Morphological divergence in a trimorphic population of Arctic charr (Salvelinus alpinus (L.)) in Skogsfjordvatn, northern Norway.

Sympatric polymorphisms are found in many freshwater fish taxa, including the salmonid Arctic charr (Salvelinus alpinus (L.)). Polymorphism is often expressed as differences in morphology, behaviour and life-history strategies, and may be driven by alternative phenotypic adaptations to resource use such as habitat and prey preferences. Morphological divergence is usually linked to different functions of the morphological trait. Here, I study the correlations between morphology and ecological function in sympatric morphs of Arctic charr. \nThe oligotrophic lake Skogsfjordvatn (Norway) has been found to inhabit a trimorphic population of Arctic charr: a littoral spawning omnivore morph (LO morph), a profundal spawning benthivore morph (PB morph) and a profundal spawning piscivore morph (PP morph). The three charr morphs reveal highly variable morphologies regarding both body- and head morphology. They also diverge in resource use (i.e. diet and habitat), life-history strategies, and into three genetic groups. \nThe LO morph appears as a typical charr found in monomorphic populations. It predominantly utilizes the littoral-pelagic habitats, has a wide diet niche and express similar life-history traits found in monomorphic charr. The other two morphs reside in the profundal habitat throughout their lifetime, and were found to diverge in morphology, prey utilization and have highly contrasting life-history strategies. The small-sized PB morph is found to have a paedomorphic appearance with a body- and head shape adapted to live close to the soft profundal bottom and to utilize benthos submerged in the sediment. The PP morph has a large, robust head and an elongated body shape strongly related to its piscivorous behaviour, predominantly utilizing small charr and three-spined sticklebacks as prey. Both the profundal morphs have large eyes, suggested as an adaptation to survival in a darker environment. All the morphs reveal morphologies that clearly are adaptations to their environmental surroundings and their foraging ecology. Thus, the study provides empirical support for incipient ecological driven speciation to be in action.