Ultrastructural Bacteriocyte Characterization among Chemosymbiotic Alviniconcha Snails from Hydrothermal Vents in the...

AbstractSymbiotic associations between deep-sea invertebrates and chemoautotrophic bacteria vary widely in terms of symbiont type, localization, and host nutritional strategy, yet host-symbiont dynamics remain poorly understood in some taxa. Here we advance knowledge of host-symbiont interactions within the snail genus Alviniconcha, which are abundant members of hydrothermal vent communities in the Indian and West Pacific Oceans. Using electron microscopy, we use comparative analyses based on the ultrastructural characteristics of the host bacteriocytes and bacterial symbionts. Despite previous assumptions that all Alviniconcha host endosymbiotic bacteria, we find that A. boucheti hosts episymbiotic bacteria, which densely populate the microvilli of bacteriocytes and appear to be endocytosed. Alviniconcha may also use species-specific organizational and nutritional strategies within bacteriocytes, supporting differences in symbiont maintenance and digestion. Alviniconcha adamantis and A. hessleri hosted endosymbionts in individual vacuoles with little evidence of symbiont digestion, A. kojimai and A. strummeri housed their symbionts similarly but with comparatively more evidence of digestion, and A. marisindica and A. boucheti stored symbionts in large multibacterial vacuoles, with the highest apparent degree of digestion in A. boucheti. Host species also differed in the density and morphotype of symbionts present, with all species hosting one to three morphotypes of symbionts resembling thiotrophic bacteria. In addition, both A. adamantis and A. hessleri harbored symbionts resembling methanotrophic bacteria, which genetic studies (16S rRNA amplicon and genomic) to date have not detected. This study provides the first genus-wide comparison of host-symbiont interactions in Alviniconcha, revealing key cellular features and advancing understanding of the diverse associations between hosts and their bacterial symbionts.