Hard Clam Mercenaria mercenariaL) throughout the Geographical Range

The published data on rates of growth of the hard clam Mercenaria mercenaria throughout its geographical range are reviewed and new data for British populations given. The data have been subjected to appropriate transformations to give comparative figures for rates of growth and these are compared. Local variations in annual growth is extreme, but the extent of the variation is similar throughout the continuous range of the species and in Europe. North of the continuous range the maximum annual increment is reduced. The seasonal distribution of growth is compared for different areas and related to environmental temperature. In the north of the range growth takes place only during the summer, further south growth is continuous through the year. An optimum temperature for growth of 20°C is indicated. Growth stops below 9°C, and above 31°C. The relationship between growth rate and temperature appears to be similar throughout the geographical range. Considerable deviations from the temperature/growth rate relationship occur indicating that other factors, particularly food availability, determine the rate of growth within the limits set by temperature. Similar data for M. campechiensis and M. campechiensis X M. mercenaria hybrids are reviewed. They indicate that growth rates in these groups continue to increase with temperature in the range 10–30°C.