DISCUSSION
Life-history type (i.e., selection line) affected fish performance under
stressful environmental conditions. Large-selected fish, which had been
selected for large body size, slow juvenile growth rate and old age at
maturity (“slow life-history type”) seemed to tolerate better high
MnSO4 concentrations than small-selected fish with small
body size, fast juvenile growth rate and young age at maturity (“fast
life-history type”; Uusi-Heikkilä et al., 2015). Large-selected fish
had higher growth rate and condition factor in elevated
MnSO4 concentrations than small-selected fish despite no
significant differences in standard metabolic rate. They were also more
active and had higher feeding probability. These results suggest, that
potentially owing to different energy allocation strategies,
personalities or stress coping styles, individuals with slow life
histories may be better able to cope with chronic environmental stress.
Therefore, we should react with caution to statements that a certain
stress response is common across all individuals within a species
(Balasch & Tort, 2019). Instead, it may depend on individual
characteristics, such as life-history types or personalities (Caizergues
et al., 2022; Prentice et al., 2022; but see Santicchia et al., 2020).
Although manganese accumulated in a similar manner to fish with both
life-history types (Figure 3), the growth response differed between
them. Fish with a selection history for slow life performed better than
fish with a selection history for fast life in terms of growth (Figure
1). The difference was absent in the control treatment, which indicates
recovery of this trait from the past selection but became visible when
individuals experienced more stressful environments (i.e., elevated
MnSO4 concentrations). Previous study has shown that
large-selected fish exhibit more variation in growth and respond to
starvation stress differently than small-selected fish (Uusi-Heikkilä et
al., 2016). These observations imply that large-selected fish are more
plastic and better able to cope with environmental stressors than
small-selected fish. Indeed, it has been suggested by other studies that
stress coping abilities are affected by the degree of individual
phenotypic plasticity (Balasch & Tort, 2019).
The mechanisms behind the higher growth rate of large-selected fish may
be related to differences in, for example, stress physiology and/or
energy metabolism. We did not detect any significant trends in the
standard metabolic rate, which neither differed between the life-history
types nor was linearly affected by the MnSO4concentration. This finding could be related to the small sample size,
high variation, as fish metabolic rates were measured in groups, and/or
to the age of measured fish (i.e., they had already exceeded the period
of fast growth). Furthermore, as the MnSO4concentrations could not be controlled in the respirometer chambers, the
measurements were done in pure water. In addition to the metabolic rate,
there might be other physiological traits (e.g., feed conversion ratio
or metabolic efficiency) that could explain the higher growth rate of
large-selected fish in high MnSO4 concentrations.
The effective Mn concentrations in our long-term experiment (measured
mean concentrations in water 0.19 and 0.41 mg L-1,
nominal concentrations 1.27 and 2.73 mg L-1) were
higher than the predicted no-effect concentrations for aquatic organisms
recommended (Peters et al., 2010; Harford et al., 2015). Acute effective
or lethal concentrations of Mn are clearly higher, for example the
effective concentration (EC10) of zebrafish embryos is 4.63 mg
L-1 (Peters et al., 2010). Mn concentration of more
than 4.5 mg L-1 (10 times higher than our highest
measured concentration) has been shown to decrease growth in brown trout
early stages (Salmo trutta ) and concentrations higher than 15.5
mg L-1 can be lethal (Stubblefield et al., 1997).
Interestingly, even very high Mn concentration (100 mg
L-1) did not affect brown trout hatching success
(Stubblefield et al., 1997). Arola et al. (2017) demonstrated slightly
lower concentrations (approx. 15-30 mg L-1) as LC50
value for whitefish (Coregonus lavaretus ) offspring, whereas
Lewis (1976) noticed only 1 mg L-1MnSO4 to increase embryonic mortality in rainbow trout.
Several studies have demonstrated different sublethal physiological
effects of Mn in fish, for example altered hematological parameters
(Aliko et al., 2018), such as decreased number of red blood cells and
hemoglobin value (Sharma & Langer, 2014) even without mortality itself
(Wepener et al., 1992). Furthermore, toxicity level of Mn is suggested
to be associated with oxidative stress (Vieira et al., 2012; but see
Baden et al., 1995). Increase in water temperature also increases the
uptake of Mn in fish (Howe et al., 2004), also raising the potential
effect of climate-induced changes in water temperature in this context.
Large-selected fish were not only able to grow faster but they were also
able to potentially allocate more energy into fat production
particularly in high concentrations indicated by higher condition factor
(Figure 1B). This process was, however, severely disrupted in the
highest concentration (7.5 mg L-1) where the condition
factor of both large- and small-selected fish was five times lower than
in other concentrations and could reflect altered metabolic homeostasis.
