| Summary: | The Thyasiridae are small marine clams common in cold waters, including around the Canadian Arctic and Eastern Canada. Some thyasirid species form nutritional symbioses with sulfur-oxidizing bacteria and may act as ecosystem engineers in sediments recovering from organic enrichment. Using thyasirids as ecological indicators requires accurate species identification, and there is a lack of taxonomic attention to thyasirid species across large geographic areas. This thesis aims to improve our understanding and ability to identify 5 thyasirid species (Thyasira cf. dunbari, T. cf. equalis, T. plana, T. cf. gouldi, and Axinopisda orbiculata) occurring in the Canadian Arctic and Eastern Canada, based on the study of multiple characters (anatomical, genetic, and symbiotic). Examination of the recently discovered T. cf. gouldi complex led to the reinstatement of a formerly synonymized species, T. plana. Chemosymbiosis is confirmed for the first time in arctic thyasirids, namely in T. cf. gouldi; other thyasirids from this region are asymbiotic. New taxonomically informative gene sequences are presented, and a re-examination of the distribution of thyasirid species notably revealed T. cf. dunbari occurrences at lower latitudes than T. dunbari. Future work is needed to improve the taxonomy of thyasirids and to inform their usefulness as indicators of organic enrichment.
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