Coexistence of cryptic rotifer species: ecological and genetic characterisation of Brachionus plicatilis

1. The coexistence of five cryptic species of the rotifer species complex Brachionus plicatilis was investigated in four coastal Mediterranean ponds. Monthly sampling was undertaken for 15 months and species were characterised using allozyme electrophoresis. 2. We describe species-diagnostic allozyme loci that can be used for rapid identification of these species. 3. The five species overlapped to some extent in their temporal use of the ponds, although some seasonal segregation was observed. 4. The match between temporal and spatial distribution and limnological conditions suggested ecological specialisation in some cases, although we found striking examples of extensive seasonal overlap. 5. Our results indicate that sympatry of cryptic rotifer species is largely because of seasonal ecological specialisation, which allows seasonal succession and partitioning of resources. The processes that might be involved in the long periods of overlapping seasonal distributions of species which are potentially competitors are discussed. This example illustrates that the 'paradox of the plankton' is more the rule than the exception.