Developmental effects of heatwave conditions on the early life stages of a coral reef fish

Heatwave duration, intensity and timing as drivers of performance in larvae of a marine invertebrate

In marine ecosystems, crustaceans face an alarming threat from the increasing frequency and intensity of marine heatwaves as their early planktonic stages are particularly temperature sensitive. While the impact of heatwaves on adult crustaceans is well-studied, their effects on larvae remain underexplored. This study focuses on heatwave effects on larvae of the European shore crab, Carcinus maenas. Through a factorial experiment, larvae were exposed to different heatwaves of varying onset timings, durations, and intensities. Survival, development duration, and dry mass decreased under intense heatwaves, with more severe effects observed when heatwaves occurred later in development, highlighting a stage-specific sensitivity to heatwave. We also identified a "region of existence" beyond which larval performance was compromised compared to baseline temperatures. This region defines the heatwave components considered "extreme" for the organism, as well as those inducing neutral or positive effects on performance. Additionally, we distinguished heatwave effects (characterised by their components) from those attributed to the average temperature experienced during the experiments. Our findings demonstrated that larval performance was lower during intense heatwaves compared to the performance expected under a constant average temperature. These findings emphasize the importance of considering heatwave timing relative to the life cycle for predicting marine population responses to climate change.

Extreme marine heatwave linked to mass fish kill in the Red Sea

Anthropogenic climate change has precipitated an increase in marine heatwaves (MHWs) that have significant and multifaceted impacts on marine ecosystems. In late August 2023, an intense heatwave coincided with a mass fish kill event on the Saudi Arabian coast of the central Red Sea. Here, we compile MHW metrics from satellite data to illustrate the mortality event was linked with the most intense period of rapid heating in the central Red Sea in recent history. Using field surveys, we quantified the impact of the event on the fish community and found that nearly 1000 fish washed ashore along a 60 km stretch of coastline. Representatives of 54 species were detected, which illustrates the impact of the MHW event on a broad range of fishes. The exact cause of mortality during the event is unknown, but likely related to temperature-induced physiological stress and associated factors. Sparse coastal monitoring limited our ability to rapidly respond to the event and document the proximate cause of mortality. This study not only sheds light on the immediate impacts of a MHW on components of a coral reef ecosystem, but also emphasizes the broader ecological consequences. Mass fish kills may have cascading impacts on ecosystem functioning by causing shifts in community structure and a decrease in biodiversity, which can undermine both the ecological functioning and economic stability of marine-dependent regions. This may be especially true for reefs already occupying a thermal niche that approaches the upper limits of many species, such as those in the Red Sea. Our study highlights the critical need for enhanced reporting mechanisms and forecasting tools to effectively document and help mitigate further impacts linked to MHW-induced mass marine die-offs.

Interspecies differences in lactate dehydrogenase and citrate synthase activity among damselfish and cardinalfish

Species with different thermal distributions, life-history traits, and behaviours have evolved physiological processes to suit energetic demands. Previous research has argued that these interspecies differences are often reflected in muscle enzyme activity that serve as proxies for aerobic and anaerobic respiration. Here, we measured the maximal enzyme activity of two enzymes, citrate synthase and lactate dehydrogenase, between two damselfish (Pomacentrus) and cardinalfish (Ostorhinchus) species. Citrate synthase was measured as a proxy for mitochondrial volume density, a marker of aerobic metabolism; lactate dehydrogenase was measured as a proxy for anaerobic energy production, a marker for anaerobic metabolism. Thermal performance curves of maximal enzyme activity were measured from 10 to 50 °C, at 10 °C intervals. Citrate synthase and lactate dehydrogenase both showed a positive correlation with temperature, that was absent of a plateau. Damselfish displayed higher levels of citate synthase maximal enzyme activity, while cardinalfish displayed a higher lactate dehydrogenase to citrate synthase ratio. Ostorhinchus doederleini, a sedentary cardinalfish, displayed higher level of lactate dehydrogenase maximal enzyme activity. Temperature coefficients (Q10) for lactate dehydrogenase showed a curved relationship, peaking at differences between 30 and 40 °C. No differences in Q10 values were observed between species, suggesting no difference in the thermal sensitivity of enzymes. Interspecies differences in maximal enzyme activity identified in this study compliments previous research, whereby more active species require higher levels of citrate synthase to fuel sustained swimming, as well as energetically demanding locomotion behaviours. Alternatively, more sedentary species possessed higher levels of lactate dehydrogenase and reliance on anaerobic metabolism, possibly due to an increased reliance on infrequent burst swimming behaviours.

