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 does it comprise a number of genetically distinct populations or taxonomically 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 separated 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 Stream; Type II individuals were smaller and became more abundant toward the eastern regions of the Gulf Stream. Significant differences in the size-frequency distributions of N. minor from different regions of the North Atlantic may result from mixtures of the two genetic types and environmental 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 individuals of one haplotype, 31 individuals of a second, and 60 unique individuals. Haplotype diversity, h, was very similar across the sampled range: h = 0.886 in samples from the FS and 0.874 for samples from the Gulf Stream Meander Region (GSMR). Nucleotide diversity, pi, was significantly greater in the FS (pi = 0.00490) than in the GSMR (0.00414), largely due to a number of genetically divergent individuals. Haplotype abundances did not differ significantly either within the regions (among FS samples, P = 0.756; among GSMR samples, P = 0.336) or between the regions (P = 0.636). Molecular genetic analysis can reveal cryptic species among marine taxa, and is particularly useful for taxa characterized by morphological similarity.
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