Setae thickening in <Emphasis Type="Italic">Daphnia magna</Emphasis> alleviates the food stress caused by the filamentous cyanobacteria

It is assumed that daphnids adjust the filter screen morphology in order to minimize the interference with cyanobacterial filaments. The aim of this study was to investigate the impact of filamentous cyanobacteria (Aphanizomenon gracile Lemmermann, Cylindrospermopsis raciborskii Woloszynska Seenaya et Subba Raju) on the thickness and length of setae of the third pair of thoracic limbs of Daphnia magna. The second objective was to assess whether the setae modifications could improve the performance of daphnids in the presence of cyanobacteria. Three clones of Daphnia magna Straus were cultured with: green algae; green algae with filaments of Cylindrospermopsis; and green algae with filaments of Aphanizomenon. The size and age of animals in the first reproduction cycle as well as the number of offspring were recorded. Setae thickness and length were measured in the central part of each endopodite. Additionally, we analyzed how the changes in setae morphology affect the fitness of experimental animals using the intrinsic rate of population increase calculated with the Euler–Lotka equation. The results showed that the thickness and length of setae increased in the presence of filamentous cyanobacteria. Moreover, cyanobacteria-induced setae thickening was positively correlated to the fitness of daphnids, which may indicate setae thickening as a phenotypic adaptation to cope with food stress caused by filamentous cyanobacteria.