Acknowledgements
We thank Jochen Dierschke and Klaus Müller for logistical support, and Sven Hessler for fieldwork on Helgoland. The study was supported with a grant from the Deutsche Forschungsgemeinschaft (DFG) awarded to CE (EI 1048/4-1), and grants from the Swedish Research Council (2018-04278) and Crafoord Foundation (2020-0728) awarded to AH.