Secondary contact of two sibling species of Eurytemora affinis: distribution,diversity and trophic role in the St Lawrence estuarine transition zone

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The St. Lawrence estuarine transition zone (ETZ), characterized by strong environmental gradients, is a secondary contact zone for two sibling species (clades) of the dominant estuarine copepod Eurytemora affinis. This study analysed pattern of distribution, genetic heterogeneity and the trophic role of the species complex of E. affinis. Sequencing of 652bp of the cytochrome oxidase I subunit (COI, mtDNA) revealed a high degree of heterogeneity in genetic structure and habitat type along the salinity gradient. The two genetically-distinct clades showed pattern of niche partitioning within the St. Lawrence ETZ by separate geographic distribution. The North Atlantic clade occupied the central portion of the St. Lawrence Middle Estuary, whereas the Atlantic clade was more prevalent along the margins, in the upstream reaches of the estuary and downstream salt marshes. The genetic population structure observed at fine spatial scales may be responsible for differentiation of trophic roles between the two genetic clades when channelling carbon from primary producers up to higher trophic levels, due to longitudinally organized distribution of primary producers and predators such as mysids and larval fishes across the St. Lawrence ETZ. Furthermore this distribution pattern might result in corresponding spatial structure in secondary production patterns. Overall, this study illustrates the overriding importance to consider cryptic intra-specific diversity in studies on ecosystem functioning