1. Department of Biology, Centre for Ecological and Evolutionary Synthesis (CEES), University of Oslo, PO Box 1050 Blindern, N‐0316 Oslo, Norway
2. Norwegian Institute for Nature Research (NINA), NO‐7485 Trondheim, Norway
3. Centre National de la Recherche Scientifique (CNRS), UMR 8013 ELICO, Université des Sciences et Technologies de Lille 1, Station Marine, 28 Avenue Foch, BP 80, F‐62930 Wimereux, France
4. Flødevigen Marine Research Station, Institute of Marine Research, NO‐4817 His, Norway