In the highest concentration, there were more individuals with very low
than high condition factor among both life-history types. One
explanation underlying this bimodal distribution could be related to
social structure in zebrafish shoals. In stressful conditions, the
competition for food might become more intense as energy demand
increases and create more pronounced social hierarchies where few
individuals dominate the resources. Among large-selected fish, there
seemed to be more of these potentially dominating individuals than among
small-selected fish. Similar decreases in condition factor in fish
exposed to high heavy metal concentrations have been reported earlier
because of reduced feed intake or metabolic activity (Baudou et al.,
2017; Eastwood & Couture, 2002; but see Dethloff et al., 2001; Farag et
al. 1998;). While the effect of environmental toxins and heavy metals on
fish condition has been well studied and demonstrated, less attention
has been paid to the interaction between heavy metal concentrations and
individual differences in life histories. Therefore, it is important to
consider the heterogeneity of stress response among individuals in an
environment where several human-induced selection pressures operate
simultaneously and potentially antagonistically (e.g., size-selective
harvesting and heavy metal exposure).
Energy demand typically increases under stress and growth may be
compromised if the animal cannot balance their energetic requirements
(Rueda-Jasso, 2004). Indeed, toxic agents may lead to an imbalance
between energy supply and demand by negatively affecting feeding
behavior either directly or indirectly (e.g., damaging sensory, and/or
nervous system; Hoskins & Volkoff, 2012). Studies have reported reduced
intake of food in cadmium-exposed fish together with altered swimming
activity (Baudou et al., 2017; Ferrari et al., 2011; Sloman et al.,
2003). In fish, appetite is commonly associated with increased swimming
activity as they search for food. The lower feeding probability (Figure
2B) likely underlies, at least partly, the lower growth rate and
condition factor of small-selected fish compared to large-selected fish
in our study. Small-selected fish were also less active (Figure 2A), and
this could indicate that they were not searching food as effectively as
large-selected fish or alternatively they were saving energy. This, in
turn, could indicate that they had lower appetite. However, differences
in feeding behavior and activity between the two life-history types were
already present in the control treatment. Large-selected fish have also
been previously shown to be more active and explorative than
small-selected fish (Uusi-Heikkilä et al., 2015), thus it seems that
these behavioral differences are correlating with other morphological
and life-history differences characterizing these different life-history
types.
Behavioral responses to stress have been described as reactive (often
characterized by freezing behavior) or proactive (e.g., highly active
fight or flight behavior). In high MnSO4 concentrations,
small-selected fish appeared to adopt a reactive behavior (less active,
low feeding probability) whereas large-selected fish behaved evidently
proactively (more active, high feeding probability). These behavioral
types are considered as adaptations for life in unstable and stable
environments, respectively; thus, reactive individuals are characterized
by higher levels of physiological stress responses than proactive
individuals (Cockrem, 2007). In vertebrates, the physiological stress
response involves activation of the hypothalamic-pituitary-adrenal (HPA)
axis, where exposure to stress stimulates secretion of glucocorticoids
(e.g., Cockrem, 2007). In turn, glucocorticoid secretion elicits a
cascade of physiological and behavioral processes that are essential to
cope with stressful events (Landys et al., 2006; Wingfield &
Ramenofsky, 1999). The stress-coping style hypothesis makes the specific
predictions that proactive individuals should have lower baseline
concentrations of glucocorticoids and a less reactive HPA axis. Although
we did not measure cortisol concentrations of the fish in the present
study, we have shown earlier that cortisol concentration in zebrafish
correlates negatively with body size and feed intake (Merino et al.,
unpublished; Uusi-Heikkilä et al., 2018).
Finally, small-selected fish have been shown to differ genetically (both
at sequence and gene expression level) from large-selected fish
(Uusi-Heikkilä et al., 2015,2017). Although gene expression profiles of
the experimental fish were not investigated in the present study, it is
possible that there were differences in certain stress-related
regulatory mechanisms between the life-history types leading to
differences in the ability to compensate the effects of the stressor.
This type of a response is often associated to chronic stress since
heavy acute stressors may result in death, and mild ones in recovery.
In the present study, we demonstrated that life-history type may affect
stress coping ability in fish. Considering the complex response to
stress becomes important when human activities are imposing different
selection pressures on wild animal populations. For example, fisheries
is often size-selective and selects for small body size and fast
life-history types (e.g., Jørgensen & Fiksen 2010; Olsen et al., 2004;
Uusi-Heikkilä et al., 2015). Harvested populations may experience other
human-induced selection pressures in their environment operating, for
example, on stress coping abilities. Therefore, if a population mostly
consists of individuals with fast life-histories and potentially low
stress coping abilities, the two selection pressures operating
antagonistically may exacerbate population decline. It is also good to
keep in mind that human-induced selection pressures are often
directional and may reduce plasticity in a population, which has been
suggested to help coping with maladaptive stressors (Balasch & Tort,
2019). While complicating the predictions how organisms may respond to
stress, these are important factors to consider when anticipating the
effects of multiple, simultaneous human-induced stressors on
heterogeneous populations.