Low O2 avoidance is associated with physiological perturbation but not exhaustion in the snapper (Pagrus auratus: Sparidae)

Integrated behavioural and physiological responses of sand smelt larvae to the effects of warming and hypoxia as combined stressors

Forecasts indicate that rising temperatures towards the future and the expansion of dead zones will change environmental suitability for fish early stages. Therefore, we assessed the chronic effects of warming (26 °C), hypoxia (<2-2.5 mg L-1) or their combination on mortality rate, growth, behaviour, energy metabolism and oxidative stress using Atherina presbyter larvae as a model species. There were no differences between the treatments in terms of mortality rate. The combination of warming and hypoxia induced faster loss of body mass (+22.7%). Warming, hypoxia or their combination enhanced boldness (+14.7-25.4%), but decreased exploration (-95%-121%), increased the time in frozen state (+60.6-80.5%) and depleted swimming speed (-45.6-50.5%). Moreover, routine metabolic rate was depleted under hypoxia or under the combination of warming and hypoxia (-56.6 and 57.2%, respectively). Under hypoxia, increased catalase activity (+56.3%) indicates some level of antioxidant defence capacity, although increased DNA damage (+25.2%) has also been observed. Larvae also exhibited a great capacity to maintain the anaerobic metabolism stable in all situations, but the aerobic metabolism is enhanced (+19.3%) when exposed to the combination of both stressors. The integrative approach showed that changes in most target responses can be explained physiologically by oxidative stress responses. Increased oxidative damages (lipid peroxidation and DNA damage) and increased interaction between antioxidant enzymes (superoxide dismutase and catalase) are associated to increased time in frozen state and decreased swimming activity, growth rates and boldness. Under all stressful situations, larvae reduced energy-consuming behaviours (e.g. depleted exploration and swimming activity) likely to stabilize or compensate for the aerobic and anaerobic metabolisms. Despite being an active small pelagic fish, we concluded that the sensitive larval phase exhibited complex coping strategies to physiologically acclimate under thermal and hypoxic stress via behavioural responses.

Early life stage mechanisms of an active fish species to cope with ocean warming and hypoxia as interacting stressors

Ocean's characteristics are rapidly changing, modifying environmental suitability for early life stages of fish. We assessed whether the chronic effects of warming (24 °C) and hypoxia (<2-2.5 mg L-1) will be amplified by the combination of these stressors on mortality, growth, behaviour, metabolism and oxidative stress of early stages of the white seabream Diplodus sargus. Combined warming and hypoxia synergistically increased larval mortality by >51%. Warming induced faster growth in length and slower gains in weight when compared to other treatments. Boldness and exploration were not directly affected, but swimming activity increased under all test treatments. Under the combination of warming and hypoxia, routine metabolic rate (RMR) significantly decreases when compared to other treatments and shows a negative thermal dependence. Superoxide dismutase and catalase activities increased under warming and were maintained similar to control levels under hypoxia or under combined stressors. Under hypoxia, the enzymatic activities were not enough to prevent oxidative damages as lipid peroxidation and DNA damage increased above control levels. Hypoxia reduced electron transport system activity (cellular respiration) and isocitrate dehydrogenase activity (aerobic metabolism) below control levels. However, lactate dehydrogenase activity (anaerobic metabolism) did not differ among treatments. A Redundancy Analysis showed that ∼99% of the variability in mortality, growth, behaviour and RMR among treatments can be explained by molecular responses. Mortality and growth are highly influenced by oxidative stress and energy metabolism, exhibiting a positive relationship with reactive oxygen species and a negative relationship with aerobic metabolism, regardless of treatment. Under hypoxic condition, RMR, boldness and swimming activity have a positive relationship with anaerobic metabolism regardless of temperature. Thus, seabreams may use anaerobic reliance to counterbalance the effects of the stressors on RMR, activity and growth. The outcomes suggests that early life stages of white seabream overcame the single and combined effects of hypoxia and warming.

Metabolic resilience of the Australasian snapper (Chrysophrys auratus) to marine heatwaves and hypoxia

Marine organisms are under threat from a simultaneous combination of climate change stressors, including warming sea surface temperatures (SST), marine heatwave (MHW) episodes, and hypoxic events. This study sought to investigate the impacts of these stressors on the Australasian snapper (C. auratus) - a finfish species of high commercial and recreational importance, from the largest snapper fishery in Aotearoa New Zealand (SNA1). A MHW scenario was simulated from 21°C (current February SST average for north-eastern New Zealand) to a future predicted level of 25°C, with the whole-animal and mitochondrial metabolic performance of snapper in response to hypoxia and elevated temperature tested after 1-, 10-, and 30-days of thermal challenge. It was hypothesised that key indicators of snapper metabolic performance would decline after 1-day of MHW stress, but that partial recovery might arise as result of thermal plasticity after chronic (e.g., 30-day) exposures. In contrast to this hypothesis, snapper performance remained high throughout the MHW: 1) Aerobic metabolic scope increased after 1-day of 25°C exposure and remained high. 2) Hypoxia tolerance, measured as the critical O₂ pressure and O₂ pressure where loss of equilibrium occurred, declined after 1-day of warm-acclimation, but recovered quickly with no observable difference from the 21°C control following 30-days at 25°C. 3) The performance of snapper mitochondria was also maintained, with oxidative phosphorylation respiration and proton leak flux across the inner mitochondrial membrane of the heart remaining mostly unaffected. Collectively, the results suggest that heart mitochondria displayed resilience, or plasticity, in snapper chronically exposed to 25°C. Therefore, contrary to the notion of climate change having adverse metabolic effects, future temperatures approaching 25°C may be tolerated by C. auratus in Northern New Zealand. Even in conjunction with supplementary hypoxia, 25°C appears to represent a metabolically optimal temperature for this species.

