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<p>The frequency of severe climatic events has increased recently, drawing increasing attention to their ecological and evolutionary impacts. In riverine systems, spates are major selective agents that shape functional phenotypes. In particular, spate-induced downstream dispersal is often phenotype dependent in riverine organisms and may lead to “spatial sorting,” a selective force arising from differential dispersal. Larger eyes and pupils enhance visual capacity, especially under dim-light conditions, and they may help stream fish to maintain their position and avoid downstream dispersal during spates, which typically involve visually challenging conditions due to turbidity and strong nocturnal flows. Spate-induced spatial sorting may therefore increase the prevalence of larger eyes and pupils in fish in upstream habitats. Here, I examined whether spatial sorting caused by spate-induced downstream dispersal affected eye and pupil sizes, thereby influencing the distribution of these phenotypes in fat minnows, <i>Rhynchocypris oxycephala jouyi</i>, a common cyprinid stream fish in western Japan. Two mark–recapture experiments were conducted during naturally occurring severe spate events. Combined analyses of the two experiments showed that relatively large eyes or pupils for body size increased the likelihood of retention in the upstream section, suggesting potential sorting pressure on visual–cognitive traits. Additionally, differences in body-size-corrected eye and pupil sizes between individuals recaptured in the release section (i.e., undispersed individuals) and all recaptured individuals were observed, although each difference was significant in only one of the experiments. These results suggest that spate-induced dispersal generates sorting pressure on visual–cognitive traits, potentially driving phenotypic shifts through the non-random spatial persistence of individuals with larger eyes or pupils. Although caution is warranted in extrapolating the results of this experimental study to natural settings, the findings provide intriguing insights into the selective mechanisms imposed by severe climatic events.</p>

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Experimental evidence that spate-induced downstream dispersal drives spatial sorting of visual–cognitive traits in a stream fish

  • 期刊论文

  • 发表日期：

    2026年1月2日

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  • Hiroyuki Yamada

摘要

The frequency of severe climatic events has increased recently, drawing increasing attention to their ecological and evolutionary impacts. In riverine systems, spates are major selective agents that shape functional phenotypes. In particular, spate-induced downstream dispersal is often phenotype dependent in riverine organisms and may lead to “spatial sorting,” a selective force arising from differential dispersal. Larger eyes and pupils enhance visual capacity, especially under dim-light conditions, and they may help stream fish to maintain their position and avoid downstream dispersal during spates, which typically involve visually challenging conditions due to turbidity and strong nocturnal flows. Spate-induced spatial sorting may therefore increase the prevalence of larger eyes and pupils in fish in upstream habitats. Here, I examined whether spatial sorting caused by spate-induced downstream dispersal affected eye and pupil sizes, thereby influencing the distribution of these phenotypes in fat minnows, Rhynchocypris oxycephala jouyi, a common cyprinid stream fish in western Japan. Two mark–recapture experiments were conducted during naturally occurring severe spate events. Combined analyses of the two experiments showed that relatively large eyes or pupils for body size increased the likelihood of retention in the upstream section, suggesting potential sorting pressure on visual–cognitive traits. Additionally, differences in body-size-corrected eye and pupil sizes between individuals recaptured in the release section (i.e., undispersed individuals) and all recaptured individuals were observed, although each difference was significant in only one of the experiments. These results suggest that spate-induced dispersal generates sorting pressure on visual–cognitive traits, potentially driving phenotypic shifts through the non-random spatial persistence of individuals with larger eyes or pupils. Although caution is warranted in extrapolating the results of this experimental study to natural settings, the findings provide intriguing insights into the selective mechanisms imposed by severe climatic events.