This study investigated whether neonate Hemiscyllium ocellatum reared under a 4\(⁰\)C higher thermal regime, as expected from the SSP5-8.5 end-of-century ocean warming scenario, showed differences in the metabolic and structural features of skeletal muscle as compared to those reared at current-day summer temperatures. While epaulette sharks tolerate high temperature, high CO2, and oxygen-limited systems, these conditions are acute and often predictable with tidal and seasonal fluctuations (Heinrich et al. 2014; Johnson et al. 2016; Gervais et al. 2018; Nay et al. 2021). When epaulette sharks were exposed to chronically elevated temperatures during their embryonic and neonatal development, they exhibited a shortened embryonic period, skeletal muscle fibers showed lower fiber density, higher satellite cell density, and increased oxidative damage to proteins compared to those reared at the current-day temperature. However, other measures of muscle structure, function, and development did not differ between the 27°C and 31°C reared sharks, which might be expected for a species that inhabits fluctuating environmental conditions. The limited differences in these properties in animals reared at 31°C may indicate a lack of muscle plasticity and an inability to acclimate, resulting in reduced aerobic scope and reduced performance (e.g., exercise protocols; Wheeler et al. 2021).
The epaulette sharks hatched earlier at 31°C, but body size was similar to those reared at 27°C. Shorter embryonic periods have been documented in several teleost and elasmobranch fishes when maintained at elevated temperatures within their thermal range (Gillooly et al. 2002; Schulte et al. 2011; Rosa et al. 2014; Schulte 2015). Exposure to high temperatures within an organism’s thermal window during embryonic development often decreases development time by increasing rates of metabolic processes, which subsequently increases organism growth rate (Gillooly et al. 2002; Zuo et al. 2012; Rosa et al. 2014). In neonate epaulette sharks, shorter embryonic developmental time at higher temperatures (up to 31°C) is associated with increased growth rates and faster yolk consumption, with the net effect of a slightly smaller body mass at hatching (Wheeler et al. 2021). While these 31°C hatchlings were smaller, they could have regained mass by ad libitum feeding throughout the rest of the experiments (Wheeler et al. 2021). However, at 32°C, growth declines rapidly and mortality increases, indicating that temperatures reaching 31°C may be the pejus temperature of growth in juvenile epaulettes (Gervais et al. 2016; Gervais et al. 2018; Wheeler et al. 2021). Similar relationships between temperature and body mass are seen in other fish species, as more energy is allocated to basal metabolic energy requirements and growth, the shorter development time may lead to smaller body sizes, including into adulthood (Daufresne et al. 2009; Sheridan and Bickford 2011; Lema et al. 2019).
Thermal windows are temperature ranges set by critical thermal limits in which an organism can successfully maintain aerobic respiration; these ranges vary by ontogenetic stage and thermal history (Schulte et al. 2011; Sinclair et al. 2016; Flynn and Todgham 2018). Early ontogenetic stages tend to have narrower thermal windows, potentially from an underdeveloped capacity of cardiorespiratory systems to deal with large changes in oxygen demand (Dahlke et al. 2020; Portner 2021). Juvenile temperate fishes may have thermal windows almost half that of adults (Rombough 1997), while tropical fishes exhibit much smaller differences between embryonic, juvenile, and adult life stages, due to the comparatively narrow thermal range of their environment (Rummer et al. 2014; Sunday et al. 2019; Bennett et al. 2021). While the determination of thermal windows is set by measures of acute thermal tolerance, the crux of thermal tolerance for some species may lie in chronic temperature exposure and those carryover effects (Wheeler et al 2021; Wheeler et al. 2022). Seasonally acclimatized adult epaulette sharks show absolute upper thermal limits of 36–39°C, compared to 32°C for juveniles under a chronic exposure (Gervais et al. 2016; Gervais et al. 2018). However, when comparing critical thermal maxima across life stages and body sizes for wild-caught epaulette sharks of Heron Island, GBR with similar thermal histories, no differences were found among juveniles, subadults, and adults (Wheeler et al. 2022).
Average skeletal muscle fiber density was slightly lower in sharks from the elevated temperature. Lower fiber density would be consistent with lower muscle cellularity during early embryonic stages, and sharks from the elevated temperature had a slightly narrower fiber size distribution, including fewer small, newly formed fibers. This may explain why fiber density was lower in the elevated temperature group, but mean fiber cross-sectional area did not change. Skeletal muscle growth in early juvenile stages of teleosts is dependent on both increasing muscle fiber size via hypertrophy and increasing fiber cellularity via hyperplasia (Johnston 2006; Johnston et al. 2011). Hypertrophy constitutes the main mechanism underpinning muscle growth in many species of fish and spanning juvenile to adulthood life stages (Johnston et al. 2011). However, hyperplasia is also necessary during embryonic and juvenile life stages for species that increase body and muscle mass by several orders of magnitude throughout ontogeny (Johnston et al. 2011; Kacperczyk et al. 2011; Priester et al. 2011). Hypertrophy and hyperplasia are strongly influenced by environmental temperature, especially during embryonic development (Macqueen et al. 2008; Carey et al. 2009; Johnston et al. 2009; Scott and Johnston 2012; Ahammad et al. 2021). In this study, epaulette sharks that exhibited a lower fiber density under elevated temperatures may also be exhibiting a reduction in hyperplasia during early life stages.
