EcoPhysioMechanics: Integrating energetics and biomechanics to understand fish locomotion under climate change

Ecological physiologists and biomechanists have been broadly investigating swimming performance in a diversity of fishes, however the connection between form, function and energetics of locomotion has been rarely evaluated in the same system and under climate change scenarios. In this perspective I argue that working within the framework of 'EcoPhysioMechanics', i.e., integrating energetics and biomechanics tools, to measure locomotor performance and behavior under different abiotic factors, improves our understanding of the mechanisms, limits and costs of movement. To demonstrate how ecophysiomechanics can be applied to locomotor studies, I outline how linking biomechanics and physiology allows us to understand how fishes may modulate their movement to achieve high speeds or reduce the costs of locomotion. I also discuss how the framework is necessary to quantify swimming capacity under climate change scenarios. Finally, I discuss current dearth of integrative studies and gaps in empirical datasets that are necessary to understand fish swimming under changing environments.

Acidification and hypoxia interactively affect metabolism in embryos, but not larvae, of the coastal forage fish Menidia menidia

Ocean acidification is occurring in conjunction with warming and deoxygenation as a result of anthropogenic greenhouse gas emissions. Multistressor experiments are critically needed to better understand the sensitivity of marine organisms to these concurrent changes. Growth and survival responses to acidification have been documented for many marine species, but studies that explore underlying physiological mechanisms of carbon dioxide (CO2) sensitivity are less common. We investigated oxygen consumption rates as proxies for metabolic responses in embryos and newly hatched larvae of an estuarine forage fish (Atlantic silverside, Menidia menidia) to factorial combinations of CO2×temperature or CO2×oxygen. Metabolic rates of embryos and larvae significantly increased with temperature, but partial pressure of CO2 (PCO2 ) alone did not affect metabolic rates in any experiment. However, there was a significant interaction between PCO2  and partial pressure of oxygen (PO2 ) in embryos, because metabolic rates were unaffected by PO2  level at ambient PCO2 , but decreased with declining PO2  under elevated PCO2 For larvae, however, PCO2  and PO2  had no significant effect on metabolic rates. Our findings suggest high individual variability in metabolic responses to high PCO2 , perhaps owing to parental effects and time of spawning. We conclude that early life metabolism is largely resilient to elevated PCO2  in this species, but that acidification likely influences energetic responses and thus vulnerability to hypoxia.

Are long-term growth responses to elevated pCO2 sex-specific in fish?

Whether marine fish will grow differently in future high pCO₂ environments remains surprisingly uncertain. Long-term and whole-life cycle effects are particularly unknown, because such experiments are logistically challenging, space demanding, exclude long-lived species, and require controlled, restricted feeding regimes—otherwise increased consumption could mask potential growth effects. Here, we report on repeated, long-term, food-controlled experiments to rear large populations (>4,000 individuals total) of the experimental model and ecologically important forage fish Menidia menidia (Atlantic silverside) under contrasting temperature (17°, 24°, and 28°C) and pCO₂ conditions (450 vs. ~2,200 μatm) from fertilization to ~ a third of this annual species’ life span. Quantile analyses of trait distributions showed mostly negative effects of high pCO₂ on long-term growth. At 17°C and 28°C, but not at 24°C, high pCO₂ fish were significantly shorter [17°C: -5 to -9%; 28°C: -3%] and weighed less [17°C: -6 to -18%; 28°C: -8%] compared to ambient pCO₂ fish. Reductions in fish weight were smaller than in length, which is why high pCO₂ fish at 17°C consistently exhibited a higher Fulton’s k (weight/length ratio). Notably, it took more than 100 days of rearing for statistically significant length differences to emerge between treatment populations, showing that cumulative, long-term CO₂ effects could exist elsewhere but are easily missed by short experiments. Long-term rearing had another benefit: it allowed sexing the surviving fish, thereby enabling rare sex-specific analyses of trait distributions under contrasting CO₂ environments. We found that female silversides grew faster than males, but there was no interaction between CO₂ and sex, indicating that males and females were similarly affected by high pCO₂. Because Atlantic silversides are known to exhibit temperature-dependent sex determination, we also analyzed sex ratios, revealing no evidence for CO₂-dependent sex determination in this species.

