LOW MITOCHONDRIAL DIVERSITY AND SMALL EFFECTIVE POPULATION SIZES OF THE COPEPODS CALANUS-FINMARCHICUS AND NANNOCALANUS-MINOR - POSSIBLE IMPACT OF CLIMATIC VARIATION DURING RECENT GLACIATION

Molecular population genetic diversity of two planktonic copepods of t he North Atlantic, Calanus finmarchicus and Nannocalanus minor (Crusta cea, Copepoda, Calanoida), was characterized using the sequence variat ion in a 350 bp region of the mitochondrial 16S rRNA gene. The subarct ic species, C. finmarchicus, shows lower population genetic diversity (haplotype diversity, h = 0.368, SD = 0.043; nucleotide diversity, pi = 0.00370, SD = 0.0026) than the temperate/subtropical species, N. min or (h = 0.824, SD = 0.024; pi = 0.00502, SD = 0.0032), Effective popul ation sizes (N-e, estimated from numbers of haplotypes) and effective female population sizes (N-f(e), estimated from nucleotide diversities ) for the two species are 10(7) to 10(10) smaller than census female p opulation sizes (N-f) estimated from observed densities and areal dist ributions. For both C. finmarchicus and N. minor, N-f similar to 10(15 ), N-e similar to 10(8), and N-f(e) similar to 10(5). We hypothesize t hat the cause of both low levels of molecular diversity and small effe ctive population sizes of the two species is the impact of glaciation during the past 20,000 years. C. finmarchicus may have experienced 75% range reduction and latitudinal displacement during the last glacial maximum at 18,000 years BP, giving rise to a genetic bottleneck; this may explain low diversity and an L-shaped distribution of pairwise hap lotype differences. In contrast, N. minor may have experienced range r eduction of only 30% and less change in latitudinal extent, with less impact of levels of molecular diversity and the shape of the pairwise difference distribution. Although marine zooplankton species are highl y abundant, conservation biologists should note that their numbers may vary significantly on climatic to evolutionary time scales, generatin g low levels of molecular genetic diversity.