Aphanizomenon gracile increases in width in the presence of Daphnia. A defence mechanism against grazing?

Filamentous cyanobacteria are frequently consumed by grazers like Daphnia, which can break filaments and make them more readily available to filter-feeders. However, various defence mechanisms against grazing have also been observed in cyanobacteria. Data concerning changes in the morphology of filamentous algae, especially their width in the presence of a grazer, are scarce. Field studies of filament morphology of cyanobacteria relate their changes to nutrient availability and temperature. Moreover, filament morphology displays significant differences in filament length and width among seasons. We hypothesised that the morphological changes in filament observed in the field – especially their width – could be a defence mechanism that is induced by the presence of a grazer, such as Daphnia. Thus, two experiments were conducted in order to test the influence of Daphnia (direct grazing and infochemicals together in the first experiment) and the chemicals it released (grazing excluded, only chemicals present in the second experiment) on Aphanizomenon gracile’s morphology, in controlled laboratory conditions. Aphanizomenon filaments became significantly shorter and thicker in both experiments. However, Daphnia’s grazing combined with excreted chemicals had stronger effect than chemicals alone. To our knowledge, this is the first report describing the shortening and thickening of filaments in the presence of Daphnia infochemicals. It seems that the Aphanizomenon filaments in the presence of Daphnia switch their growing mode and invest more heavily in width than length. Our results support the hypothesis that Daphnia is at least partly responsible for the changes in filament width observed in the field. This could be a strategy that helps Aphanizomenon to withstand grazer’s pressure during early stages of a bloom.