GROWTH RATES OF BATHYMODIOLIDS FROM GAS SEEPS IN THE GULF OF MEXICO DERIVED FROM POLONIUM-210/RADIUM-226 RATIOS

Deep-sea mytilids (genus Bathymodiolus) harboring intracellular microbial symbionts are common inhabitants of hydrothermal vents and hydrocarbon seeps. In general, growth rates of deep-sea biota are significantly slower than that of shallow water biota but whether or not the unique chemosymbiotic pathway of the bathymodiolids affects their growth rates is unknown. Here we present the results of a recent study whose objective is to determine the growth rates of bathymodiolids in their natural habitat. Eleven evenly spaced sections were produced from a 73 mm long calcareous shell of a Bathymodiolus childressi mussel sampled alive at a water depth of 694 m in the Green Canyon-272 of the Gulf of Mexico. Two independent radiometric methods were applied in the study (i) AMS C-14, and (ii) Po-210/Ra-226 ratios. Radiocarbon measurements yield apparent ages of 1050 yrs to 2080 yrs and provide clear evidence of dates compromised by incorporation of "old" carbon derived from methane oxidation. The Po-210/Ra-226 ratios increase progressively from 0.07 in the shell margin to 0.60 in the umbo and show a linear relationship with the cumulative shell length. The data indicate that the mussel was 17.6 years old at sampling time and yield a mean linear growth rate of 4.1 mm/yr. The radiometric growth rate is in good agreement with the growth rate of 6.3 mm/yr measured for seep mussels in a controlled environment where methane was the sole carbon and energy source. Hence the growth rates of bathymodiolid mussels are about 50 times faster than deep-sea clams relying solely on filter-feeding nutritional strategy.