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The Snake River Pilose Crayfish, Pacifastacus connectens (Faxon 1914), previously known only from the middle Snake River drainage in south-central Idaho west to the Harney Basin in southeastern Oregon, was discovered in the lower Deschutes River in north-central Oregon co-occurring with Signal Crayfish, Pacifastacus leniusculus (Dana 1852). Two groups of obligate ectosymbionts (i.e., entocytherid ostracods and branchiobdellidans) associated with these 2 Pacifastacus species displayed asymmetrical patterns of host switching, with host exchange primarily occurring from P. leniusculus onto P. connectens. Our results have implications for understanding patterns of host specificity over short ecological timescales but also raise questions about the persistence of these patterns over time.

Northern crayfish, Orconectes virilis (Hagen, 1870), were collected from 89 sites across Alberta, Saskatche - wan, Manitoba, Montana, North Dakota, and Minnesota. The entocytherid ostracod Thermastrocythere riojai (Hoff, 1943) was found on O. virilis at 45 of the 89 sites, distributed primarily in the eastern and southern portion of the study area. These observations of T. riojai greatly extend the known range of the species. The widespread distribution of T. riojai suggests that the dearth of entocytherid records from other parts of Canada is a result of nontargeted sampling rather than true absence. In addition, we report on observations of 3 noteworthy associations of oligochaetes with their crayfish hosts. RESUMEN.—Se colectaron especimenes del acocil Orconectes virilis (Hagen, 1870) en 89 sitios a lo largo de Alberta, Saskatchewan, Manitoba, Montana, North Dakota y Minnesota. Encontramos el ostracodo entociterido Thermastrocythere riojai (Hoff, 1943) en O. virilis en 45 de los 89 sitios, distribuidos principalmente en la parte sur y la parte este del area de estudio. Estas observaciones de T. riojai amplian considerablemente el area de distribucion conocida de esta especie. La distribucion extensa de T. riojai sugiere que la escasez de registros de entociteridos en otras partes de Canada es el resul- tado del muestreo inespecifico y no de una ausencia verdadera. Ademas, reportamos observaciones de 3 asociaciones sobre- salientes entre los oligoquetos y los acociles huespedes.

Orconectes virilis (northern crayfish) were collected from 67 sites in Alberta, Saskatchewan, Manitoba, and far-western Ontario, Canada, and yielded 2 species of branchiobdellidans, Cambarincola vitreus and Cambarincola chirocephalus. This is the first report of branchiobdellidans in the Prairie Provinces. Cambarincola vitreus was distributed across the study area, but C. chirocephalus appeared to be restricted to southeastern Saskatchewan, southern Manitoba, and the site in western Ontario. Neither branchiobdellidan species was observed on crayfish in the Beaver River or the South Saskatchewan River and associated tributaries upstream (west) of Saskatoon, Saskatchewan, despite multiple sampling at these locations.