1. Department of Biology, Ecology and Evolution; University of Fribourg; 1700 Fribourg Switzerland
2. Zoological Institute and Zoological Museum, Biocenter Grindel; Hamburg University; Hamburg Germany
3. Tvärminne Zoological Station; FIN-10900 Hanko Finland
4. Centre d'Ecologie Fonctionnelle et Evolutive - UMR 5175; 34293 Montpellier Cedex 5 France