Interactive effects of predation risk and parasitism on the circadian rhythm of foraging activity in the great pond snail Lymnaea stagnalis (Gastropoda: Lymnaeidae)

Four experiments were serially carried out conducted to study foraging activity in the freshwater gastropod Lymnaea stagnalis in the absence and presence of predators (crayfish or fish) under daylight and in the dark. The snails were either parasitized by digenean trematodes (Diplostomum pseudospathaceum or Plagiorchis elegans) or remained unparasitized. The snails had an evident circadian foraging rhythm: they preferred diurnal foraging notwithstanding the predators and parasites. However, the snail’s parasite status and the predation risk had an interaction effect on the number of feeding events per snail individual so that especially the snails parasitized by D. pseudospathaceum were more active in the presence of a fish predator than in the absence of predators. Some of the snails responded to cues from a crayfish predator by crawling out of the water. The results suggest that D. pseudospathaceum may be capable of manipulating the behaviour of its first intermediate snail host in ways that enhance its success of transmission to the parasite’s second intermediate fish host. The results also indicate that the effects of biotic stressors, such as predation risk and parasitism, on the foraging pattern in the snails L. stagnalis may be interactive and highly specialized rather than generalized.