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.