In the North Atlantic and the Arctic Ocean, four species of the copepod genus Calanus dominate the zooplankton biomass. Because of their morphological resemblance, knowledge of their respective distribution range has long been biased by misidentification, until the recent use of molecular tools uncovered numerous areas of sympatry. As hybridization between Calanus finmarchicus and Calanus glacialis has been claimed in the East-Canadian Arctic based on microsatellites, we investigated further the potential for interbreeding in newly uncovered areas of sympatry. Calanus species and stage composition were analyzed during winter in two Norwegian subarctic fjords, using molecular markers developed specifically for species identification and hybrid detection between C. finmarchicus and C. glacialis. Overall, C. glacialis were the most abundant throughout the winter, followed by C. finmarchicus and Calanus hyperboreus with only a few records of Calanus helgolandicus. The presence of C. glacialis, C. hyperboreus, and C. finmarchicus' nauplii was recorded, indicating that these species reproduce locally. In January and February, the simultaneous occurrence of males and females of both C. finmarchicus and C. glacialis suggested a potential for interspecies mating. However, genetic admixture tests performed on all 1126 individuals revealed no signal of hybridization, implying a strong reproductive isolation mechanism. We conclude that no evidence supports a potential for hybridization between C. finmarchicus and C. glacialis.

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