1. Marine Biological Association, The Laboratory, Citadel Hill, Plymouth, PL1 2PB United Kingdom, and Ecole Normale Supérieure de Lyon, Laboratoire de Biologie Cellulaire et Moléculaire, F-69364, Lyon Cedex 07 France
2. Marine Biological Association, The Laboratory, Citadel Hill, Plymouth, PL1 2PB United Kingdom