Morphological and trophic divergence of lake and stream minnows
(Phoxinus phoxinus)
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 individual
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 adaptions to a
specific foraging mode. Herein, I studied the divergence pattern of the
European minnow (Phoxinus phoxinus), a common freshwater fish that has
paid little attention despite its large distribution. In many
Scandinavian mountain lakes, they are considered as being invasive and
were found to pose threats to the native fish populations due to dietary
overlap. Minnows were recently found to show phenotypic adaptions in
lake versus stream habitats, but the question remained if this
divergence pattern is related to trophic niche partitioning. 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 zooplankton and a more streamlined body
shape with an 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 positive relationship of the proportion of zooplankton in
the gut and morphological features present in the lake minnows. The
results of this study highlight the habitat-specific divergence pattern
in morphology and resource use in this ubiquitous freshwater fish, which
may promote contrasting inter-specific interactions in the respective
food webs.