Growth rate of a vesicomyid clam from the 21° N East Pacific Rise hydrothermal area

Previously, we determined the growth rate of a vesicomyid clam from the Galápagos spreading centre by using natural radionuclide analyses of serially sampled shell layers¹. The growth rate was ∼4 cm yr ^-1 measured along the 12 cm length of the axis of maximum growth, yielding an age for the clam of 3-4 yr, assuming this rapid rate for the entire age of the clam. We noted that the linear growth rate was actually highest in the last year of the life of the organism; this could indicate a change in food supply, which in turn, in this chemically driven ecosystem, would be an index of hydrothermal activity. We have now determined the growth history of a vesicomyid clam from the 21° N East Pacific Rise (EPR) site using ²²⁸Ra, ²²⁶Ra and ²²⁸Th measurements described earlier¹. The ²¹⁰Pb (half life 22.3 yr)-²¹⁰Po (half life 138 days) couple is useful only for 2 yr, at which time secular equilibrium is attained, whereas ²²⁸Ra (half life 5.7 yr) and its progeny, ²²⁸Th (half life 1.9 yr), provide information on a longer time scale. The ²²⁸Th/²²⁸Ra activity ratio reaches the transient equilibrium value of 1.5 in ∼10yr and the ²²⁸Ra decay can be followed for ∼30 yr. We report that the average growth rate along the axis of maximum growth (10 cm) is 0.27 cm yr^-1 resulting in an estimated age of 27 yr for the clam, about 10 times older than a clam of comparable size from the Galápagos spreading centre. There is evidence for changes in growth rate with time.