Elevated pCO2 and Hypoxia Alter the Acid-Base Homeostasis of Developing Sheepshead Minnows, Cyprinodon variegatus

Low dissolved oxygen and increased acidification are two environmental variables that concomitantly change in an estuarine environment, both of which are exacerbated by nutrient pollution and subsequent eutrophication. To better understand how estuarine residents compensate for daily fluctuations in these environmental variables, the interactive effects of acidification and hypoxia were assessed in developing sheepshead minnows (Cyprinodon variegatus) using a 2 by 2 factorial design over a 42-day exposure. Embryos were exposed to either acidic (partial pressure of CO₂, pCO₂, ~2000 μatm), hypoxic (reduced dissolved oxygen, ~2 mg l^-1), or combined acidic and hypoxic conditions and monitored for development, hatch rate, and survival. Changes in oxygen consumption, anaerobic metabolism, oxidative stress, and acid-base balance were evaluated at three life stages (embryo, larval, and juvenile fish) to discern if and how fish compensate for these stressors during development. The combination of acidification and hypoxia delayed hatching in embryos and significantly decreased oxygen consumption (p<0.001) in all three life-stages. Neither acidification, hypoxia, nor the combination of the stressors impacted the anaerobic metabolism or oxidative stress of juvenile fish, but acid-base equilibrium was disrupted by all three treatments in larval fish. Elevated carbonic anhydrase activity was observed in the multi-stress treatment in embryos and larval fish, but not in juvenile fish. These results show that developing sheepshead minnows can re-establish cellular homeostasis in compensating to acidified and hypoxic waters.

Negative Effects of Diurnal Changes in Acidification and Hypoxia on Early-Life Stage Estuarine Fishes

Estuaries serve as important nursery habitats for various species of early-life stage fish, but can experience cooccurring acidification and hypoxia that can vary diurnally in intensity. This study examines the effects of acidification (pH 7.2–7.4) and hypoxia (dissolved oxygen (DO) ~ 2–4 mg L−1) as individual and combined stressors on four fitness metrics for three species of forage fish endemic to the U.S. East Coast: Menidia menidia, Menidia beryllina, and Cyprinodon variegatus. Additionally, the impacts of various durations of exposure to these two stressors was also assessed to explore the sensitivity threshold for larval fishes under environmentally-representative conditions. C. variegatus was resistant to chronic low pH, while M. menidia and M. beryllina experienced significantly reduced survival and hatch time, respectively. Exposure to hypoxia resulted in reduced hatch success of both Menidia species, as well as diminished survival of M. beryllina larvae. Diurnal exposure to low pH and low DO for 4 or 8 h did not alter survival of M. beryllina, although 8 or 12 h of daily exposure through the 10 days posthatch significantly depressed larval size. In contrast, M. menidia experienced significant declines in survival for all intervals of diel cycling hypoxia and acidification (4–12 h). Exposure to 12-h diurnal hypoxia generally elicited negative effects equal to, or of greater severity, than chronic exposure to low DO at the same levels despite significantly higher mean DO exposure concentrations. This evidences a substantial biological cost to adapting to changing DO levels, and implicates diurnal cycling of DO as a significant threat to fish larvae in estuaries. Larval responses to hypoxia, and to a lesser extent acidification, in this study on both continuous and diurnal timescales indicate that estuarine conditions throughout the spawning and postspawn periods could adversely affect stocks of these fish, with diverse implications for the remainder of the food web.