Exposure to sub-optimal temperature during early development decreases hypoxia tolerance in juvenile Fundulus heteroclitus

Cross-protection occurs when exposure to one stressor confers heightened tolerance against a different stressor. Alternatively, exposure to one stressor could result in reduced tolerance against other stressors. Although cross-protection has been documented in a wide range of taxa at juvenile and adult life stages, whether early developmental exposure to a stressor confers cross-protection or reduced tolerance to other stressors later in life through developmental plasticity remains largely unexplored. In this study, we examined whether altered temperature during embryonic development results in developmental plasticity in upper thermal tolerance or hypoxia tolerance using a small topminnow, Fundulus heteroclitus, and examined potential underlying molecular mechanisms. We incubated embryos at one of two ecologically relevant temperatures (20°C or 26°C) until hatch. Once hatched, fish were raised at a common temperature of 20°C for 1 year, and tolerance was assessed in both juveniles (6 months) and early adults (1 year). Developmental temperature had no significant effect on thermal tolerance (CTmax) in juvenile fish, or on the transcript abundance of thermal tolerance-related genes (constitutive heat shock proteins, hsc70, hsp90b). In contrast, reduced developmental temperature decreased hypoxia tolerance but increased transcript levels of the hypoxia inducible factor hif1α in juvenile fish but the effects were less evident in older fish. Overall, we found no indication of developmental plasticity for thermal tolerance, but there was evidence of negative impacts of lower developmental temperature on hypoxia tolerance in juveniles associated with changes in gene expression, providing evidence of developmental plasticity across stressors and levels of organization.

The effects of marine heatwaves on a coral reef snapper: insights into aerobic and anaerobic physiology and recovery

Marine heatwaves (MHWs) are increasing in frequency and intensity. Coral reefs are particularly susceptible to MHWs, which cause mass coral bleaching and mortality. However, little is known about how MHWs affect coral reef fishes. Here, we investigated how MHWs affect the physiology of a coral reef mesopredator, Lutjanus carponotatus. Specifically, we exposed mature adults to two different MHW intensities, +1°C (29.5°C) and + 2°C (30.5°C) and measured physiological performance at 2 and 4 weeks of exposure and at 2 weeks post-exposure. At these time points, we measured oxygen consumption at rest and after a simulated fishing capture event, recovery time, excess post-exercise oxygen consumption (EPOC) and associated biochemical markers in the blood (baseline lactate, post-capture lactate, glucose, haemoglobin levels and haematocrit proportion). We found that 2 weeks of exposure to MHW conditions increased resting oxygen consumption (+1°C = 23%, +2°C = 37%), recovery time (+1°C = 62%, +2°C = 77%), EPOC (+1°C = 50%, +2°C = 68%), baseline lactate (+1°C = 27%, +2°C = 28%), post-capture lactate (+1°C = 62%, +2°C = 109%) and haemoglobin levels (+1°C = 13%, +2°C = 28%). This pattern was maintained at 4 weeks of exposure except for post-capture lactate which was reduced (+1°C = -37%, +2°C = 27%). In combination, these results suggest a greater reliance on anaerobic glycolysis to maintain homeostasis in MHW conditions. At 2 weeks post-exposure, when compared to control fish, we found that capture oxygen consumption was increased (+1°C = 25%, +2°C = 26%), recovery rate was increased (+2°C = 38%) and haemoglobin was still higher (+1°C = 15%, +2°C = 21%). These results show that MHW conditions have direct physiological demands on adult coral reef snapper and ecologically relevant residual effects can last for at least 2 weeks post-MHW; however, individuals appear to recover from the negative effects experienced during the MHW. This provides new insight into the effects of MHWs on the physiological performance of coral reef fishes.

Shark critical life stage vulnerability to monthly temperature variations under climate change

In a 10-month experimental study, we assessed the combined impact of warming and acidification on critical life stages of small-spotted catshark (Scyliorhinus canicula). Using recently developed frameworks, we disentangled individual and group responses to two climate scenarios projected for 2100 (SSP2-4.5: Middle of the road and SSP5-8.5: Fossil-fueled Development). Seasonal temperature fluctuations revealed the acute vulnerability of embryos to summer temperatures, with hatching success ranging from 82% for the control and SSP2-4.5 treatments to only 11% for the SSP5-8.5 treatment. The death of embryos was preceded by distinct individual growth trajectories between the treatments, and also revealed inter-individual variations within treatments. Embryos with the lowest hatching success had lower yolk consumption rates, and growth rates associated with a lower energy assimilation, and almost all of them failed to transition to internal gills. Within 6 months after hatching, no additional mortality was observed due to cooler temperatures.

