Species composition and abundance of copepods in the morphologically cryptic genus Pseudocalanus in the Bering Sea

Copepods in the genus Pseudocalanus are abundant in temperate-boreal waters of the Northern Hemisphere. However, their relative distributions are not well known because many species are morphologically cryptic. Here, Pseudocalanus species abundance and distribution in the southeastern Bering Sea from March to June, 2010, were determined using samples spanning the middle (50-100 m depth) and outer (>100 m depth) shelf domains. Based on analysis of mitochondrial COI sequences of 637 adult females, four species were identified; two are temperate (Pseudocalanus mimus, Pseudocalanus newmani) and two arctic (Pseudocalanus minutus, Pseudocalanus acuspes). Average genetic divergence among species was 14.7±4%. Species abundance and distribution differed significantly between the outer and middle shelf domains. For example, the temperate species P. mimus was more abundant in the outer shelf domain of the SE Bering Sea, while the arctic species P. acuspes was more abundant in the middle shelf domain. Genetic diversity within species varied. Haplotype and nucleotide diversities were higher in temperate than arctic species. No genetic structure was observed in the temperate species P. newmani or the arctic P. minutus across the outer and middle shelf domains, suggesting high levels of gene flow and connectivity across the shelf. Differences in the abundance, distribution and genetics among Pseudocalanus species in the Bering Sea highlight their unique ecological dynamics and evolutionary histories and suggest that they may have distinct responses to future environmental change.