Juvenile divergence in adaptive traits among seven sympatric fish eco-morphs arises before moving to different lacustrine habitats

Identifying the mechanisms initiating sympatric diversification in vertebrates has remained a conceptual challenge. Here we analyze an assemblage of sympatric charr (Salvelinus malma) morphs from landlocked Lake Kronotskoe basin as a model to uncover the divergence pathways in freshwater fishes during the early life history stages. All morphs have distinct developmental biology, but a similar developmental rate retardation compared to the ancestor. Our study reveals that adult morphological differences, which acquire functionality at maturation, originate in the early juvenile stages due to heterochrony in skeletogenesis and allometric changes triggered by variation in metabolic activity. The craniofacial differences among the morphs result from asynchronous development of several skeletal modules. The accelerated ossification of teeth-armed bones occurs in predatory feeding morphs, while cranial cover ossification is promoted in benthivorous morphs. These contrasting growth patterns ha...