Introduction
Natural selection can evoke adaptive phenotypic divergence of
populations (1-3), which can lead to the formation of distinct
populations or ecotypes (4, 5), and might even initiate speciation (6).
In freshwater fish species, underlying ecological factors responsible
for diversifying patterns in populations may include different predation
regimes (7, 8), parasite occurrence (9), or different hydrodynamic
conditions (10, 11). An environment that is characterized either by
running or standing water may trigger divergence in fish populations
with regard to morphological, physiological, developmental, or
behavioral traits (12). To reduce the drag in the current, stream fishes
often have a more streamlined body shape (13), which has been found,
e.g. in pumpkinseeds (Lepomis gibbosus ) or rock bass
(Ambloplites rupestris ) (14). Furthermore, divergence can be
based on resource use, often referred to as trophic polymorphism (15,
16). Often, adaptations to different habitat types are intertwined with
trophic niche partitioning (17). For example, in three-spined
stickleback (Gasterosteus aculeatus ) (e.g. 18) and juvenile
sockeye salmon (Oncorhynchus nerka ) (19), the lake ecotypes have
a more streamlined body, as this morphology can be seen as beneficial to
forage at relatively high speed and to cover larger areas in the lake
habitat. In contrast, a deeper body generally supports a higher
maneuverability in a structurally complex stream habitat when searching
for the more cryptic benthic prey (20, 21).
In this study, I investigate the patterns of divergence in the European
minnow (Phoxinus phoxinus ), a common freshwater fish that has
paid little attention despite its large distribution (22, 23). It is an
understudied fish species, despite its profound ecological impact when
introduced to new areas, where it can become invasive and has the
potential to modify native ecosystems (24-26). Minnows were recently
found to show phenotypic adaptions in lake versus stream habitats:
Collin and Fumagalli (27) studied minnow populations in Switzerland, and
Ramler, Palandacic (28) investigated minnows in Northern Italy and the
Danube basin. The studies found opposing results: Collin and Fumagalli
(27) found stream minnows being more streamlined, a body form that is
beneficial to reduce the drag in the current. In contrast, Ramler,
Palandacic (28) reported that a streamlined body form was more
pronounced in lake minnows compared to stream minnows and lake minnows
also had larger heads compared to stream minnows. This might be due to
habitat-induced changes in head structures linked to different modes of
foraging, as it is known, for example from European perch (Perca
fluviatilis ) (e.g. 29, 30). However, evidence on trophic niche
divergence, incorporating morphological adaptations in minnows
inhabiting lake versus stream habitats is missing.
In this study, I have analyzed stomach contents to understand the
trophic niches of minnows in the lake Ånnsjön, Central Sweden and its
tributaries. This method has the advantage to provide a direct insight
into the foraging ecology, proving information on ingested prey with a
high taxonomic resolution (31, 32). By combining the resource use
assessment with morphological analyses by geometric morphometrics, I
aimed to link the changes in body morphology to the individual resource
use in the respective habitats. Streams are generally characterized by a
lower abundance of zooplankton (e.g. 33). Therefore, I predict that in
stream minnows, the dietary contribution of zooplankton would be lower
compared to lake minnows. Furthermore, I predict that there is a
relationship between morphological distance and dietary preference,
indicating a specific morphology when consuming specific prey.