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.

The finding of Breviconia andrei sp. nov. in the Russian Bering Sea enabled the clear-cut phylogenetic characterization of the former monotypic genus Breviconia Conroy-Dalton & Huys, 2000 as a monophylum. Comparison of the new species with B. australis (George, 1998) and other members of the subfamily Ancorabolinae Sars, 1909 yielded four autapomorphies that unambiguously support the monophyletic state of Breviconia: (1) an elongated and approximately 90°-curved mandibular gnathobase, (2) reduction of the maxillar endopod, (3) maxillar endites carrying 2 instead of 3 setae, and (4) loss of the minute seta on the maxillipedal claw. For B. andrei sp. nov., two autapomorphies could be detected, namely, (1) the development of dorsal tubercles on the P5-bearing body somite and (2) the remarkable elongation of the first endopodal segment of the first swimming leg that is twice as long as the whole exopod. Of particular interest is the presence of a 3-segmented endopod in the third swimming leg of the male of B. andrei sp. nov. It disproves the current assumption that the Ancorabolinae are characterized by (among others) the derived presence of an only 2-segmented endopod in the male’s third swimming leg.