Spring evolution of Pseudocalanus spp. abundance on Georges Bank based on molecular discrimination of P-moultoni and P-newmani

The planktonic copepod sibling species Pseudocalanus moultoni and P. newman i (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 char acters, but are readily identified and distinguished by simple, rapid, and inexpensive molecular protocols based on sequence variation of mitochondria l DNA (mtDNA). DNA sequence variation of a portion of the mitochondrial cyt ochrome oxidase I (mtCOI) confirmed the presence of P. moultoni and P. newm ani on Georges Bank; the mtCOI sequences were used to design species-specif ic oligonucleotide primers for use in a competitive multiplexed species-spe cific polymerase chain reaction (PCR). Species-specific PCR was used to det ermine the relative abundances of the two species in sub-samples of zooplan kton collections from US GLOBEC Georges Bank Study Broadscale Surveys from February to June, 1997. Based on monthly visualizations, we inferred the sp ring evolution of the two species' distributions and abundances on Georges Bank. Both species' overall abundances increased from February to May or Ju ne: 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 13,8 54 m(-2) in subsurface waters on the Northeast Peak in April. The Peak in d istribution of P. moultoni shifted from Georges Basin in Apt-ii, to the nor thern edge of the Bank in May, to the center of the Bank in June. In contra st, P. newmani was more abundant to the south and east of the Bank. Beginni ng in Apl-il, P. newmani occurred on the Bank but was less abundant and les s widely-distributed than P. moultoni; P. newmani abundance peaked in May a nd declined somewhat in June. Females of the species differed in their patt erns of distribution and abundance, with P, moultoni always the more abunda nt 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/o r from advective transport from adjacent regions. In contrast, P. newmani m ay 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 dynamic s causing these distributions. (C) 2000 Elsevier Science Ltd. All rights re served.