Physiological mechanisms linking cold acclimation and the poleward distribution limit of a range-extending marine fish

Extensions of species' geographical distributions, or range extensions, are among the primary ecological responses to climate change in the oceans. Considerable variation across the rates at which species' ranges change with temperature hinders our ability to forecast range extensions based on climate data alone. To better manage the consequences of ongoing and future range extensions for global marine biodiversity, more information is needed on the biological mechanisms that link temperatures to range limits. This is especially important at understudied, low relative temperatures relevant to poleward range extensions, which appear to outpace warm range edge contractions four times over. Here, we capitalized on the ongoing range extension of a teleost predator, the Australasian snapper Chrysophrys auratus, to examine multiple measures of ecologically relevant physiological performance at the population's poleward range extension front. Swim tunnel respirometry was used to determine how mid-range and poleward range edge winter acclimation temperatures affect metabolic rate, aerobic scope, swimming performance and efficiency and recovery from exercise. Relative to 'optimal' mid-range temperature acclimation, subsequent range edge minimum temperature acclimation resulted in absolute aerobic scope decreasing while factorial aerobic scope increased; efficiency of swimming increased while maximum sustainable swimming speed decreased; and recovery from exercise required a longer duration despite lower oxygen payback. Cold-acclimated swimming faster than 0.9 body lengths sec-1 required a greater proportion of aerobic scope despite decreased cost of transport. Reduced aerobic scope did not account for declines in recovery and lower maximum sustainable swimming speed. These results suggest that while performances decline at range edge minimum temperatures, cold-acclimated snapper are optimized for energy savings and range edge limitation may arise from suboptimal temperature exposure throughout the year rather than acute minimum temperature exposure. We propose incorporating performance data with in situ behaviour and environmental data in bioenergetic models to better understand how thermal tolerance determines range limits.

A mechanistic oxygen- and temperature-limited metabolic niche framework

The abundance and distribution of fishes and other water-breathing ectotherms are partially shaped by the capacities of individuals to perform ecologically relevant functions, which collectively determine whole-organism performance. Aerobic scope (AS) quantifies the capacity of the cardiorespiratory system to supply tissues with oxygen for fuelling such functions. Aquatic hypoxia and water temperature are principal environmental factors affecting the AS of water-breathing ectotherms. Although it is intuitive that animal energetics will be of ecological significance, many studies argue against a hypothesized overarching link between AS, whole-organism performance, and shifts in the abundance and distribution of water-breathing ectotherms with environmental change. Consequently, relationships between AS and ecologically relevant performance traits must be established for individual species. This article proposes a mechanistic framework for integrating and correlating experimental traits for assessing the AS, anaerobic capacity (AC) and range boundaries of water-breathing ectotherms exposed to progressive aquatic hypoxia and rising water temperature. The framework also describes cardiorespiratory thermal tolerance and proposes an empirical definition of the mechanism underlying the critical thermal maximum in species with oxygen-dependent upper thermal limits. Incorporating performance traits, exemplified with preference and avoidance responses, may provide information about the role of metabolism in shaping whole-organism performance, and the potential applicability of AS and AC in species distribution models. This article is part of the theme issue 'Physiological diversity, biodiversity patterns and global climate change: testing key hypotheses involving temperature and oxygen'.

Clownfish in hypoxic anemones replenish host O2 at only localised scales

The clownfish-anemone association exemplifies a symbiosis where both members benefit from nutrient exchange and protection from predators. Clownfish also perform aeration-like behaviour in their host anemones at night, but it is not yet known whether this is stimulated by the onset of hypoxia, and whether both members benefit from O₂ replenishment. Oxygen at 3 distances above the sea anemone Entacmaea quadricolor (0.2, 1.2 and 2.2 cm) therefore was measured under 3 light levels (photon flux density = 0, 55 and 110 µmol m⁻² s⁻¹), with and without the anemonefish Amphiprion frenatus. Hypoxia (O₂ < 50% air saturation) was recorded in the anemone, but only at 0.2 cm away from the anemone surface under dark conditions when A. frenatus was absent. This localised layer of hypoxia was eliminated by the presence of A. frenatus exhibiting aeration-like behaviour. Respirometry revealed that A. frenatus is extremely hypoxia tolerant (S_crit = 14.3% at 25 °C), suggesting that aeration behaviour does not provide a major metabolic advantage to clownfish because they do not breathe water at 0.2 cm and are not metabolically constrained by O₂ at distances ≥ 1.2 cm. That the aeration behaviour of A. frenatus facilitates only the metabolism of its O₂-conforming host reveals a unique aspect of this symbiotic relationship.