Organ health and development in larval kingfish are unaffected by ocean acidification and warming

Anthropogenic CO₂ emissions are causing global ocean warming and ocean acidification. The early life stages of some marine fish are vulnerable to elevated ocean temperatures and CO₂ concentrations, with lowered survival and growth rates most frequently documented. Underlying these effects, damage to different organs has been found as a response to elevated CO₂ in larvae of several species of marine fish, yet the combined effects of acidification and warming on organ health are unknown. Yellowtail kingfish, Seriola lalandi, a circumglobal subtropical pelagic fish of high commercial and recreational value, were reared from fertilization under control (21 °C) and elevated (25 °C) temperature conditions fully crossed with control (500 µatm) and elevated (1,000 µatm) pCO₂ conditions. Larvae were sampled at 11 days and 21 days post hatch for histological analysis of the eye, gills, gut, liver, pancreas, kidney and liver. Previous work found elevated temperature, but not elevated CO₂, significantly reduced larval kingfish survival while increasing growth and developmental rate. The current histological analysis aimed to determine whether there were additional sublethal effects on organ condition and development and whether underlying organ damage could be responsible for the documented effects of temperature on survivorship. While damage to different organs was found in a number of larvae, these effects were not related to temperature and/or CO₂ treatment. We conclude that kingfish larvae are generally vulnerable during organogenesis of the digestive system in their early development, but that this will not be exacerbated by near-future ocean warming and acidification.

Diel and tidal pCO2 × O2 fluctuations provide physiological refuge to early life stages of a coastal forage fish

Coastal ecosystems experience substantial natural fluctuations in pCO₂ and dissolved oxygen (DO) conditions on diel, tidal, seasonal and interannual timescales. Rising carbon dioxide emissions and anthropogenic nutrient input are expected to increase these pCO₂ and DO cycles in severity and duration of acidification and hypoxia. How coastal marine organisms respond to natural pCO₂ × DO variability and future climate change remains largely unknown. Here, we assess the impact of static and cycling pCO₂ × DO conditions of various magnitudes and frequencies on early life survival and growth of an important coastal forage fish, Menidia menidia. Static low DO conditions severely decreased embryo survival, larval survival, time to 50% hatch, size at hatch and post-larval growth rates. Static elevated pCO₂ did not affect most response traits, however, a synergistic negative effect did occur on embryo survival under hypoxic conditions (3.0 mg L⁻¹). Cycling pCO₂ × DO, however, reduced these negative effects of static conditions on all response traits with the magnitude of fluctuations influencing the extent of this reduction. This indicates that fluctuations in pCO₂ and DO may benefit coastal organisms by providing periodic physiological refuge from stressful conditions, which could promote species adaptability to climate change.

High sensitivity of a keystone forage fish to elevated CO2 and temperature

Sand lances of the genus Ammodytes are keystone forage fish in coastal ecosystems across the northern hemisphere. Because they directly support populations of higher trophic organisms such as whales, seabirds or tuna, the current lack of empirical data and, therefore, understanding about the climate sensitivity of sand lances represent a serious knowledge gap. Sand lances could be particularly susceptible to ocean warming and acidification because, in contrast to other tested fish species, they reproduce during boreal winter months, and their offspring develop slowly under relatively low and stable pCO₂ conditions. Over the course of 2 years, we conducted factorial pCO₂ × temperature exposure experiments on offspring of the northern sand lance Ammodytes dubius, a key forage species on the northwest Atlantic shelf. Wild, spawning-ripe adults were collected from Stellwagen Bank National Marine Sanctuary (Cape Cod, USA), and fertilized embryos were reared at three pCO₂ conditions (400, 1000 and 2100 μatm) crossed with three temperatures (5, 7 and 10 ˚C). Exposure to future pCO₂ conditions consistently resulted in severely reduced embryo survival. Sensitivity to elevated pCO₂ was highest at 10 ˚C, resulting in up to an 89% reduction in hatching success between control and predicted end-of-century pCO₂ conditions. Moreover, elevated pCO₂ conditions delayed hatching, reduced remaining endogenous energy reserves at hatch and reduced embryonic growth. Our results suggest that the northern sand lance is exceptionally CO₂-sensitive compared to other fish species. Whether other sand lance species with similar life history characteristics are equally CO₂-sensitive is currently unknown. But the possibility is a conservation concern, because many boreal shelf ecosystems rely on sand lances and might therefore be more vulnerable to climate change than currently recognized. Our findings indicate that life history, spawning habitat, phenology and developmental rates mediate the divergent early life CO₂ sensitivities among fish species.

