Feeding dynamics of the invasive calanoid copepod, Pseudodiaptomus inopinus, in two northeast Pacific estuaries

: The Asian calanoid copepod Pseudodiaptomus inopinus , first observed in the Columbia River Estuary in the early 1990s, has since become the dominant copepod species in many estuaries along the US Pacific coast, but its feeding behavior has not been previously studied. In October 2019 and 2020, when P. inopinus was at peak seasonal abundance, we conducted incubation experiments with this species feeding on natural microplankton prey assemblages sampled from 2 invaded estuaries: the Chehalis River estuary, Washington, and the Yaquina River estuary, Oregon. In both estuaries, diatoms were the most numerically abundant prey group, with 11− 15 μm Chaetoceros sp. and 21−25 μm Cyclotella sp. dominating the Chehalis and Yaquina estuaries, respectively. Diatom and ciliate biomass were highest in both estuaries, with all prey cells in the Yaquina estuary typically larger than those in the Chehalis estuary. P. inopinus fed omnivorously on microplankton prey, with a preference for prey >20 μm and occasional avoidance of cyanobacteria and cells <10 μm. Ingestion rates were highest on ciliates and diatoms. The omnivory of P. inopinus may contribute to its success as an invader in northeast Pacific estuaries.