Uncovering hidden species: hatching diapausing eggs for the analysis of cladoceran species richness

Effective methods for the detection of species are highly needed in biodiversity research. The assessment of richness is especially difficult for short-lived aquatic organisms, like plankton. Because of the high degree of spatial and temporal variation in plankton community composition, the compilation of complete plankton species lists requires intensive sampling programs. Many large-scale studies on planktonic organisms are based on single occasion samplings and inevitably fail to detect a substantial fraction of the species. In the present study, we assessed species richness of Cladocera (Branchiopoda, Crustacea) in 88 European lakes by repeated sampling of the active communities and by an alternative method based on the hatching of diapausing eggs from the lake sediments. Identification of hatchlings obtained from egg bank samples enabled the detection of about twice as many cladoceran species than identification of an equal number of individuals retrieved from active community samples. Loglinear analyses revealed a strong species by method interaction, suggesting that both methods are highly complementary and should preferably be applied in combination. Some species were underrepresented in the active community samples, probably because of the lack of spatial and temporal integration in these samples. Other species were underrepresented in the hatching assemblages as a result of species-specific propensities to produce diapausing eggs or hatch from such eggs.