Pectoral fins from juvenile rainbow trout (Oncorhynchus mykiss) were used in a bioassay, the object of which was to quantify the effect of simulated shadow stimuli on the transmission success of the cercariae. For a period of 60 min, parasites and fins were exposed to a sequence of computer-controlled shadow stimuli, continuous light or total darkness, and subsequently the number of infections was counted. Trials were conducted with cercariae 1, 12 and 24 h old. Within the constraints of the experimental procedure, shadows were found to increase significantly the number of infections achieved by the 12-h-old parasites. As the cercariae aged, their ability to infect host tissue declined. The relevance of these results to transmission in the natural environment is discussed.

The cercariae of Cryptocotyle lingua have a brief but active life during which they do not feed. In order to maximize the probability of infection they must respond to a variety of host-related stimuli yet conserve their finite energy reserves by minimizing unnecessary muscular activity. In laboratory experiments, simulated shadows representing the passage of a fish host were found to increase population activity levels. Evidence was also found for a relationship between shadow duration and the duration of subsequent bursts of swimming activity. Adaptation to continuous shadowing was recorded, its onset and magnitude being dependent on both the frequency and duration of shadows delivered. As cercariae aged they became less responsive to shadow stimuli, especially those of short duration.