Supplemental description of Gyrodactylus olsoni (Monogenea van Beneden, 1858) from Gillichthys mirabilis (Gobiidae) including molecular phylogeny and ecology.

Additional morphology, new sequence (482 bp SSU rRNA gene, 902 bp ITS Region), and ecological data for Gyrodactylus olsoni Mizelle and Kritsky, 1967 parasitizing longjaw mudsucker, Gillichthys mirabilis Cooper (Gobiiformes: Gobiidae) from coastal waters of California, are provided. The parasite was found on 96% of 617 mudsuckers collected from Tijuana Estuary to Arcata Bay, California. It was present on gills, fins, and body surfaces. Samples from gills and fins shared identical sequence data indicating a lack of site specificity for the parasite. The new material matched the description of G. olsoni in features of the haptoral hard parts and MCO, but trichrome-stained specimens and scanning electron microscopy showed a pair of shallow tissue pads immediately anterior to the folded anchor roots. The pads, which appear partially hardened on the curved inner surface typically with two thin marginal ribs each, likely serve as a cushioning brace for any forward action of the folded anchor roots. Similar pads have only occasionally been reported in Gyrodactylus, with one species being G. proterorhini Ergens, 1967 from gobies in European freshwaters. Based on available sequences, the SSU rRNA gene was most similar to G. curemae Conroy and Conroy, 1985 from a mullet in coastal Venezuela, while the ITS was most similar to G. proterorhini. The relationship of G. olsoni to other species in the genus is discussed with support of a molecular phylogeny that finds G. olsoni to be a member of a widespread, mostly marine lineage that has radiated along with gobiid and blenniid fishes during their global radiation. Gyrodactylus olsoni has not yet been found on other species of gobies.