Combined effects of ocean acidification and temperature on larval and juvenile growth, development and swimming performance of European sea bass (Dicentrarchus labrax)

Ocean acidification and ocean warming (OAW) are simultaneously occurring and could pose ecological challenges to marine life, particularly early life stages of fish that, although they are internal calcifiers, may have poorly developed acid-base regulation. This study assessed the effect of projected OAW on key fitness traits (growth, development and swimming ability) in European sea bass (Dicentrarchus labrax) larvae and juveniles. Starting at 2 days post-hatch (dph), larvae were exposed to one of three levels of PCO2 (650, 1150, 1700 μatm; pH 8.0, 7.8, 7.6) at either a cold (15°C) or warm (20°C) temperature. Growth rate, development stage and critical swimming speed (Ucrit) were repeatedly measured as sea bass grew from 0.6 to ~10.0 (cold) or ~14.0 (warm) cm body length. Exposure to different levels of PCO2 had no significant effect on growth, development or Ucrit of larvae and juveniles. At the warmer temperature, larvae displayed faster growth and deeper bodies. Notochord flexion occurred at 0.8 and 1.2 cm and metamorphosis was completed at an age of ~45 and ~60 days post-hatch for sea bass in the warm and cold treatments, respectively. Swimming performance increased rapidly with larval development but better swimmers were observed in the cold treatment, reflecting a potential trade-off between fast grow and swimming ability. A comparison of the results of this and other studies on marine fish indicates that the effects of OAW on the growth, development and swimming ability of early life stages are species-specific and that generalizing the impacts of climate-driven warming or ocean acidification is not warranted.

Experimental assessments of marine species sensitivities to ocean acidification and co-stressors: how far have we come?

Experimental studies assessing the potential impacts of ocean acidification on marine organisms have rapidly expanded and produced a wealth of empirical data over the past decade. This perspective examines four key areas of transformative developments in experimental approaches: (1) methodological advances; (2) advances in elucidating physiological and molecular mechanisms behind observed CO2effects; (3) recognition of short-term CO2variability as a likely modifier of species sensitivities (Ocean Variability Hypothesis); and (4) consensus on the multistressor nature of marine climate change where effect interactions are still challenging to anticipate. No single experiment allows predicting the fate of future populations. But sustaining the accumulation of empirical evidence is critical for more robust estimates of species reaction norms and thus for enabling better modeling approaches. Moreover, advanced experimental approaches are needed to address knowledge gaps including changes in species interactions and intraspecific variability in sensitivity and its importance for the adaptation potential of marine organisms to a high CO2world.

Robust quantification of fish early life CO2 sensitivities via serial experimentation

Despite the remarkable expansion of laboratory studies, robust estimates of single species CO₂ sensitivities remain largely elusive. We conducted a meta-analysis of 20 CO₂ exposure experiments conducted over 6 years on offspring of wild Atlantic silversides (Menidia menidia) to robustly constrain CO₂ effects on early life survival and growth. We conclude that early stages of this species are generally tolerant to CO₂ levels of approximately 2000 µatm, likely because they already experience these conditions on diel to seasonal timescales. Still, high CO₂ conditions measurably reduced fitness in this species by significantly decreasing average embryo survival (-9%) and embryo+larval survival (-13%). Survival traits had much larger coefficients of variation (greater than 30%) than larval length or growth (3-11%). CO₂ sensitivities varied seasonally and were highest at the beginning and end of the species' spawning season (April-July), likely due to the combined effects of transgenerational plasticity and maternal provisioning. Our analyses suggest that serial experimentation is a powerful, yet underused tool for robustly estimating small but true CO₂ effects in fish early life stages.