Variation in ephippial buoyancy in Daphnia pulicaria

1. Freshwater zooplankton often produce diapausing eggs to survive environmental stress. The diapausing eggs of Daphnia (Crustacea, Cladocera) are encased in an ephippium that either floats at the surface or sinks to the sediment. These two types of ephippia may represent different strategies between spatial (floaters) and temporal (sinkers) dispersal of offspring. 2. We observed floating and sinking characteristics of ephippia obtained from eight lakes. We then conducted an experiment with 26 Daphnia pulicaria clones obtained from six of these lakes and observed the production of buoyant and non-buoyant ephippia under high and low food conditions. 3. Ephippia were more often non-buoyant than buoyant both from females caught in nature and those reared in the laboratory. The experiment revealed that each clone was able to produce both types of ephippia, but that there was considerable among-clone variation in the percentage of non-buoyant ephippia produced. 4. We conclude that production of non-buoyant versus buoyant ephippia may be driven by both genetic and environmental factors.