Nuclear density and position influence skeletal muscle growth (Priester et al. 2011; Jimenez and Kinsey 2012). Nuclear recruitment allows for an increase in muscle fiber size, and the position of nuclei within an individual fiber determines diffusional distances for nuclear substrates and products, such as mRNA and large proteins (Priester et al. 2011; Jimenez and Kinsey 2012). The myonuclear domain describes the volume of muscle fiber that an individual nucleus controls (Koumans and Akster 1995; Priester et al. 2011). Myonuclear domain was positively correlated with body mass and fiber size for neonate epaulette sharks, which is consistent with the effects of growth seen in other vertebrates and under conditions such as aging and muscle atrophy (Van der Meer et al. 2011; Jimenez and Kinsey 2012; Hiebert and Anderson 2020). As muscle fibers grow via hypertrophy, subsarcolemmal nuclei are recruited to the fiber via satellite cell fusion, which allows muscle cells to maintain a relatively small myonuclear domain and short diffusional distances for nuclear substrates and products, thus permitting continued growth (Hall and Ralston 1989; Priester et al. 2011). While our analyses found no changes in nuclear recruitment to muscle fibers, other studies examining temperate and cold-water teleost fishes have seen an increase in myonuclear density with elevated temperatures. When Atlantic salmon, Salmo salar, eggs are reared at temperatures 3–4°C higher than their natal tributary thermal regime, fry exhibit higher nuclear densities and hypertrophic muscle growth shortly after hatching (Johnston et al. 2000). Similarly, juvenile Atlantic herring (Clupea harengus), juvenile pacu (Piaractus mesopotamicus), and larval European sea bass (Dicentrarchus labrax) reared under elevated temperatures exhibit higher myonuclear densities, as well as increased hypertrophic and hyperplastic growth rates (Johnston et al. 1998b; de Assis et al. 2004; Alami-Durante et al. 2006). The lack of such an effect in epaulette sharks may reflect differences in tropical species responses, especially when species are already living near their pejus temperatures.
Temperature can influence the timing and rate of myogenesis (Johnston et al. 1998b; Johnston et al. 2001; Macqueen et al. 2008; Johnston et al. 2009). Early myogenesis is a critical period for satellite cell proliferation and differentiation within an embryonic fish's dermomyotome (Steinbacher et al. 2006; Steinbacher et al. 2011; Keenan and Currie 2019; Zhang et al. 2021). In the current study, satellite cell density was higher in neonate sharks reared at the elevated temperature regime, and muscle fibers associated with satellite cells were smaller in cross-sectional area. This relationship suggests that sharks maintained under the elevated temperature exhibited more satellite cells yet to fuse with existing fibers, which is supported by the smaller size of muscle fibers associated with satellite cells. This observation may be due to the shorter embryonic period observed in sharks reared under the elevated temperature regime (Zhu et al. 2014). In this case, it might be expected that the muscle fibers in sharks reared at 31°C would have fewer nuclei (greater myonuclear domain), but this was not observed. Whether temperature effects on satellite cells are transient or influence long term muscle growth is an open question, as previous studies have shown that changes in hyperplastic growth at hatching and early juvenile stages has species-specific effects on body size and locomotory performance (Nathanailides et al. 1995; de Assis et al. 2004; Carey et al. 2009; Steinbacher et al. 2011; de la Serrana et al. 2012; Takata et al. 2018).
At 31°C, neonate epaulette sharks reduced aerobic performance compared to sharks under 27°C by exhibiting reduced maximum metabolic rates and aerobic scopes and longer recovery times from exhaustion (Wheeler et al. 2021). Organism oxygen uptake is a proxy for metabolic rate and aerobic scope generally increases with increasing temperature until temperatures exceed the normal physiological range and begin to approach lethal levels (Clark et al. 2013). This decline in aerobic scope may indicate that, at maximum activity levels, neonate epaulette sharks are unable to maintain adequate oxygen supplies to meet the aerobic demands of swimming (Wheeler et al. 2021). Maintaining oxygen supply to meet increased demand under elevated temperatures is considered a major factor influencing the overall survival of organisms (Portner and Knust 2007; Portner 2021). Beyond aerobic scope, fishes may modify structures associated with cardiorespiratory capacity and peripheral oxygen circulation such as the heart, gills, and vasculature, under chronic or more frequent increases in temperature (Franklin et al. 2007; Akbarzadeh et al. 2014; Ekstrom et al. 2017; Nyboer and Chapman 2018). In some cases, limited structural change in the gills of animals reared in elevated temperatures has been suggested to contribute to reduced aerobic capacity (Bowden et al. 2014; Liu et al. 2015; Takata et al. 2018). Previous research has also demonstrated that higher temperatures can impact intracellular muscle fiber oxygenation and capillary supply (Johnston 1982; Egginton and Sidell 1989; Egginton 1998). However, the present study found no effect of elevated temperature on muscle capillary density; although there was a noticeable, yet not statistically significant, reduction in capillary density in sharks from the elevated temperature group (Fig. 2D).
