No impact of a presumed manipulative parasite on the responses and susceptibility of fish to simulated predation.

Parasites manipulating their host to facilitate trophic transmission is a widespread and diverse phenomenon. Trematode eye‐flukes in the family Diplostomidae infect a variety of fish species as metacercariae, many residing in the eyes. A recently described diplostomid, Tylodelphys darbyi, from the South Island of New Zealand has been found to infect common bully Gobiomorphus cotidianus, an endemic freshwater fish. Within the fish, the metacercariae move about freely in the liquid parts of the eye and are quite large. We hypothesized that increasing intensity of T. darbyi infection will result in increasing visual impairment, thus reducing the ability of G. cotidianus to identify and react to a predatory threat. To test this hypothesis, we performed experiments to (a) examine the fish's reaction to a purely visual predator cue and (b) test their ability to avoid simulated predation under natural levels of infection. Among the 64 fish used in our experiments, T. darbyi had a prevalence of 98.7% with an average of 17.6 worms per fish. However, there was no relationship between T. darbyi intensity and either the fish's reaction to a visual predator stimulus or their ability to escape a simulated predator. Our findings indicate that despite being present in large numbers in the eyes of its fish host, the parasite appears incapable of improving its chances of trophic transmission to its avian definitive host. The results also suggest that the fish G. cotidianus could be using other senses (e.g., olfaction and lateral line) to compensate for visual impairment, and detect and respond to predators. [ABSTRACT FROM AUTHOR]