MOLECULAR DIVERSITY OF THE COPEPOD, NANNOCALANUS MINOR - GENETIC-EVIDENCE OF SPECIES AND POPULATION-STRUCTURE IN THE NORTH-ATLANTIC OCEAN

The abundant calanoid copepod, Nannocalanus minor, is widespread from the Florida Straits (FS), throughout the Gulf Stream and the Sargasso Sea, to the eastern North Atlantic Ocean. Does the species represent a single, randomly-mating population across this extensive region, or d oes it comprise a number of genetically distinct populations or taxono mically distinct forms? What are patterns and pathways of dispersal of the copepod across the North Atlantic? These questions were addressed using population genetic analysis of DNA sequence variation of a 440 base-pair region of the mitochondrial 16S rRNA gene. This analysis sep arated N. minor into two genetically distinct types (distinguished by 10% sequence difference) that may represent the previously described N . m. forma major and N.m. forma minor. The two genetic types differed in size range and in geographic distribution: Type I individuals were larger and were most abundant in the western regions of the Gulf Strea m; Type II individuals were smaller and became more abundant toward th e eastern regions of the Gulf Stream. Significant differences in the s ize-frequency distributions of N. minor from different regions of the North Atlantic may result from mixtures of the two genetic types and e nvironmental differences in food availability. Within N. minor Type I, mtDNA sequence variation defined 68 haplotypes among 155 individuals. The haplotype frequency distribution was skewed: there were 40 indivi duals of one haplotype, 31 individuals of a second, and 60 unique indi viduals. Haplotype diversity, h, was very similar across the sampled r ange: h = 0.886 in samples from the FS and 0.874 for samples from the Gulf Stream Meander Region (GSMR). Nucleotide diversity, p(i), was sig nificantly greater in the FS (p(i) = 0.00490) than in the GSMR (0.0041 4), largely due to a number of genetically divergent individuals. Hapl otype abundances did not differ significantly either within the region s (among FS samples, P = 0.756; among GSMR samples, P = 0.336) or betw een the regions (P = 0.636). Molecular genetic analysis can reveal cry ptic species among marine taxa, and is particularly useful for taxa ch aracterized by morphological similarity.