Animals at early juvenile stages often exhibit high metabolic rates and maintenance costs compared to adults due to constant macromolecular turnover to induce growth and rapid ontogenetic change (Monaghan et al. 2009; Metcalfe and Alonso-Alvarez 2010; Rosa et al. 2014; Boltana et al. 2017). Lipid peroxidation and DNA oxidative damage, which are both reversible processes, did not differ between the temperature groups, but protein carbonylation, which is irreversible, was higher in animals reared at the elevated temperature (Fig. 5C). While lipid peroxidation and low levels of hydrogen peroxide play an important role in promoting cell signaling, growth, resistance to oxidative stress, and increased lifespan (Wildburger et al. 2009; Milkovic et al. 2015; Else 2017), protein carbonylation is most often associated with oxidative damage and cellular dysfunction (Barreiro and Hussain 2010). Juvenile brown trout, Salmo trutta, and Atlantic salmon, S. salar, exhibit similar responses to the current observations for H. ocellatum, with an accumulation of damaged proteins during rapid ontogenetic growth under stressful conditions (Morgan and Metcalfe 2001; Almroth et al. 2010). While three-spined stickleback, Gastrosteus aculeatus, show higher oxidative DNA damage in skeletal muscle upon warm temperature acclimation during the winter breeding season (Kim et al. 2019).
Higher levels of oxidative damage may translate to greater maintenance costs (Squier 2001; Stern 2017; Neurohr et al., 2021). In gilthead sea bream larvae, Sparus aurata, both the stress protein Hsp70 and protein carbonylation expression increase at moderately elevated temperatures, and larvae eventually increased ubiquitin expression at temperatures reaching their critical thermal limits (Madeira et al. 2016). Hsp70 is an indicator of temperature stress in ectotherms and is highly influential in thermal plasticity (Ali et al. 2003; Madeira et al. 2018; Pan et al. 2021). Increases in Hsp70 expression were documented in all tissues in response to non-optimal elevated temperatures for Japanese flounder, Paralichthys olivaceus (Liu et al. 2015). In eastern brook trout, Salvelinus fontinalis, Hsp70 expression depends on relative water level (depth) and temperature (Nguyen et al. 2021). However, in this study, there was no evidence of increased protein turnover in the epaulette shark via the major protein degradation pathways or in expression of Hsp70, which regulates the unfolded protein response and is involved in both chaperone-mediated autophagy and ubiquitin proteasome regulation (Fig. 6). The lack of an effect of temperature on Hsp70 in epaulette sharks in this study is perhaps not surprising, considering that 31°C is near, but does not appear to exceed, the normal thermal window for this species (Wheeler et al., 2021). However, this relationship may change when temperatures reach 32°C, since at this temperature, juvenile thermal performance declines and mortality increases (Gervais et al. 2018; Wheeler et al. 2021).
Climate change vulnerability will not be consistent across all organisms and individual factors may not have consistent consequences (Somero 2010; Kingsolver et al. 2013; Gunderson and Stillman 2015; Gunderson et al. 2017). The current study addressed potential climate change impacts using only temperature; yet impacts on marine organisms will more likely involve synergistic changes to temperature, dissolved oxygen, pH, salinity, and other biogeochemical and physicochemical variables (IPCC 2022). When fish muscle function is compromised by environmental stressors, such as extreme temperature, pH, oxygen, or salinity changes, this may manifest structurally as muscle atrophy, physiologically as a depression in energy metabolism, and functionally as an eventual loss of motor function from reduced muscle contraction or hyperactivity, leading to reduced swimming performance or escape responses, whole animal spasms or lack of responses to stimuli (Randall and Brauner 1991; Domenici et al. 2019; Rossi and Wright 2020; Vilmar and Di Santo 2022). Yet, often when studies assess responses to environmental variability, few encompass a combination of morphological, physiological, functional, and behavioral changes (Di Santo 2022). Likewise, many studies are based on singular acute exposures within one life stage which may not allow enough time to induce morphological adaptation. Rather, these acute responses entail balancing short-lived metabolic processes, as opposed to long-term maintenance and survival that would occur under repeated or chronic exposure throughout ontogeny leading to acclimation and eventual adaptation in structure, function, and overall behavior (Pörtner and Farrell 2008; Pörtner and Peck 2010; Somero 2012; Cook et al. 2013). The impact on an organism depends on the duration and severity of the changing variables and the species, its life history, and ontogenetic stage (Domenici et al. 2019).
This study provides the first analysis of muscle morphology and physiology of an elasmobranch species under chronic ocean warming. Translating changes in muscle morphology to functionality is the next step in understanding potential impacts of long-term environmental variability across biological scales of locomotion and behavior. The epaulette shark may serve as a model for many other tropical, benthic elasmobranch species under a changing climate. Considering the increasing loss of marine species richness in equatorial regions as a result of ocean warming, further examination of the potential effects of both acute and chronic temperature change on the morphology, physiology, performance, and behavior of the epaulette shark and other tropical elasmobranch species is warranted.