Intraspecific variation and structuring of phenotype in a lake-dwelling species are driven by lake size and elevation

Abstract

The fragmented, heterogeneous and relatively depauperate ecosystems of recently glaciated lakes present contrasting ecological opportunities for resident fish. Across a species, local adaptation may induce diverse and distinct phenotypic responses to various selection pressures. We tested for intraspecific phenotypic structuring by population in a common native lake-dwelling fish species across a medium-scale geographic region with considerable variation in lake types. We investigated potential lake-characteristic drivers of trophic morphology. Using geometric morphometric techniques, we quantified the head shapes of 759 adult brown trout (Salmo trutta L.) from 28 lakes and reservoirs across Scotland. Multivariate statistical analyses showed that almost all populations differed from one another. Trout from larger and deeper lakes had deeper, but shorter heads, and smaller eyes. Higher elevation lakes were associated with fish with shorter heads and jaws. Within-population shape variation also differed by population, and was positively correlated with lake surface area and depth. Trout within reservoirs differed subtly from those in natural lakes, having larger eyes, shorter jaws and greater variability. This study documents an extraordinary morphological variation between and within populations of brown trout, and demonstrates the role of the extrinsic environment in driving phenotypic structuring over a medium-scale and varied geographic area.

Patterns, processes and conservation management consequences of intraspecific diversity, illustrated by fishes from recently glaciated lakes

“…Similarly, structuring of phenotype may reflect evolutionary responses to different local environments (Bolnick et al, 2011; Koene et al, 2020; Recknagel et al, 2017), especially when structuring establishes between discrete habitats, both in allopatry and sympatry. The fragmented nature of freshwater systems is especially likely to promote allopatric structuring between catchments within obligate freshwater species with limited powers of dispersal, such as many freshwater fishes (Adams et al, 2016; Bush & Adams, 2007).…”

“…Similarly, structuring of phenotype may reflect evolutionary responses to different local environments (Bolnick et al, 2011;Koene et al, 2020;Recknagel et al, 2017), especially when structuring establishes between discrete habitats, both in allopatry and sympatry.…”

Identifying and conserving sympatric diversity in trout of the genus Salmo, with particular reference to Lough Melvin, Ireland

“…Specific environmental conditions (e.g. lake depth and altitude) may also contribute to changes in morphology, especially of the head (Koene et al, 2020). Identical ecomorphological adaptations and associated genetic and gene regulation changes can occur in unconnected lakes as a result of similar selection pressures, and thus, the evolution of similar phenotypes in different lakes is a common phenomenon (Eaton et al, 2021;Jacobs et al, 2020;Rougeux et al, 2019;Taylor, 1999).…”

“…Eurasian trout raised in an enriched hatchery environment differ in their colouration and spotting patterns from siblings reared in a conventional hatchery set up (Yaripour et al, 2020). When Eurasian trout populations are examined for morphometrics on a multivariate basis almost all populations are generally found to be distinct (Koene et al, 2020). Unfortunately, Salmo species are still being described based solely on environmentally labile characters, without demonstration of heritability and adaptation, and ratios to standardise for size, which are long known to be inappropriate due to allometric growth (Phillips, 1983).…”

“…Sympatric diversity appears to occur more in lakes rather than rivers, and in lakes that were formed by the last glaciation relative to older ones (Koene et al, 2020). Lakes typically have a greater diversity of habitats than rivers facilitating ecological segregation; this ecological opportunity and spawning diversity increase with lake size and depth (Doenz et al, 2019; Recknagel et al, 2017).…”