Introduction
The brain is the anatomical structure that determines the information
processing capacity and behavioral adaptation of animals (Kotrschalet al. , 1998; Pike et al. , 2018). Brain size is driven by
the trade-off between benefits provided by cognitive skills and costs
for its development and maintenance (Boogert et al. , 2018;
Morand-Ferron et al. , 2015). Previous studies have shown that the
fish brain often responds to selection pressure as a modular organ, in
that only specific region controlling required cognitive skills under
the selection will increase their volume, while the brain regions that
are not used can reduce their volume as an energy saving adaptation
(Kotrschal et al. , 2017; Pike et al. , 2018; Fong et al.
2021). Response of brain to physical habitat complexity has been
proposed as one of the key drivers shaping brain morphology across
different fish species, with more complex habitats selecting for larger
brains and, particularly, for the larger brain regions that facilitate
spatial navigation and complex decision making (i.e.,telencephalon), perception of visual cues (i.e., optic tectum),
and motor coordination (i.e., cerebellum) (Kotrschal et
al. , 1998; Pollen et al. , 2007). The changes in brain morphology
of fishes can be evolutionary (Kotrschal et al. , 1998; Pollenet al. , 2007) as well as plastic (e.g., Näslund et
al., 2012; Triki et al. , 2019). However, the association of
physical habitat complexity and brain morphology in wild fishes has been
much less studied at intraspecific than interspecific level.
A study on three-spined sticklebacks Gasterosteus aculeatus have
shown differences in brain morphology among populations from lake and
stream habitats (Ahmed et al. , 2017), but these differences were
not always consistent with the prediction that individuals from more
physically complex stream habitat have larger telencephalon than
individuals from less complex lake habitat (Ahmed et al. , 2017).
Lake fish can also experience high habitat complexity in lakes with
developed littoral zone, but fish in lakes with simple shoreline should
generally experience lower physical habitat complexity than stream
dwelling conspecifics (Park & Bell 2010; Ahmed et al. , 2017).
Therefore, what drives the differences in brain morphology among lake
and stream dwelling populations of fishes, remains an open question.
Development of brain morphology is inherently linked to supply of energy
and nutrients, particularly of omega-3 long-chain polyunsaturated fatty
acids (n-3 LC-PUFA) (Pilecky et al. , 2021). The availability of
these nutrients differs across ecosystems, and they are more available
to fish in lake habitats than in streams (Heissenberger et al. ,
2010). High amount of dietary n-3 LC-PUFA is also typical for diet of
hatchery reared fish (Heissenberger et al. , 2010), which are
raised in habitats with extremely low physical complexity (Näslundet al., 2012). Therefore, a comparison of brain morphology across
individuals from stream, lake, and hatchery habitat can provide an
insight into intraspecific responses of brain size and morphology to
habitat quality in freshwater fishes.
Brown trout, Salmo trutta L., is a good model species for such
comparative study, because genetically and phenotypically different
populations of brown trout occur in lake and stream habitats (Jonsson &
Jonsson 2011). A previous study has shown differences in brain
morphology of anadromous and stream resident brown trout, which were
suggested to be driven by differences in sex specific reproduction
strategies rather than by physical habitat complexity (Kolm et al.
2009). Brown trout, like other salmonids, are also often reared in
extremely simple hatchery environments (Heissenberger et al. ,
2010; Näslund et al., 2012). Some evidence suggests that
hatchery-reared individuals have limited cognitive skills caused by a
plastic response of their brain to the simplicity of the habitat in
which they have developed (e.g., Näslund et al., 2012), and by an
evolutionary response to the artificial selection pressure on the
hatchery strains (e.g., Fleming et al., 2000). In this study, we
aim to compare brain morphology of brown trout from stream, lake, and
hatchery environments in order to test how brain morphology varies
across environments that differ in physical habitat complexity and
quality of available diet.