A comparison of bivalve (Calyptogena magnifica) growth at two deep-sea hydrothermal vents in the eastern Pacific

Analyses of specimens of a common deep-sea hydrothermal vent bivalve, Calyptogena magnifica, from two hydrothermal vent sites in the eastern Pacific, have been utilized to develop a mathematical model of the growth of this species based on accurate measurements of: (1) in situ rates of dissolution of the aragonitic outer granular shell layer; and (2) the thickness of the outer shell layer remaining at any given point in time at various distances from the umbo. The model permits the variances of each independently measured variable to be combined into a final confidence limit on age at a given size, making possible statistical comparisons of the calculated parameters of the von Bertalanffy growth equation. This model, in turn, provides a powerful tool for quantifying temporal and spatial variability in rates of biological processes both within and between deep-sea hydrothermal vent communities. Ontogenetic growth curves for C. magnifica specimens at both hydrothermal sites suggest that growth rates of this species are several orders of magnitude greater than those reported for the one bivalve (Tindaria callistiformis) analysed to date from a deep-sea, non-vent habitat. These data provide additional evidence that biological processes at submarine hydrothermal vents along the mid-oceanic ridge system proceed at rates that are extremely rapid for a deep-sea environment and are comparable with those from some shallow water temperate environments. Rates of shell dissolution decreased markedly with increasing distance from vent fluids. In a 20 cm distance spanning 10 cm into a bivalve assemblage atop an active vent at Rose Garden (Galapagos Rift) to 10 cm outside the assemblage, dissolution rates of the outer granular layer of C. magnifica declined 100-fold (from 355.4 to 3.5 μm y^-1, respectively) in in situ exposures of approximately 210 days. At distances ranging from 1 to 6 m away from active vent sites, no measurable thickness (<1 μm) of C. magnifica shells had dissolved. The latter dissolution rates are far below estimates reported in the literature for shells of this species exposed in situ at comparable depths in the eastern Pacific and these results have profound implications for estimating 'residence' times of empty shell valves at inactive vent sites.