Decrypting the Cryptic: A Multidisciplinary Approach to Elucidating Pseudocalanus Diversity and Morphological Variation off the Oregon Coast

Pseudocalanus is a genus of cryptic marine copepods commonly found of off the Oregon coast. Previous work has noted significant changes in Pseudocalanus abundance with season, along with co-occuring changes in the overall zooplankton assemblage. These changes are associated with seasonal shifts in oceanographic conditions in the Northern California Current, as well as with basin scale events such as the Pacific Decadal Oscillation and El Niño/Southern Oscillation and as such are an important indicator of source waters for the ecosystem. A wide variation in body size of Pseudocalanus has been observed during the 23-year time series taken along the Newport Hydrographic Line off central Oregon, which could indicate the presence of multiple species or morphological variation in response to food availability. It is unclear morphologically if multiple species of Pseudocalanus occur off of Oregon, and whether species and body size vary with season, latitude, or distance offshore. To the authors’ knowledge, no systematic study has been made of variation in the body size of Pseudocalanus. We combined morphological and molecular approaches by measuring and DNA barcoding 361 individual specimens of Pseudocalanus sp. collected in different seasons and at different locations (from northern California to northern Washington) in 2018-19. Morphological results showed significant variation in body size seasonally and spatially, with larger copepods generally appearing in summer and nearer inshore. Sequencing found exclusively Pseudocalanus mimus. New analysis of a time series containing Pseudocalanus body sizes since 1996 also showed notable interannual variation corresponding with major oceanographic events. These results may yield a more sophisticated understanding of the bioenergetics of the Coastal Pacific, as well as the seasonal zooplankton assemblage shift – insights which are critical to supporting Pacific fisheries, and projecting potential impacts of a changing climate on our oceans.