Predation and reproductive performance in two pelagic typhloplanid turbellarians

We investigated feeding and reproductive performance of coexisting pelagic turbellarians from experiments on predation rates of Mesostoma ehrenbergii and M . craci as a function of food ( Daphnia similis , three levels) and temperature (4 levels) during 10 days. Flatworms were collected from the pelagic of a subtropical lake in Brazil. Growth was more rapid at higher temperatures: more prey were consumed, and more eggs produced. M . craci and particularly M . ehrenbergii fitted a linear mixed-effects model and showed a type II functional response. M . craci was the more stenothermic of the two. Intrageneric predation also occurred: M . ehrenbergii fed on M . craci , but not vice versa. After a first clutch of subitaneous eggs, M . ehrenbergii produced resting eggs only. In M . craci an intermediate type of eggs hatched some time after release, survived passage through the gut of M . ehrenbergii , but did not resist drying. By primarily selecting cladoceran prey, M . ehrenbergii can make coexistence of both flatworms possible. As population density of M . ehrenbergii increases, it turns to producing resting and non-viable subitaneous eggs, thus limiting its population size. In nature, these processes structure the zooplankton community, while avoiding extinction of prey and predator.