Invasion of Lake Victoria by the Large Bodied Herbivorous Cladoceran Daphnia Magna

Abstract

Zooplankton collections from offshore Lake Victoria during the early to mid 1990s contained a Ctenodaphnia species that is morphologically similar to Daphnia monacha (D. lumholtzi var. monacha) which is present in Lakes Edward and Albert. Direct comparison of the Lake Victoria specimens with representative D. monacha from the Rift Valley lakes reveals that the taxa are not the same, however. Instead, the Lake Victoria animals conform more closely to the morphology of Daphnia magna. The large body size of D. magna makes it unlikely that it was overlooked by previous investigators of the lake. Instead, it appears that food web alterations of Lake Victoria created occasional conditions of relaxed planktivory by fish to such a degree that an herbivore of unusually large size could establish viable populations. The observation runs counter to prevailing paradigms about the small size structure of tropical zooplankton communities compared with those in temperate zones. The observation is consistent with claims that biological interactions rather than physical and chemical factors are the typical determinants of zooplankton size structure in the tropics.