Effects of thermal effluents on parasites and commensals of Physa gyrina Say (Mollusca: Gastropoda) and their interactions at Lake Wabamun, Alberta

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Abstract
Effects of thermal effluents on the dynamics of larval helminth parasites and on populations of Chaetogaster limnaei limnaei and Chaetogaster limnaei vaghini (Annelida: Oligochaeta) harbored by Physa gyrina (Mollusca: Pulmonata) were studied at Lake Wabamun, Alberta. Thermal effluents provided the necessary conditions to maintain digenean parasite transmission throughout the year between definitive and intermediate hosts, and increased the prevalence of certain parasites, especially the metacercarial stages.At temperatures above 24 °C prevalences of C. l. limnaei and C. l. vaghini decreased in the lake, and at these temperatures in the laboratory C. l. limnaei abondoned the snails and perished. Under experimental conditions the number of Echinoparyphium recurvatum larvae penetrated Physa gyrina was inversely related to the number of C. l. limnaei present. It was also observed that these oligochaetes actively ingested digenean larvae. Elimination of C. l. limnaei caused by thermal effluents has augmented the metacercarial infections.An ecological model depicting the interactions of Physa gyrina – digeneans – Chaetogaster l. limnaei is presented. This is a complex system in which water temperature acts as a main regulating factor. The three main components of the system interact with each other and are influenced by various external factors, resulting in a dynamic ecological system.