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

Abstract

The seasonal succession and cyclomorphosis of Bosmina longirostris and Bosmina fatalis (also known as Sinobosmina fatalis) were investigated in the shallow eutrophic Lake Suwa from 1996 to 1999. Two different invertebrate predators, cyclopoid copepods and 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 spring and fall. The two species also showed different seasonal changes in morphology. During summer, when the density of the invertebrate predator Leptodora was high, B. fatalis changed its morphology and maintained long features, with elongation of the antennules and mucrones. This species also maintained the protruding shape of the antennules by increasing the angles between the antennules and the body, and between two antennules, during the same period. The seasonal differences and magnitude of changes in morphology were more marked for larger individuals. However, such marked development of protruding structures was not observed in B. longirostris, which showed only slight changes in the antennule and mucrone lengths related to the density of copepods. The results suggest that the morphological traits of B. fatalis are a morphological defence against Leptodora, while those of B. longirostris are a response to copepods. In Lake Suwa, the predators show different patterns of seasonal changes in density. Different morphological traits and the resulting interactions with invertebrate predators seem to be important in determining the reciprocal succession of the two Bosmina species.