We wish to thank two anonymous reviewers for their suggestions that have greatly improved this paper. We thank the scientists at the Northeast Fisheries Science Center (NMFS) for collecting our samples. I.G. Pola and J.L. Horreo collaborated in laboratory tasks. G.M.S. has been supported by the Spanish Agency of International Cooperation AECI. F.J. received support from a Hatch grant. This study has been supported by the Spanish National Project CGL2009-08279.