Does phylogenetic relatedness imply similar manipulative ability in parasites?

Abstract Many parasites can alter the behaviour of their hosts in a manner beneficial to the parasite (parasitic manipulations). Obviously, examples of non-manipulative parasites also exist; however, their number might be underestimated because of publication bias. Trematodes from the Diplostomidae family that infect fish eyes are often considered manipulators. However, only one eye-dwelling species (Diplostomum pseudospathaceum) of this family has been shown to do so in controlled laboratory conditions. We studied experimentally whether another common diplostomid species (Tylodelphys clavata) manipulated host defensive behaviour using salmonids (Salvelinus malma) reared and infected in the laboratory. We tested fish activity, depth preference, and dip net avoidance (common fish defensive traits) in different lighting conditions. Although the experimental design was identical to those used earlier for D. pseudospathaceum, no manipulative abilities were detected in T. clavata. Infected fish did not differ from control ones in terms of the expression of the defensive behavioural traits tested. Interestingly, fish activity was confounded by fish size in control but not infected fish; however, this pattern does not appear to be a manipulation. Our results show that even closely related parasites occupying similar habitat niches can differ dramatically in their ability to manipulate host behaviour.