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.