Differential effects of parental and developmental temperatures on larval thermal adaptation in oviparous and viviparous model fish species

Phenotypic plasticity has been identified as a major mechanism of response to changing temperatures. Parental effects are potentially important drivers of ecological and evolutionary dynamics, while developmental plasticity also plays a key role in generating phenotypic variation. However, little is known of the interaction between parental effects and developmental plasticity on the thermal phenotypes of fishes with different reproductive modes (i.e. oviparous vs. viviparous). To understand the contributions of inter- and intra-generational plasticity of thermal phenotypes (preferred temperature, avoidance temperatures, critical thermal thresholds) in fishes with different reproductive modes, we carried out a factorial experiment in which both breeding parents and offspring were exposed to lower (22 °C) or higher (28 °C) temperatures, using zebrafish (Danio rerio) and guppies (Poecilia reticulata) as representative oviparous and viviparous species. We found that offspring thermal preference and avoidance of both species were significantly influenced by parental effects and developmental plasticity, with higher thermal preference and avoidance consistent with higher background (parental) temperature treatments. However, parental effects were only found to impose significant effect on the thermal tolerances of guppies. The findings suggest that phenotypic plasticity, both within and across generations, may be an important mechanism to adapt to rapid climate changes, and that future temperature fluctuations may impose more profound effects on viviparous fish species in general.

Developmental plasticity in thermal tolerance: Ontogenetic variation, persistence, and future directions

Understanding the factors affecting thermal tolerance is crucial for predicting the impact climate change will have on ectotherms. However, the role developmental plasticity plays in allowing populations to cope with thermal extremes is poorly understood. Here, we meta-analyse how thermal tolerance is initially and persistently impacted by early (embryonic and juvenile) thermal environments by using data from 150 experimental studies on 138 ectothermic species. Thermal tolerance only increased by 0.13°C per 1°C change in developmental temperature and substantial variation in plasticity (~36%) was the result of shared evolutionary history and species ecology. Aquatic ectotherms were more than three times as plastic as terrestrial ectotherms. Notably, embryos expressed weaker but more heterogenous plasticity than older life stages, with numerous responses appearing as non-adaptive. While developmental temperatures did not have persistent effects on thermal tolerance overall, persistent effects were vastly under-studied, and their direction and magnitude varied with ontogeny. Embryonic stages may represent a critical window of vulnerability to changing environments and we urge researchers to consider early life stages when assessing the climate vulnerability of ectotherms. Overall, our synthesis suggests that developmental changes in thermal tolerance rarely reach levels of perfect compensation and may provide limited benefit in changing environments.

Trans- and Within-Generational Developmental Plasticity May Benefit the Prey but Not Its Predator during Heat Waves

Theoretically, parents can adjust vital offspring traits to the irregular and rapid occurrence of heat waves via developmental plasticity. However, the direction and strength of such trait modifications are often species-specific. Here, we investigated within-generational plasticity (WGP) and trans-generational plasticity (TGP) effects induced by heat waves during the offspring development of the predator Phytoseiulus persimilis and its herbivorous prey, the spider mite Tetranychus urticae, to assess plastic developmental modifications. Single offspring individuals with different parental thermal origin (reared under mild or extreme heat waves) of both species were exposed to mild or extreme heat waves until adulthood, and food consumption, age and size at maturity were recorded. The offspring traits were influenced by within-generational plasticity (WGP), trans-generational plasticity (TGP), non-plastic trans-generational effects (TGE) and/or their interactions. When exposed to extreme heat waves, both species speeded up development (exclusively WGP), consumed more (due to the fact of WGP but also to TGP in prey females and to non-plastic TGE in predator males), and predator females got smaller (non-plastic TGE and WGP), whereas prey males and females were equally sized irrespective of their origin, because TGE, WGP and TGP acted in opposite directions. The body sizes of predator males were insensitive to parental and offspring heat wave conditions. Species comparisons indicated stronger reductions in the developmental time and reduced female predator-prey body size ratios in favor of the prey under extreme heat waves. Further investigations are needed to evaluate, whether trait modifications result in lowered suppression success of the predator on its prey under heat waves or not.

