Stable Carbon Isotopic Evidence for Carbon Limitation in Hydrothermal Vent Vestimentiferans

Stable carbon isotope composition (delta(13)C values) can be used to evaluate an animal's source of nutritional carbon. Most animals with chemoautotrophic endosymbionts have quite negative tissue delta(13)C values due to discrimination against (13)C associated with chemoautotrophic assimilation of inorganic carbon. However, the delta(13)C values of hydrothermal vent (HTV) vestimentiferans are significantly higher than the values reported for non-HTV vestimentiferans or other invertebrates with chemoautotrophic endosymbionts. Tissue delta(13)C values of two species of HTV vestimentiferans increase with increasing size of the animals. This relation supports the hypothesis that the relatively high delta(13)C values are the result of inorganic carbon limitation during carbon fixation. A more favorable relation between gas exchange and carbon fixation in the smaller individuals is expected, due to differences in the geometric scaling of gas-exchange surfaces and trophosome volume.