On the biology of Rhabdochona rotundicaudatum and R. cascadilla (Nematoda: Thelazioidea) in stream fishes from southern Ontario, Canada

Abstract

The seasonal pattern of parasite acquisition, maturation, and loss of Rhabdochona cascadilla Wigdor, 1918 and Rhabdochona rotundicaudatum Byrne, 1992 collected in common shiners (Notropis cornutus Mitchill, 1817) from the Eramosa River in southern Ontario, Canada, was followed. Adult R. cascadilla and R. rotundicaudatum appeared in large numbers in late spring. Females of both species matured quickly and contained embryonated eggs in June and July. Prevalence and intensity of R. cascadilla tapered off throughout the summer but never reached zero. Prevalence and intensity of R. rotundicaudatum decreased rapidly through the summer and only one adult was recovered after August 1. Rapid acquisition of worms by fish in late spring seems to be facilitated by the convergence of the intermediate and definitive hosts in space and time. Convergence with common shiner and the intermediate host of R. rotundicaudatum, Ephemera simulans Walker, 1853 (Ephemeroptera), occurs in the stream riffle areas during late May and early June. Results suggest that the intermediate host of R. cascadilla and the common shiner also converge during the spring, but because the intermediate host of R. cascadilla was not found, this could not be confirmed. Also, the presence and rapid maturation of adult R. rotundicaudatum are probably related to their acquisition as adults from the mayfly intermediate host. These findings support the hypothesis that the presence of precocial nematode parasites in the intermediate host represents a transmission strategy that accelerates gametogenesis in the final host in order to take advantage of the limited time and space available for transmission.