Population genetic diversity and structure of the euphausiids Thysanoessa inermis and T. raschii in the Arctic Ocean: inferences from COI barcodes

Euphausiids are an ecologically significant and abundant group of marine zooplankton that form key links between primary producers and consumers in pelagic food webs throughout the world ocean. The euphausiid species, Thysanoessa inermis and T. raschii, have boreal-Arctic distributions, occurring in the North Atlantic, North Pacific, and Arctic Oceans. The species differ in depth ranges and habitat preferences: T. raschii is found in coastal waters on continental shelf habitats, while T. inermis is abundant in slope and deep water regions. Population genetic analysis based on DNA sequence variation of the mitochondrial cytochrome oxidase I (COI) barcode region was carried out for identified specimens of T. inermis and T. raschii collected in the Arctic (Beaufort/Chukchi and Norwegian Seas, Svalbard Area) and North Atlantic (Gulf of St. Lawrence, Labrador Sea, Iqaluit, Hudson Bay). Populations of T. inermis in the N. Atlantic showed high connectivity, but were genetically isolated from the Beaufort/Chukchi Sea population. Population genetic diversity of T. inermis showed high haplotype and nucleotide diversity and no departures from neutral expectations. In contrast, T. raschii showed lower haplotype and nucleotide diversity, with highly significant departures from neutral expectations. A possible hypothesis is that T. raschii experienced a significant historical demographic event (e.g., population bottleneck), while T. inermis maintained a stable population over recent evolutionary history. The results provide new insights into population dynamics and implications for responses to climate change of these key euphausiid species for the Arctic Ocean.