Parents exposed to warming produce offspring lower in weight and condition

The parental environment can alter offspring phenotypes via the transfer of non-genetic information. Parental effects may be viewed as an extension of (within-generation) phenotypic plasticity. Smaller size, poorer physical condition, and skewed sex ratios are common responses of organisms to global warming, yet whether parental effects alleviate, exacerbate, or have no impact on these responses has not been widely tested. Further, the relative non-genetic influence of mothers and fathers and ontogenetic timing of parental exposure to warming on offspring phenotypes is poorly understood. Here, we tested how maternal, paternal, and biparental exposure of a coral reef fish (Acanthochromis polyacanthus) to elevated temperature (+1.5°C) at different ontogenetic stages (development vs reproduction) influences offspring length, weight, condition, and sex. Fish were reared across two generations in present-day and projected ocean warming in a full factorial design. As expected, offspring of parents exposed to present-day control temperature that were reared in warmer water were shorter than their siblings reared in control temperature; however, within-generation plasticity allowed maintenance of weight, resulting in a higher body condition. Parental exposure to warming, irrespective of ontogenetic timing and sex, resulted in decreased weight and condition in all offspring rearing temperatures. By contrast, offspring sex ratios were not strongly influenced by their rearing temperature or that of their parents. Together, our results reveal that phenotypic plasticity may help coral reef fishes maintain performance in a warm ocean within a generation, but could exacerbate the negative effects of warming between generations, regardless of when mothers and fathers are exposed to warming. Alternatively, the multigenerational impact on offspring weight and condition may be a necessary cost to adapt metabolism to increasing temperatures. This research highlights the importance of examining phenotypic plasticity within and between generations across a range of traits to accurately predict how organisms will respond to climate change.

The alternative splicing landscape of a coral reef fish during a marine heatwave

Alternative splicing is a molecular mechanism that enables a single gene to encode multiple transcripts and proteins by post-transcriptional modification of pre-RNA molecules. Changes in the splicing scheme of genes can lead to modifications of the transcriptome and the proteome. This mechanism can enable organisms to respond to environmental fluctuations. In this study, we investigated patterns of alternative splicing in the liver of the coral reef fish Acanthochromis polyacanthus in response to the 2016 marine heatwave on the Great Barrier Reef. The differentially spliced (DS; n = 40) genes during the onset of the heatwave (i.e., 29.49°C or +1°C from average) were related to essential cellular functions such as the MAPK signaling system, Ca(2+) binding, and homeostasis. With the persistence of the heatwave for a period of one month (February to March), 21 DS genes were detected, suggesting that acute warming during the onset of the heatwave is more influential on alternative splicing than the continued exposure to elevated temperatures. After the heatwave, the water temperature cooled to ~24.96°C, and fish showed differential splicing of genes related to cyto-protection and post-damage recovery (n = 26). Two-thirds of the DS genes detected across the heatwave were also differentially expressed, revealing that the two molecular mechanisms act together in A. polyacanthus to cope with the acute thermal change. This study exemplifies how splicing patterns of a coral reef fish can be modified by marine heatwaves. Alternative splicing could therefore be a potential mechanism to adjust cellular physiological states under thermal stress and aid coral reef fishes in their response to more frequent acute thermal fluctuations in upcoming decades.

Intraspecific variation in tolerance of warming in fishes

Intraspecific variation in key traits such as tolerance of warming can have profound effects on ecological and evolutionary processes, notably responses to climate change. The empirical evidence for three primary elements of intraspecific variation in tolerance of warming in fishes is reviewed. The first is purely mechanistic that tolerance varies across life stages and as fishes become mature. The limited evidence indicates strongly that this is the case, possibly because of universal physiological principles. The second is intraspecific variation that is because of phenotypic plasticity, also a mechanistic phenomenon that buffers individuals' sensitivity to negative impacts of global warming in their lifetime, or to some extent through epigenetic effects over successive generations. Although the evidence for plasticity in tolerance to warming is extensive, more work is required to understand underlying mechanisms and to reveal whether there are general patterns. The third element is intraspecific variation based on heritable genetic differences in tolerance, which underlies local adaptation and may define long-term adaptability of a species in the face of ongoing global change. There is clear evidence of local adaptation and some evidence of heritability of tolerance to warming, but the knowledge base is limited with detailed information for only a few model or emblematic species. There is also strong evidence of structured variation in tolerance of warming within species, which may have ecological and evolutionary significance irrespective of whether it reflects plasticity or adaptation. Although the overwhelming consensus is that having broader intraspecific variation in tolerance should reduce species vulnerability to impacts of global warming, there are no sufficient data on fishes to provide insights into particular mechanisms by which this may occur.

