A Mimic's Mimic: Redescription of Elysia corinneae (Ev. Marcus, 1973), One of Three Halimeda-Eating Elysia (Gastropoda: Heterobranchia) Misclassified as Bosellia Due to Fused Parapodia

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The Thyasiridae are small bivalves commonly found in organically enriched sediments in cold marine waters. Within this family, some species form nutritional symbioses with sulfur-oxidizing bacteria. Thyasirid species identification is mainly based upon shell characters that are often considered to display broad intra-specific variation. Recent work that examined multiple characters, including gene sequences and symbiont presence, has revealed a complex of closely related taxa resembling Thyasira gouldii (Philippi, 1845), a species with a presumed pan-arctic distribution, including off the coast of Newfoundland, Canada. We investigated the anatomical, genetic, and symbiotic characters of thyasirids resembling T. gouldii from Bonne Bay, Newfoundland as well as Saglek Bank and Frobisher Bay, Nunavut in the Canadian Arctic, and are here designating asymbiotic members as belonging to Thyasira plana (Verrill and Bush, 1898), a species previously synonymized with T. gouldii. Other symbiotic specimens are designated T. gouldii based on comparisons (soft anatomy, shell features) with specimens from the type locality. We note overlap in shell shape between T. plana and T. gouldii. Across our sample set, T. gouldii can attain a larger size and is genetically distinct from the asymbiotic T. plana. Both T. gouldii and T. plana appear to reach larger shell sizes and have larger prodissoconchs at higher latitudes. Our findings suggest that T. gouldii, as currently defined, represents a complex of multiple species. Properly defining this common species across its purported geographic range will require further examination of presumed T. gouldii specimens, especially using genetic approaches.