Divergent functional traits in three sympatric Arctic charr Salvelinus alpinus morphs are not coupled with the age of the lineage divergence

Three genetically discrete morphs of Arctic charr in Loch Rannoch, Scotland originated from a recent divergence within the lake (in situ) (piscivore and benthivore morphs) and from secondary contact of two older lineages (ex situ; a planktivore-piscivore/benthivore divergence). To test if the expression of traits with strong functional roles was linked to the age of the divergence, fin and gill anatomy, and dentition were quantified and compared across morphs. Five additional working hypotheses suggesting a rank order of trait expression amongst morphs were also tested. The planktivorous morph had more rays in the dorsal and pectoral fins, longer gill rakers (but not more) as well as a smaller gill cavity than the other two morphs. The piscivorous morph had more palatine teeth and longer teeth on the mandible, pre-maxillary and glossohyal bones, and a larger buccal cavity. These differences indicate a differential response to selection in these functional anatomical features most likely related to morph foraging specialisms. Notably, between-morph divergences in the expression of these traits were not simply linked to the length of divergence between morphs and have arisen equally quickly in the recent (in situ) divergence as they have in older, ex situ divergences.