Calanus InDel genotypes from: No evidence for hybridization between Calanus finmarchicus and C. glacialis in a subarctic area of sympatry

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 C. 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. Calanusspecies 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, Calanus glacialis were the most abundant throughout the winter, followed by C. finmarchicus and C. hyperboreus with only a few records of C. 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 inter-species 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 and suggest that the initial claim of hybridization between Calanus species may have resulted from the use of non-appropriate molecular tools.