Ingestion and incorporation of freshwater diatoms by Daphnia pulicaria: do morphology and oxylipin production matter?

In order to explain differences in the growth and reproduction of Daphnia pulicaria fed various freshwater diatoms, we measured ingestion rates and carbon incorporation for six cultured diatom species: the single-celled Stephanodiscus hantzschii, Stephanodiscus minutulus and Cyclotella meneghiniana, and the colony-forming Asterionella formosa, Fragilaria capucina and Fragilaria sp. Two of the colony-forming species, when damaged, produced polyunsaturated aldehydes (oxylipins) that have been found to impair the reproduction of marine copepods. We tested two hypotheses: (i) feeding and incorporation rates are affected by diatom morphology; and (ii) polyunsaturated aldehydes act as feeding deterrents. Daphnia body length versus ingestion rate regressions differed for single-celled and colony-forming diatoms. Ingestion rates for single-celled diatoms showed clear size dependencies and high correlation coefficients, while the dependency was weak for colony-forming diatoms and individual variability was high. This difference was not observed for carbon incorporation rates, which showed low variability for all diatoms. Asterionella formosa yielded the lowest incorporation rates due to low incorporation efficiency, while all other diatoms were incorporated at similar rates. Thus, morphological differences of the diatoms had no effect on carbon uptake by Daphnia. The presence or absence of polyunsaturated aldehydes did not cause different ingestion rates; hence the aldehydes are not feeding deterrents.