Different life-history trade-offs of two Daphnia species (Cladocera, Crustacea) under natural conditions as the response to predation and competition

Daphnia longispina (O. F. Mu¨ ller 1785) and Daphnia cucullata (G. O. Sars 1862), despite coexisting in similar densities in the lake, showed different demographic cycles. To search the causes of such variances between the two Daphnia, selected population parameters (density, body proportions, reproductive output) were correlated with the density of five dominant and potentially competitive species of Cladocera. The same parameters were estimated in relation to predation pressure of vertebrates (planktivorous fish) and invertebrates (Leptodora kindtii (Focke 1844)). The obtained results show that competition had no apparent impact on life-histories of Daphnia. Different strategies of the two Daphnia resulted from the uneven effect of invertebrate and vertebrate predators. D. longispina that was affected by planktivorous fish and Leptodora kindtii invested in reproduction and carried big egg-clutches. D. cucullata, which was insignificantly influenced by fish, displayed low reproductive output but changed their body proportions in the process of cyclomorphosis when occurring with L. kindtii. Other life-history trade-offs altered during the year, in accordance with the intensity of predation pressure.