Fish-induced life-history shifts in the cladocerans Daphnia and Simocephalus: are they positive or negative responses?

The cladocerans Daphnia and Simocephalus were exposed to fish smells under different food conditions, and their life-history parameters were analysed. They showed similar responses to fish smells and food shortage: lowered juvenile growth rate, reduced mature size, elongated maturation time, reduced brood size and increased offspring size. The life-history changes induced by the fish smell might be a result of deleterious effects of the factor. Although it has been considered that fish-induced life-history shifts in Daphnia are positive responses to predators, our results suggest that they are negative responses of animals that show some other positive responses to fish, such as behavioural and morphological changes to reduce mortality due to predation. Synergism was detected in the effects of the fish smell and food deficiency, which suggests that fish smell reduces the tolerance of Daphnia to an environmental stress, food shortage.