Maintenance of Pseudocalanus spp. on Georges Bank
A. Bucklin and D. J. McGillicuddy
The planktonic copepod sibling species, Pseudocalanus moultoni
and P. newmani (Crustacea, Copepoda), are abundant in
waters over Georges Bank from late winter until mid-summer and are
thought to reproduce throughout this period. The two species cannot
be reliably distinguished using morphological characters, but are
readily identified and distinguished by simple, rapid, and
inexpensive molecular protocols based on sequence variation of
mitochondrial DNA (mtDNA). DNA sequence variation of a portion of
the mitochondrial cytochrome oxidase I (mtCOI) confirmed the
presence of P. moultoni and P. newmani on
Georges Bank; the mtCOI sequences were used to design
species-specific oligonucleotide primers for use in a competitive
multiplexed species-specific Polymerase Chain Reaction (PCR).
Species-specific PCR was used to determine the relative abundances
of the two species in sub-samples of zooplankton collections from
U.S. GLOBEC Georges Bank Study Broad-scale Surveys from February to
June, 1997 and January to June, 1999. Based on monthly
visualizations, we inferred the spring-summer evolution of the two
species' distributions and abundances on Georges Bank. During both
years, both species' overall abundances increased during these
months. The two species differed in that the peak in distribution
of P. moultoni was most frequently on the crest of the
Bank, while P. newmani was more abundant to the south and
east of the Bank. The spring-summer increase of P.
moultoni may result from the persistence of reproducing
individuals over the Bank and/or from advective transport from
adjacent regions. In contrast, P. newmani may be
transported to Georges Bank from upstream populations on the Scotian
Shelf and Browns Bank. The processes responsible for the observed
patterns cannot be determined from this series of monthly snap-shots
alone; ongoing studies use numerical models to examine the
biological and physical dynamics causing these distributions.