INTRODUCTION
Animals experience various abiotic and biotic stressors in their environment and often many of them simultaneously. These stressors challenge individuals and require them to adjust their behavior and/or physiology (e.g., Killen et al., 2013; Kolonin et al., 2022; Øverli et al., 2006). Individuals vary in their reactions to stress, hence there is also variation in stress-associated physiological and behavioral traits (Kolonin et al., 2022; Koolhaas et al., 1999). The variation in stress-coping can be a consequence of the amount of trait variation individuals harbor (plasticity) and the level of sensitivity they have (Engell Dahl et al., 2021; Radley et al., 2015). This has been demonstrated not only among species but also within species. An example of the former comes from Lake Tahoe where larvae of a native fish species were shown to be better able to cope with increased ultraviolet radiation by increasing their pigmentation than those of non-native species (Gevertz et al., 2012). Individual variation towards confinement stress has been demonstrated in rainbow trout (Oncorhynchus mykiss ; Pottinger et al., 1992). Sticklebacks (Gasterosteus aculeatus ) originating from different populations showed different stress reactions towards predator cue and confinement (Bell et al., 2010).
Different life-history strategies can also affect individuals’ ability to respond to stress. Animals with ‘fast life histories’ (fast juvenile growth, early maturation, small adult body size, and reduced life span) are suggested to invest less in functions that are not directly related to growth and reproduction yet require a considerable amount of energy, such as detoxification (Congdon et al., 2001). Thus, individuals with fast life histories can have lower stress-coping capabilities than ones with ‘slow life histories’ (slow juvenile growth, late maturation, large adult body size, and extended life span). This hypothesis has been tested only by a handful of studies. In an experiment where four damselfly species were exposed to pesticides, Debecker and colleagues (2016) demonstrated a stress response in the fastest-living species, in terms of pesticide-induced effect on the covariance between life history and boldness. Fast growing sticklebacks exposed to thermal stress had higher stress response in oxidative DNA damage (Kim et al., 2018) and birds with fast life histories suffered more from oxidative stress than birds with slow life histories (Vágási et al., 2019). Humans are changing environment at an increasing pace and exposing animals to different extrinsic stressors (Ceballos et al., 2015; Coleman & Williams, 2002; Häder et al., 2020). Therefore, it is important to understand individual variation in stress-coping styles, what might be the mechanisms underlying these and what are the consequences of stress for populations consisting of individuals with, for example, different life-history types. Indeed, stress coping is of fundamental importance to fitness and understanding individual differences in coping ability has become a paramount task in stress research (e.g., Bartolomucci et al., 2005; Cavigelli & McClintock, 2003; Korte et al., 2005).
Heavy metal and mineral pollution in aquatic environments are a global threat to fish populations (e.g., Kakade et al., 2023; Zamora-Ledezma et al., 2021). Particularly manganese (Mn) and manganese sulfate (MnSO4) are contaminants elevated in aquatic ecosystems due to anthropogenic activities, such as mining and metal industry (Arola et al., 2017, 2019), tilling of acid sulfate soils (Nyman et al., 2023), and from wastewater and sewage systems (Howe et al., 2004). Short term exposures to Mn and MnSO4 have been shown to reduce growth in brown trout (Salmo trutta ) at early life stages (Stubblefield et al., 1997) and increase larval and egg mortality in rainbow trout and whitefish (Coregonus lavaretus ; Arola et al., 2017; Lewis, 1976). Although at high concentrations Mn is known to be harmful for aquatic organisms (Barnhoorn, 1999; Howe et al. 2004; Pinsino et al. 2010; Stubblefield et al. 1997) very little is known about the long-term effects of moderately elevated (i.e., sublethal) concentrations of Mn to fish life histories, physiology, and behavior. Mn occurs naturally in surface waters at concentrations of 0.01-1.0 mg L-1 (Lydersen et al., 2002) and the long-term freshwater environmental quality standards for Mn determined by species sensitivity distribution varies from 0.073 (0.033-0.466) mg L-1 in acid soft waters in Australia (Harford et al., 2015) to 0.062-0.123 mg L-1 in the UK (Peters et al., 2010).
In this study, we demonstrate long-term effects of elevated concentration of MnSO4 to various phenotypic traits in fish with fast and slow life histories. To study the mechanisms of potential effects of MnSO4 on growth and condition factor, we additionally investigated its effects on physiology (metabolic rate) and behavior (activity and feeding). We used zebrafish (Danio rerio ) populations which had been selected for body size (Uusi-Heikkilä et al., 2015). This directional selection resulted in two different life-history types: fast life-history fish (selected for small body size; hereafter small-selected fish) and slow life-history fish (selected for large body size; hereafter large-selected fish). To understand the effect of size selection on fish ability to cope with abiotic stress, we exposed both small- and large-selected fish to manganese sulfate in laboratory environment and monitored them from embryo to adulthood.