Thicker filaments of are more harmful to than thinner .

BACKGROUND

Filamentous cyanobacteria are known to negatively affect the life history of planktonic herbivores through mechanical interference with filtering apparatus. Here, we hypothesise that not only the length but also thethickness of cyanobacterial filaments is an important factor shaping the life history of .

RESULTS

To test our hypothesis, we cultured with non-toxin-producing strains of either or.The former possesses wide filaments, whereas the latter has thinner filaments. The strain of has two morphological forms differing in filament widths. The exposure to the thicker filaments caused a stronger body-length reduction in females at maturity and a greater decrease in offspring number than exposure to the thinner filaments. The width of filaments, however, did not significantly affect the length of newborns. The analysis of mixed thick and thin filament width distribution revealed that reduces the number of thinner filaments, while the proportion of thicker ones increases.Also, the effects of cyanobacterial exudates alone were examined to determine whether the changes in lifehistory were indeed caused directly by the physical presence of morphologically different filaments and not by confounding effects from metabolite exudation. This experiment demonstrated no negative effects of both and exudates.

CONCLUSIONS

To our knowledge, this is the first study that demonstrates that the thickness of a cyanobacterial filament might be an important factor in shaping 's life history. At a given biomass, thicker filaments of were more detrimental to than thinner ones of . There was also a strong interaction between species of the cyanobacterium and filament biomass, where species with thicker filaments and at higher biomass had the strongest negative impact on life history.