Slow growth rate of a deep-sea clam determined by 228 Ra chronology ( growth bands / ages of deep-dwelling organisms with hard parts )

The age of a deep-sea clam, Tindaria caifistiformis, from 3800 m depth has been determined by MRa (6.7 year half-life) chronology of separated size fractions of a captured population. A length of 8.4 mm is attained in about 100 years. Shells of this size fraction show about 100 regularly spaced bands, indicating that the growth feature may be an annual one. There are several reports indicating that the rate of metabolism on the deep ocean floor is extremely slow-generally one to two orders of magnitude slower than the rate for comparable organisms in shallow water systems (1-4). From small brood size and the high proportion of adult size classes observed for deep ocean benthos Grassle and Sanders (5) have postulated life histories with low rates of reproduction, growth, and mortality. The deep sea is an environment devoid of light, virtually invariant in ambient high pressure and low temperature, and probably subject to a slow food supply rate. These facts provide the basis for expectation of slow metabolic rates at depth but they also make the interpretation of growth features in the hard parts of deep-sea organisms difficult. The aim of our study was to explore the possibility of radiometrically determining the rates of growth of deep-sea benthic organisms that secrete a hard mineral shell. Our initial effort reported here is on a deep-sea clam, Tindaria callistiformis, an exclusively deep-sea species with regular growth bands. Our technique is to use the 228Ra (half-life = 6.7 years) concentrations in a number of growth stages and to infer from these data the growth rate of the organism. Radium-228 is produced from the decay of long-lived 232Th, which is found ubiquitously in clay-rich sediments at a concentration of about 10 parts per million although virtually absent in sea water. The 228Ra produced by a-decay is subject to release from sediments by diffusion or physical transport via the pore waters of the sediment into the overlying water. The distribution of 228Ra in the ocean after the initial study by Moore (6, 7) has been the subject of several intensive investigations (8, 9). Fig. 1 shows a profile from Trier et al. (9) for a North Atlantic Ocean station in the general vicinity of the site from which the clams used in this study were recovered. Radium-228 in surface waters of the deep ocean is derived from the sediment-water interactions in shallow coastal regions, especially where physical mixing knd bioturbation rates are high. Using 228Ra concentrations found in different layers of coral, Dodge and coworkers (10, 11) were able to date corals and determine that for several species the growth bands were indeed annual. Furthermore, by comparing the 228Ra/Ca ratio with 210Pb dating, they were able to demonstrate that this ratio was sensibly invariant in the surface ocean over at least 30 years. The 228Ra concentration at the water-sediment interface of the deep ocean floor is also high. Its concentration there is expected to be stabilized as well or better than in the surface waters. Thus the systematic variation in 28Ra concentration in the shells as a function of size will tell us about the growth properties of that species, providing that the shells belong to a single species of molluscs, that they were derived from animals that were alive at the time of capture, and that all the shells came from the same sample.