Genetic isolation discovered among previously described sympatric morphs of a meiobenthic copepod

We sampled female Nannopus palustris Brady, 1880, from the North Inlet Estuary, South Carolina, U.S.A., monthly for a year to determine seasonal variation in a female polymorphism previously described. Two fat morphs (one with a notched and one with a straight basal region of the terminal seta of the caudal ramus) and a thin-bodied, previously unknown, morph were present sympatrically in different relative frequencies throughout the year. Total abundances peaked in February and were lowest in the summer. All three morphs differed significantly in width, but only females with notched terminal setae were significantly shorter in body length than thin and fat females. Genetic analyses of mitochondrial (cytochrome b) and nuclear (D3 expansion segment of 28S rDNA) genes demonstrated that the females with notched terminal setae were distinct from the thin and fat females with straight terminal setae, and the latter two shared common alleles. Concordance between mitochondrial and nuclear gene trees supports that notched females are a separate species from the thin and fat forms with straight terminal setae. We have yet to find a male with a notched basal region of its principle terminal setae or any male sharing alleles in common with notched females.