Fisheries, low oxygen and climate change: how much do we really know?

As a result of long-term climate change, regions of the ocean with low oxygen concentrations are predicted to occur more frequently and persist for longer periods of time in the future. When low levels of oxygen are present, this places additional pressure on marine organisms to meet their metabolic requirements, with implications for growth, feeding and reproduction. Extensive research has been carried out on the effects of acute hypoxia, but far less on long-term chronic effects of low oxygen zones, especially with regard to commercially important fishes and shellfishes. To provide further understanding on how commercial species could be affected, the results of relevant experiments must support population and ecosystem models. This is not easy because individual effects are wide-ranging; for example, studies to date have shown that low oxygen zones can affect predator-prey relationships as some species are able to tolerate low oxygen more than others. Some fishes may move away from areas until oxygen levels return to acceptable levels, while others take advantage of a reduced start response in prey fishes and remain in the area to feed. Sessile or less mobile species such as shellfishes are unable to move out of depleted oxygen zones. Some species can tolerate low oxygen levels for only short periods of time, while others are able to acclimatize. To advance the knowledge-base further, a number of promising technological and modelling-based developments and the role of physiological data within these, are proposed. These include advances in remote telemetry (tagging) and sensor technologies, trait-based analyses to provide insight into how whole assemblages might respond in the future, research into long-term adaptability of species, population and ecosystem modelling techniques and quantification of economic effects. In addition, more detailed oxygen monitoring and projections are required to better understand the likely temporal and local-scale changes in oxygen.

Gill structural change in response to turbidity has no effect on the oxygen uptake of a juvenile sparid fish

Turbidity as a result of increased suspended sediments in coastal waters is an environmental stress of worldwide concern. Recent research on fish suggests that detrimental changes to gill structure can occur in turbid waters, with speculation that these alterations diminish fitness variables, such as growth and development, by negatively impacting the O₂ uptake capacity (respiration) of fish. Specifically to address this unknown, the impact of turbid water on the gill structure, somatic growth rate and O₂ uptake rates of a juvenile sparid species (Pagrus auratus) was addressed following exposure to five different turbidity treatments (<10, 20, 40, 60 or 80 nephelometric turbidity units) for 30 days. Significant gill structural change was apparent with a progressive increase in turbidity and was quantified as a reduction in lamellar density, as well as an increase in basal hyperplasia, epithelial lifting and increased oxygen diffusion distance across the lamellae. The weight of control fish did not change throughout the experiment, but all fish exposed to turbid waters lost weight, and weight loss increased with nephelometric turbidity units, confirming that long-term turbidity exposure is detrimental to growth productivity. The growth of fish could be impacted in a variety of ways, but the specific hypothesis that structural alteration of the gills impairs O₂ uptake across the gills and limits growth fitness was not supported because there was no measurable difference in the standard metabolic rate, maximal metabolic rate, aerobic metabolic scope or critical oxygen saturation limit of fish measured in clear water after 30 days of exposure. Although impaired O₂ uptake as a result of structurally adjusted gills is unlikely to be the cause of poor fish growth, the exact mechanism by which growth productivity is affected in turbid conditions remains unclear and warrants further investigation.

Interactions between oxygen homeostasis, food availability, and hydrogen sulfide signaling

The ability to sense and respond to stressful conditions is essential to maintain organismal homeostasis. It has long been recognized that stress response factors that improve survival in changing conditions can also influence longevity. In this review, we discuss different strategies used by animals in response to decreased O(2) (hypoxia) to maintain O(2) homeostasis, and consider interactions between hypoxia responses, nutritional status, and H(2)S signaling. O(2) is an essential environmental nutrient for almost all metazoans as it plays a fundamental role in development and cellular metabolism. However, the physiological response(s) to hypoxia depend greatly on the amount of O(2) available. Animals must sense declining O(2) availability to coordinate fundamental metabolic and signaling pathways. It is not surprising that factors involved in the response to hypoxia are also involved in responding to other key environmental signals, particularly food availability. Recent studies in mammals have also shown that the small gaseous signaling molecule hydrogen sulfide (H(2)S) protects against cellular damage and death in hypoxia. These results suggest that H(2)S signaling also integrates with hypoxia response(s). Many of the signaling pathways that mediate the effects of hypoxia, food deprivation, and H(2)S signaling have also been implicated in the control of lifespan. Understanding how these pathways are coordinated therefore has the potential to reveal new cellular and organismal homeostatic mechanisms that contribute to longevity assurance in animals.