Spring evolution of Pseudocalanus spp. abundance on Georges Bank based on molecular discrimination of P. moultoni and P. newmani
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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 US GLOBEC Georges Bank Study Broadscale Surveys from February to June, 1997. Based on monthly visualizations, we inferred the spring evolution of the two species’ distributions and abundances on Georges Bank. Both species’ overall abundances increased from February to May or June: maximum abundance of P. moultoni was 38,061 m(-2) in surface waters on the crest of Georges Bank in June; maximum abundance of P. newmani was 13,854 m(-2) in subsurface waters on the Northeast Peak in April. The Peak in distribution of P. moultoni shifted from Georges Basin in Apt-ii, to the northern edge of the Bank in May, to the center of the Bank in June. In contrast, P. newmani was more abundant to the south and east of the Bank. Beginning in Apl-il, P. newmani occurred on the Bank but was less abundant and less widely-distributed than P. moultoni; P. newmani abundance peaked in May and declined somewhat in June. Females of the species differed in their patterns of distribution and abundance, with P, moultoni always the more abundant species on the crest of the Bank. The spring 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. (C) 2000 Elsevier Science Ltd. All rights reserved.