Probing for depth-gradient diversification in the riverine-spawning Dolly Varden: Insights from a recently discovered ecomorph assemblage

The interplay of species could affect the pathways of adaptive diversification in lacustrine fishes. For instance, sockeye salmon is highly effective in transporting biomass from the ocean to the lakes providing abundant food supply for resident dwellers. The largest Asian population of sockeye salmon (>1.5 million spawners annually) reproduces in Lake Kurile (Russia). Here we investigate the possibility of the rapid in-lake diversification of the tributary-spawning charr (genus Salvelinus, Salmonidae) that colonised the Lake Kurile habitats >100 m depth and gained access to decomposing sockeye carcasses and eggs. The results confirm the trifurcation of the Lake Kurile charr along with the depth gradient into three reproductively isolated morphs: anadromous Dolly Varden utilising littoral and tributary resources, the resident slope charr and the profundal charr. No feeding specialisation among the morphs was found as the lake is rich in sockeye remains irrespective of depth. Thus, the in-depth specialisation of the charrs did not result in the archetypical tropho-morphological adaptations described for the majority of the deep-water species of fishes. Only the size reduction and some paedomorphic peculiarities characterise the charr permanently dwelling in the 100-300 m depth zone. The lack of morphological differences among the morphs could be explained by the short period of evolutionary diversification. The divergence event is estimated to have occurred after the close of the glacial period in the middle of Holocene, which corresponds to the time of the lake formation. A bottleneck and subsequent rapid expansion can be traced in the demographic history of the profundal morph, suggesting the possibility of modern sympatric diversification in fish without a trophic resource partitioning.