Dactylogyrus simplexus Monaco and Mizelle, 1955, occurs on the gills of fathead minnows (Pimephales promelas). Previous research on parasites of fathead minnows from 3 converging Nebraska streams, Elk Creek, Oak Creek, and West Oak Creek, shows that fish in each stream constitute distinct populations. To determine whether their parasites had diverged structurally in response to such isolation, or in response to seasonal change, we searched for patterns of intraspecific morphometric variation among D. simplexus. Over 3 collection dates in fall 2007, spring 2008, and fall 2009, 203 D. simplexus were collected from Elk and West Oak Creeks. We ran 1-way ANOVA to compare differences in 15 distinct point-to-point measurements of sclerotized parts across sites and collection dates. Significant differences were found in some D. simplexus measurements between Elk and West Oak Creeks for all 3 collection dates, but the characteristics that differed and the trend of variation between the creeks were not consistent over time. Dactylogyrus simplexus from both Elk and West Oak Creeks showed consistent patterns of variation over time for 5 measurements, including hamulus gap width, bar length, marginal hook length, sickle length, and sickle width. In conclusion, D. simplexus demonstrate consistent patterns of seasonal variation, but not spatial.

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