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Planktivorous fish positively select Daphnia bearing advanced embryos

The top-down effect of fish predation on reproduction success of Daphnia females was investigated in an outdoor mesocosm experiment with natural (oligotrophic) food conditions. The planktivorous fish, sunbleak (Leucaspius delineatus, 4.9 individuals m–3) was introduced into a half of mesocosms after the first sampling of zooplankton to compare daphnid life-history traits with and without fish predators. Our results showed selective fish predation on the daphnid females with advanced developmental stages of embryos. The Daphnia populations exposed to fish exhibited a lower proportion of females with advanced embryos over those with earlier embryonal stages. Fish predators obviously were attracted to more visible females bearing larger embryos, with well-developed pigmented eyes. Simultaneously, we found a smaller daphnid body length in the mesocosms with fish than in those that were fishless. However, the daphnid clutch size did not reflect their body-length pattern and, surprisingly, decreased regardless of fish presence or absence since the first sampling day, probably owing to deterioration of food quality (mean seston carbon:phosphorus ratio ~238). Nevertheless, this selective elimination of the females with advanced embryos, together with the overall decrease in daphnid fitness, can strengthen the deceleration of its population growth under fish predation.