1. Centre for Ecological and Evolutionary Syntheses (CEES), Department of Biosciences; University of Oslo; P. O. Box 1066 Blindern N-0316 Oslo Norway
2. Institute of Marine Research; Flødevigen Marine Research Station; Nye Flødevigveien 20 N-4817 His Norway
3. Centre for Coastal Research, Department of Natural Sciences; University of Agder; P. O. Box 422 N-4604 Kristiansand Norway