Symbiosis of the Hydrothermal Vent Gastropod Ifremeria nautilei (Provannidae) With Endobacteria-Structural Analyses and Ecological Considerations.

The gastropod Ifremeria nautilei lives in high abundance around deep-sea hydrothermal vents of the Western Pacific. The filaments of its ctenidium are very long and have a rigid axis with a hemocoelic vessel and a strongly ciliated epithelium. The flattened part of each filament largely consists of bacteriocytes that are distally filled with numerous gram-negative bacteria. The bacteria lie one by one in vacuoles that seem to be part of an interconnected tubular system. Some of the apical vacuoles regularly showed what could be openings to the ambient seawater. This special topological arrangement of the bacteria suggests that in a morphological series mirroring the supposed evolutionary pathway from extra- to intracellular symbioses, I. nautilei might correspond to an intermediate stage. The high sulfur content and the low stable carbon isotope values measured in this study, combined with corresponding data from the literature, indicate that I. nautilei is the host partner in a thiotrophic chemoautotrophic bacterial symbiosis. The importance of this symbiosis for the nutrition of the gastropod is underlined by the reduced size of the host's stomach. Unlike specimens of I. nautilei from the Manus Basin (Galchenko et al., 1992), the inspected specimens from the North Fiji Basin did not contain any methanotrophic bacteria in addition to the thiotrophic type. From the disparity in results, it may be concluded that this host species can develop different patterns of symbiosis either as an adaptation to local variances of hydrothermal vent fluid chemistry or as a consequence of genetic differentiation in the host.