Individual aerobic performance and anaerobic compensation in a temperate fish during a simulated marine heatwave

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.

Do you remember? Within-generation and transgenerational heat stress memory of recurring marine heatwaves in threespine stickleback

Marine heatwaves can have major and lasting effects on organism physiology and species persistence. Such temperature extremes are increasing in frequency, with consecutive heatwave events already occurring within the lifetime of many organisms. Heat stress memory (thermal priming) by individuals is a potential within-generation response to cope with recurring marine heatwaves. However, whether this form of biological memory can be inherited across generations is not well known. We used a three-generation experiment to investigate individual and transgenerational effects of single and recurring marine heatwaves on fitness-related traits using stickleback (Gasterosteus aculeatus) as a model species. We exposed adults (both sexes) to heatwaves and assessed female reproductive output in both the parent and offspring generation, and offspring (both sexes) survival, growth and behaviour to establish a holistic picture of potential heatwave effects on ectothermic fish. Exposure to single, extreme heatwaves lowered reproductive output, decreased offspring exploratory behaviour, impeded capacity to respond to further thermal stress and reduced long-term survival. However, prior experience of heatwaves (heat stress memory) mitigated some of these effects at both an individual (growth) and transgenerational (fecundity) level, indicating that species experiencing increasing heatwave frequency as part of ongoing climate change may cope better than previously thought.

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.

Asparagopsis taxiformis as a Novel Antioxidant Ingredient for Climate-Smart Aquaculture: Antioxidant, Metabolic and Digestive Modulation in Juvenile White Seabream (Diplodus sargus) Exposed to a Marine Heatwave

The increasing frequency and duration of marine heatwaves (MHWs) due to climate change pose severe threats to aquaculture, causing drastic physiological and growth impairments in farmed fish, undermining their resilience against additional environmental pressures. To ensure sustainable production that meets the global seafood demand and animal welfare standards, cost-effective and eco-friendly strategies are urgently needed. This study explored the efficacy of the red macroalga Asparagopsis taxiformis on juvenile white seabream Diplodus sargus reared under optimal conditions and upon exposure to a MHW. Fish were fed with four experimental diets (0%, 1.5%, 3% or 6% of dried powdered A. taxiformis) for a prophylactic period of 30 days (T30) and subsequently exposed to a Mediterranean category II MHW for 15 days (T53). Biometric data and samples were collected at T30, T53 and T61 (8 days post-MHW recovery), to assess performance indicators, biomarker responses and histopathological alterations. Results showed that A. taxiformis supplementation improved catalase and glutathione S-transferase activities and reduced lipid peroxidation promoted by the MHW, particularly in fish biofortified with 1.5% inclusion level. No histopathological alterations were observed after 30 days. Additionally, fish biofortified with 1.5% A. taxiformis exhibited increased citrate synthase activity and fish supplemented with 1.5% and 3% showed improved digestive enzyme activities (e.g., pepsin and trypsin activities). Overall, the present findings pointed to 1.5% inclusion as the optimal dosage for aquafeeds biofortification with A. taxiformis, and confirmed that this seaweed species is a promising cost-effective ingredient with functional properties and great potential for usage in a climate-smart context.

Molecular and behavioural responses of the mussel Mytilus edulis exposed to a marine heatwave

Marine heatwaves (MHW) threaten marine organisms and tend to increase in frequency and intensity. We exposed the blue mussel Mytilus edulis to a MHW lasting 23 days, including two 10-d periods of thermal intensity increase of +5 °C (20 °C-25 °C) interspersed by 1 day back to 20 °C, followed by a 4-d recovery period. We investigated behaviour responses of mussels and gene expression changes relative to the circadian rhythm (Per), oxidative stress (SOD), cellular apoptosis (CASP3), energy production (ATPs), and general stress response (hsp70). Results showed that the MHW disturbed the valve activity of mussels. Particularly, mussels increased the number of valve micro-closures, showing a stressful state of organisms. Mussels also decreased Per, CASP3, ATPs, and Hsp70 gene expression. Some behavioural and molecular effects persisted after the MHW, suggesting a limited recovery capacity of individuals. This work highlighted the vulnerability of M. edulis to a realistic MHW.

Effect of marine heatwaves on juvenile greater amberjack (Seriola dumerili)

Marine heatwaves (MHWs) have increased in frequency, intensity, and duration in recent years causing significant impacts on marine organisms and fisheries. This study explores the physiological changes of juvenile greater amberjacks (Seriola dumerili) that cope with MHWs. Results showed that physiological parameters were significantly affected by the intensity, duration of MHWs or interaction of two factors (P < 0.05). Repeated MHWs in which water temperatures were increased (24 °C to 28 °C and 32 °C) resulted in changes in enzyme activity levels (catalase (CAT), superoxide dismutase (SOD), and glutathione (GSH)), as well as the level of malondialdehyde (MDA) for antioxidant defense, immune function (acid phosphatase (ACP), alkaline phosphatase (ALP), and lysozyme (LYZ)), and energy metabolism (including triglycerides (TG), glucose (GLU), aspartate aminotransferase (GOT), alanine aminotransferase (ALT), lactate dehydrogenase (LDH), and succinate dehydrogenase (SDH)). The activities of enzymes, including those associated with antioxidant defense, immune function, and energy metabolism, changed significantly in relation to short-term MHWs, indicating a thermal stress response. When S. dumerili were exposed to repeated-MHWs, thermal stress responses increased at 28 °C (T28) and decreased at 32 °C (T32). These results exhibited the inability of S. dumerili to acclimate to severe thermal stress from MHWs. This study examined S. dumerili responses to MHWs and assessed the physiological adaptation of juvenile greater amberjacks to MHWs.