Stable carbon isotope composition (δ13C values) can be used to evaluate an animal's source of nutritional carbon. Most animals with chemoautotrophic endosymbionts have quite negative tissue δ13C values due to discrimination against 13C associated with chemoautotrophic assimilation of inorganic carbon. However, the δ13C values of hydrothermal vent (HTV) vestimentiferans are significantly higher than the values reported for non-HTV vestimentiferans or other invertebrates with chemoautotrophic endosymbionts. Tissue δ13C values of two species of HTV vestimentiferans increase with increasing size of the animals. This relation supports the hypothesis that the relatively high δ13C 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.
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