Abstract
Phenotypic divergence in response to divergent natural selection between environments is a common phenomenon in species of freshwater fishes. Intraspecific differentiation is often pronounced between individuals inhabiting lakes versus stream habitats. The different hydrodynamic regimes in the contrasting habitats may promote a variation of body shape, but this could be intertwined with morphological adaptations to a specific foraging mode.Herein, I studied the divergence pattern of the European minnow (Phoxinus phoxinus), a common freshwater fish that has received little attention despite its large distribution. In many Scandinavian mountain lakes, European minnows are considered as being invasive and were found to pose threats to the native fish populations due to resource competition. Minnows were recently found to show phenotypic adaptations in lake versus stream habitats, but the question remained if this divergence pattern is related to differences in resource use. I therefore studied the patterns of minnow divergence in morphology (i.e., using geometric morphometrics) and trophic niches (i.e., using stomach content analyses) in the lake Ånnsjön and its tributaries to link the changes in body morphology to the feeding on specific resources. Lake minnows showed a strong reliance on benthic Cladocera and a more streamlined body shape with a more upward facing snout, whereas stream minnows fed on macroinvertebrates (larvae and adults) to a higher degree and had a deeper body with a snout that was pointed down. Correlations showed a significant relationship of the proportion of macroinvertebrates in the gut and morphological features present in the stream minnows. The results of this study highlight the habitat‐specific divergence pattern in morphology and resource use in this ubiquitous freshwater fish. Consequently, interspecific interactions of invasive minnows and the native fish population could differ in the respective food webs and resource competition could target different native fish species in the contrasting habitats.
Highlights
Natural selection can evoke adaptive phenotypic divergence of populations (Endler, 1986; Rundle & Nosil, 2005; Schluter, 2000), which can lead to the formation of distinct populations or ecotypes (Svanbäck & Bolnick, 2005, 2007), and might even initiate speciation (Hendry, 2009)
Divergence can be based on resource use, often referred to as trophic polymorphism (Skúlason & Smith, 1995; Smith & Skúlason, 1996)
The contribution of benthic Cladocera was significantly higher in lake locations compared with streams (Mann–Whitney U: Z1 = −3.807, p < .001, Table 2, Figure 4), whereas no significant difference could be found in the contribution of pelagic zooplankton between minnows caught in the lake and streams
Summary
Natural selection can evoke adaptive phenotypic divergence of populations (Endler, 1986; Rundle & Nosil, 2005; Schluter, 2000), which can lead to the formation of distinct populations or ecotypes (Svanbäck & Bolnick, 2005, 2007), and might even initiate speciation (Hendry, 2009). To forage within larger areas in the lake habitat may generate a more streamlined body, whereas swimming and maneuvering in the structurally complex stream habitat while searching for the more cryptic benthic prey will be supported by a deeper body (Anderson, 1984; Ehlinger, 1989; Robinson & Parsons, 2002) Such trophic polymorphism has been reported, for example, in lake and stream ecotypes of the three-spined stickleback (Gasterosteus aculeatus) (Berner et al, 2008; Hendry, Taylor, & McPhail, 2002) and juvenile sockeye salmon (Oncorhynchus nerka) (Pavey, Nielsen, Mackas, Hamon, & Breden, 2010). I predict that there is a relationship between morphology and dietary preference, indicating a specific body form when consuming specific prey
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