Body shape and fin size in juvenile Atlantic salmon (Salmo salar): effects of temperature during embryogenesis

Temperature during egg incubation and early development influences later life stages of fishes, potentially influencing survival. Throughout its distribution, Atlantic salmon (Salmo salar Linnaeus, 1758) have experienced population declines, and in view of ongoing global warming, we tested if temperature during the earliest developmental stages modified body shape and fin size when temperatures averaged 2.6 vs. 5.6 °C. This temperature difference simulates increases predicted in climate change scenarios. Based on previous studies, we hypothesized that salmon originating from eggs subjected to cold incubation temperatures would have slimmer bodies and larger pectoral and dorsal fins than salmon from eggs that experienced warmer temperatures. After hatching, the juveniles were raised for 1 year under identical temperatures, after which we measured their body shape and fin areas. We found no support for our hypothesis regarding body shape. Indeed, we found the opposite, with cold-incubated salmon having deeper bodies than warm-incubated salmon. For fin size, the pectoral fins of cold-incubated salmon were larger than for warm-incubated salmon as predicted, but there was no difference in dorsal fin size. These results suggest that global warming may lead to altered body shape and fin size, possibly affecting swimming performance, and thus raise questions about the ecological consequences of the changes.

Sex- and time-specific parental effects of warming on reproduction and offspring quality in a coral reef fish

Global warming can disrupt reproduction or lead to fewer and poorer quality offspring, owing to the thermally sensitive nature of reproductive physiology. However, phenotypic plasticity may enable some animals to adjust the thermal sensitivity of reproduction to maintain performance in warmer conditions. Whether elevated temperature affects reproduction may depend on the timing of exposure to warming and the sex of the parent exposed. We exposed male and female coral reef damselfish (Acanthochromis polyacanthus) during development, reproduction or both life stages to an elevated temperature (+1.5°C) consistent with projected ocean warming and measured reproductive output and newly hatched offspring performance relative to pairs reared in a present-day control temperature. We found female development in elevated temperature increased the probability of breeding, but reproduction ceased if warming continued to the reproductive stage, irrespective of the male's developmental experience. Females that developed in warmer conditions, but reproduced in control conditions, also produced larger eggs and hatchlings with greater yolk reserves. By contrast, male development or pairs reproducing in higher temperature produced fewer and poorer quality offspring. Such changes may be due to alterations in sex hormones or an endocrine stress response. In nature, this could mean female fish developing during a marine heatwave may have enhanced reproduction and produce higher quality offspring compared with females developing in a year of usual thermal conditions. However, male development during a heatwave would likely result in reduced reproductive output. Furthermore, the lack of reproduction from an average increase in temperature could lead to population decline. Our results demonstrate how the timing of exposure differentially influences females and males and how this translates to effects on reproduction and population sustainability in a warming world.

Differential sensitivity to warming and hypoxia during development and long-term effects of developmental exposure in early life stage Chinook salmon

Warming and hypoxia are two stressors commonly found within natural salmon redds that are likely to co-occur. Warming and hypoxia can interact physiologically, but their combined effects during fish development remain poorly studied, particularly stage-specific effects and potential carry-over effects. To test the impacts of warm water temperature and hypoxia as individual and combined developmental stressors, late fall-run Chinook salmon embryos were reared in 10 treatments from fertilization through hatching with two temperatures [10°C (ambient) and 14°C (warm)], two dissolved oxygen saturation levels [normoxia (100% air saturation, 10.4-11.4 mg O₂/l) and hypoxia (50% saturation, 5.5 mg O₂/l)] and three exposure times (early [eyed stage], late [silver-eyed stage] and chronic [fertilization through hatching]). After hatching, all treatments were transferred to control conditions (10°C and 100% air saturation) through the fry stage. To study stage-specific effects of stressor exposure we measured routine metabolic rate (RMR) at two embryonic stages, hatching success and growth. To evaluate carry-over effects, where conditions during one life stage influence performance in a later stage, RMR of all treatments was measured in control conditions at two post-hatch stages and acute stress tolerance was measured at the fry stage. We found evidence of stage-specific effects of both stressors during exposure and carry-over effects on physiological performance. Both individual stressors affected RMR, growth and developmental rate while multiple stressors late in development reduced hatching success. RMR post-hatch showed persistent effects of embryonic stressor exposure that may underlie differences observed in developmental timing and acute stress tolerance. The responses to stressors that varied by stage during development suggest that stage-specific management efforts could support salmon embryo survival. The persistent carry-over effects also indicate that considering sub-lethal effects of developmental stressor exposure may be important to understanding how climate change influences the performance of salmon across life stages.