Trophic Relationships Between a Seep Calamyzine Worm and Its Vesicomyid Clam Host Inferred From Multiple Lines of Isotopic Evidence

ABSTRACT Calamyzine worms are often found in mantle cavities of large chemosymbiotic bivalves in deep‐sea chemosynthesis‐based ecosystems, but the nature of trophic interactions between the worms and their clam hosts remains largely obscure. Here, we combine bulk tissue carbon and nitrogen stable isotope analyses with radiocarbon and nitrogen compound‐specific isotope analysis of amino acids (CSIA‐AA) to investigate the trophic relationship between the calamyzine worm Shinkai semilonga and its host vesicomyid clam, Archivesica kawamurai , from a methane seep in Nankai Trough, Japan. The trophic position (TP Glu/Phe ) of S. semilonga (2.6) was approximately one unit higher than that of the host clam gill (1.5), the organ where the clam hosts its symbiotic bacteria. Our results indicate that S. semilonga is a parasite feeding directly on the gill tissue and assimilating both host cells and the sulphur‐oxidising bacteria. Furthermore, the host clam mantle exhibited a higher than‐predicted TP Glu/Phe (2.5 rather than 2.0), suggesting that A. kawamurai may supplement its symbiont‐derived nutrition by filter‐feeding on particulate organic matter. These findings shed light on the interactions between calamyzine worms and their host bivalves, demonstrating the usefulness of CSIA‐AA in resolving complex trophic interactions in deep‐sea chemosynthetic communities.