Habitat usage by the cryptic copepods Pseudocalanus moultoni and P. newmani on Georges Bank (Northwest Atlantic)

•Cryptic copepod species, Pseudocalanus moultoni and P. newmani, were discriminated by PCR based on mitochondrial COI.•Cryptic species show distinct patterns of distribution and abundance on Georges Bank during January to June, 1999.•Pseudocalanus moultoni concentration in deep layers may aid retention and reduce advective loss from the Bank.•Pseudocalanus newmani concentration in surface waters makes species subject to wind-driven transport.•Discrimination of cryptic species is necessary to understand and predict changes in zooplankton diversity and pelagic ecosystem function.

The cryptic copepod species, Pseudocalanus moultoni and P. newmani, co-occur on Georges Bank and in the Gulf of Maine (Northwest Atlantic); even recent studies have reported results and conclusions based on examination of the combined species. Species-specific PCR (SS-PCR) based on mitochondrial cytochrome oxidase I (COI) sequence divergence was used in this study to discriminate the species. Species-specific descriptions of habitat usage and predicted patterns of transport and retention on Georges Bank were made by mapping distributions and calculating abundances of each species from January to June, 1999 for four vertical strata (0–15 m, 15–40 m, 40–100 m, and 0–100 m) and five regions (Northern Flank, Bank Crest, Northeast Peak, Southern Flank, and Slope Water) identified on the basis of bathymetry and circulation. Patterns of distribution and abundance for the two species during January to June, 1999 were largely consistent with those described based on vertically integrating mapping and analysis for the same period in 1997 by McGillicuddy and Bucklin (2002). The region-specific and depth-stratified analyses allowed further discrimination in habitat usage by the species and confirmed the distinctive patterns for the two species. The observed differences between the species in abundances among the five regions and three depth strata over Georges Bank impact their transport trajectories. The concentration of P. moultoni in deep layers likely explains the higher rates of retention and lower rates of advective loss of this species from the Bank, compared to P. newmani, which may be more subject to wind-driven transport in the surface layer. Accurate identification and discrimination of even closely-related and cryptic species is needed to ensure full understanding and realistic predictions of changes in diversity of zooplankton and the functioning of pelagic ecosystems.

Graphical abstractThe cryptic copepod species, Pseudocalanus moultoni and P. newmani, co-occur on Georges Bank and in the Gulf of Maine (Northwest Atlantic). Distributions and abundances were mapped for each species from January to June, 1999 using species-specific PCR (SS-PCR) based on DNA sequences for a COI gene region to discriminate the species. Analysis of depth-stratified species-specific abundances and environmental parameters in five regions of the Bank revealed differences in three-dimensional habitat usage by the cryptic species (see figure below). Concentrations of P. moultoni were found in deep layers, resulting in higher rates of retention and lower rates of advective loss of this species from the Bank; in contrast, P. newmani is more subject to wind-driven transport in the surface layer. There were subtle species-specific differences in associations with temperature and salinity, based on multilinear regression. We hypothesize that differing behaviors and vertical distributions of P. moultoni and P. newmani – especially in the complex hydrographic regime of the Bank – may result in the species’ differential transport and retention over Georges Bank. It is critically important that cryptic species be routinely and accurately discriminated when examining and predicting species- and community-level responses of zooplankton to bio-physical forcing and climate change.Figure optionsDownload full-size imageDownload as PowerPoint slide