Plastic phenotypes of antennule shape in Bosmina longirostris controlled by physical stimuli from predators

Three antennule morphotypes in the small cladoceran Bosmina longirostris have been considered to be hereditarily rigid phenotypes. However, we found that monoclonal populations of Bosmina collected from a Japanese lake were composed of individuals with all three antennule types when grown under laboratory conditions. This suggests that each of the morphotypes represents a phenotype of the species. A morphotype named pellucida observed in the monoclonal cultures has been considered to be effective in avoiding copepod predations. We therefore hypothesized that the morphotype is induced by factors from the predator, cyclopoid copepod. To test this hypothesis, a laboratory experiment was conducted in which the monoclonal individuals were exposed to the kairomones of the copepod Acanthocyclops with or without physical contacts with the predator. The morphotype was not altered when Bosmina was exposed to the predator kairomone alone, but to direct predation by the copepod. This suggests that the antennule type of B. longirostris is controlled by physical stimuli from the copepod. The individual death rate of B. longirostris resulting from a copepod strike is low, even though the population dynamics of the animals are often governed by copepod predation. This may have made the physical stimuli of attempted predations useful factors for the prey Bosmina.