Translocation Experiments: Bosmina Responses to Copepod Predation

Abstract

Regional surveys of the small cladoceran Bosmina longirostris reveal a complex morphological pattern. Body size and shape vary greatly, with the most pronounced differences found in the posterior mucrones and anterior antennules. Some lakes have short—featured clones, most have intermediates, while a few have very long—featured clones. Neighboring lakes of similar morphometry can have short— or long—featured clones. Possible explanations for the geographic patterns include: (1) initial colonization events, (2) competitive release following colonization, (3) hybridization between formerly isolated breeding stocks, and (4) natural selection from resident predators, particularly omnivorous copepods. Field translocation experiments demonstrated the importance of Epischura predation in determining the success of long—featured clones. Prey risk was both size— and shape—dependent, with fitness determined largely through the differential survivorship of early instars. Increased fitness associated with longer features was evident at several levels: (1) during clonal recruitment from resting eggs, (2) in the success of translocated stocks, and (3) as developmental responses within exposed prey populations. In long—term exposures to Epischura, feature lengths of short—featured stocks increased by several standard deviations. Mucro length showed the greatest response, although antennule lengths and egg lengths also showed significant increases. During the Epischura lacustris—Bosmina longirostris interaction, responses were clearly the joint effect of two separate phenomena: selective responses (due to differential mortality) and chemical induction (developmental plasticity). Mucro and antennule responses were accompanied by significant declies in coefficients of variation.