Seasonal and reciprocal succession and cyclomorphosis of two Bosmina species (Cladocera, Crustacea) co-existing in a lake: their relationship with invertebrate predators

The seasonal succession and cyclomorphosis of Bosmina longirostrisand Bosmina fatalis (also known as Sinobosmina fatalis) wereinvestigated in the shallow eutrophic Lake Suwa from 1996 to1999. Two different invertebrate predators, cyclopoid copepodsand Leptodora kindtii, co-existed with Bosmina in the lake.A pattern of seasonal succession of the two species was apparent,with B. fatalis dominant in summer and B. longirostris in springand fall. The two species also showed different seasonal changesin morphology. During summer, when the density of the invertebratepredator Leptodora was high, B. fatalis changed its morphologyand maintained long features, with elongation of the antennulesand mucrones. This species also maintained the protruding shapeof the antennules by increasing the angles between the antennulesand the body, and between two antennules, during the same period.The seasonal differences and magnitude of changes in morphologywere more marked for larger individuals. However, such markeddevelopment of protruding structures was not observed in B.longirostris, which showed only slight changes in the antennuleand mucrone lengths related to the density of copepods. Theresults suggest that the morphological traits of B. fatalisare a morphological defence against Leptodora, while those ofB. longirostris are a response to copepods. In Lake Suwa, thepredators show different patterns of seasonal changes in density.Different morphological traits and the resulting interactionswith invertebrate predators seem to be important in determiningthe reciprocal succession of the two Bosmina species.