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Abstract
The genus Unionicola Haldeman in the family Unionicolidae is distributed worldwide, with more than half of the known species recorded as parasites of freshwater mussels (Mollusca: Bivalvia: Unionoida). Re-evaluation of the morphology and biogeography of the genus in Africa, Australia and South America extends our knowledge of host selection and host-specificity among Unionicola. New host and geographic records for Unionicola (Kovietsatax) walkeri Viets are Velesunio angasi in Western Australia. The subgenus Australatax Vidrine is re-evaluated. A new subgenus, Hyricola n. subgen., is erected for Australian members of the subgenus Australatax sensu lato. Discrete differences in leg chaetotaxy, pedipalp morphology, and female genital field chaetotaxy clearly separate the newly redefined Australatax from Hyricola. These changes provide an opportunity to re-evaluate all the gill mites in the Southern Hemisphere.