Effects of ocean acidification on metabolic performance in coral reef fishes

Given the dramatic changes in atmospheric conditions over the 400MY evolutionary history of the fishes, physiological tolerance to elevated CO2 may not be unexpected. However, the most speciose genera of coral reef fishes radiated relatively recently (23MYA) – during a period of low CO2. And, although based on only a few studies so far, the physiological effects of elevated CO2 on coral reef fishes are mixed. In some species, metabolic performance is negatively affected by near-future CO2 levels. However, other species exhibit either no change or even enhanced scope for aerobic performance. The reasons for this variation could be related to differences in lifestyle and habitat use, which could influence CO2 tolerance. Another possibility is that whole organism responses in some species may not be sensitive enough to pick up the fine-scale adjustments made at the tissue and cellular levels. Identifying changes at key sites related to oxygen transport, oxygen consumption, and energy production in response to elevated CO2 over both acute and prolonged timescales linking back to the organism’s life history are crucial. Mechanisms that may be influencing physiological changes at the whole organism level could be potential targets of natural selection and adaptation to future ocean conditions

Effects of ocean acidification on metabolic performance in coral reef fishes

Given the dramatic changes in atmospheric conditions over the 400MY evolutionary history of the fishes, physiological tolerance to elevated CO2 may not be unexpected. However, the most speciose genera of coral reef fishes radiated relatively recently (23MYA) – during a period of low CO2. And, although based on only a few studies so far, the physiological effects of elevated CO2 on coral reef fishes are mixed. In some species, metabolic performance is negatively affected by near-future CO2 levels. However, other species exhibit either no change or even enhanced scope for aerobic performance. The reasons for this variation could be related to differences in lifestyle and habitat use, which could influence CO2 tolerance. Another possibility is that whole organism responses in some species may not be sensitive enough to pick up the fine-scale adjustments made at the tissue and cellular levels. Identifying changes at key sites related to oxygen transport, oxygen consumption, and energy production in response to elevated CO2 over both acute and prolonged timescales linking back to the organism’s life history are crucial. Mechanisms that may be influencing physiological changes at the whole organism level could be potential targets of natural selection and adaptation to future ocean conditions

What if you can't sense your enemy... and your enemy is an invasive predator?

It turns out that the lionfish—an invasive fish species that is especially plaguing western Atlantic waters these days—is even more of a threat than we originally thought. Some of its prey species, like small coral reef fishes, cannot identify the lionfish as a threat

Physiology can contribute to better understanding, management, and conservation of coral reef fishes

Coral reef fishes, like many other marine organisms, are affected by anthropogenic stressors such as fishing and pollution and, owing to climate change, are experiencing increasing water temperatures and ocean acidification. Against the backdrop of these various stressors, a mechanistic understanding of processes governing individual organismal performance is the first step for identifying drivers of coral reef fish population dynamics. In fact, physiological measurements can help to reveal potential cause-and-effect relationships and enable physiologists to advise conservation management by upscaling results from cellular and individual organismal levels to population levels. Here, we highlight studies that include physiological measurements of coral reef fishes and those that give advice for their conservation. A literature search using combined physiological, conservation and coral reef fish key words resulted in similar to 1900 studies, of which only 99 matched predefined requirements. We observed that, over the last 20 years, the combination of physiological and conservation aspects in studies on coral reef fishes has received increased attention. Most of the selected studies made their physiological observations at the whole organism level and used their findings to give conservation advice on population dynamics, habitat use or the potential effects of climate change. The precision of the recommendations differed greatly and, not surprisingly, was least concrete when studies examined the effects of projected climate change scenarios. Although more and more physiological studies on coral reef fishes include conservation aspects, there is still a lack of concrete advice for conservation managers, with only very few published examples of physiological findings leading to improved management practices. We conclude with a call to action to foster better knowledge exchange between natural scientists and conservation managers to translate physiological findings more effectively in order to obtain evidence-based and adaptive management strategies for the conservation of coral reef fishes

Root effect haemoglobins in fish may greatly enhance general oxygen delivery relative to other vertebrates

The teleost fishes represent over half of all extant vertebrates; they occupy nearly every body of water and in doing so, occupy a diverse array of environmental conditions. We propose that their success is related to a unique oxygen (O2) transport system involving their extremely pH-sensitive haemoglobin (Hb). A reduction in pH reduces both Hb-O2 affinity (Bohr effect) and carrying capacity (Root effect). This, combined with a large arterial-venous pH change (ΔpHa-v) relative to other vertebrates, may greatly enhance tissue oxygen delivery in teleosts (e.g., rainbow trout) during stress, beyond that in mammals (e.g., human). We generated oxygen equilibrium curves (OECs) at five different CO2 tensions for rainbow trout and determined that, when Hb-O2 saturation is 50% or greater, the change in oxygen partial pressure (ΔPO2) associated with ΔpHa-v can exceed that of the mammalian Bohr effect by at least 3-fold, but as much as 21-fold. Using known ΔpHa-v and assuming a constant arterial-venous PO2 difference (Pa-vO2), Root effect Hbs can enhance O2 release to the tissues by 73.5% in trout; whereas, the Bohr effect alone is responsible for enhancing O2 release by only 1.3% in humans. Disequilibrium states are likely operational in teleosts in vivo, and therefore the ΔpHa-v, and thus enhancement of O2 delivery, could be even larger. Modeling with known Pa-vO2 in fish during exercise and hypoxia indicates that O2 release from the Hb and therefore potentially tissue O2 delivery may double during exercise and triple during some levels of hypoxia. These characteristics may be central to performance of athletic fish species such as salmonids, but may indicate that general tissue oxygen delivery may have been the incipient function of Root effect Hbs in fish, a trait strongly associated with the adaptive radiation of teleosts

Too turbid for nemo: suspended sediment impacts gills and favours pathogenic bacteria in clownfish larvae

The Great Barrier Reef (GBR), arguably the most pristine and best-managed coral reef in the world, has experienced a 400-800% increase in sediment inputs over the past 200 years. Further increases due to port expansions, dredging, shipping, and continued coastal agricultural and industrial development are inevitable. Increased sedimentation and turbidity impacts species composition and distribution patterns, but the underlying mechanisms are unknown. We examined the gill morphology and gill microbiome of clownfish (Amphiprion percula) larvae upon exposure to sediment concentrations common in coastal waters of the GBR. Gills exhibited excessive mucous and protective cell layers, resulting in a 56% thicker gill epithelium compared to fish from control conditions. Such changes could impact oxygen transport, which is key to critical life history activities essential to support biological fitness. We also found a shift from 'healthy' to pathogenic bacterial communities on the gills, which could increase disease susceptibility. The impact of suspended sediments at the gill may represent an underlying mechanism contributing to the negative effects that suspended sediments have on fish assemblages. Our findings underscore the necessity for future coastal development plans to consider the adverse effects of suspended sediments on fish recruitment, and consequently fish populations, and ecosystem health

Too turbid for nemo: suspended sediment impacts gills and favours pathogenic bacteria in clownfish larvae

The Great Barrier Reef (GBR), arguably the most pristine and best-managed coral reef in the world, has experienced a 400-800% increase in sediment inputs over the past 200 years. Further increases due to port expansions, dredging, shipping, and continued coastal agricultural and industrial development are inevitable. Increased sedimentation and turbidity impacts species composition and distribution patterns, but the underlying mechanisms are unknown. We examined the gill morphology and gill microbiome of clownfish (Amphiprion percula) larvae upon exposure to sediment concentrations common in coastal waters of the GBR. Gills exhibited excessive mucous and protective cell layers, resulting in a 56% thicker gill epithelium compared to fish from control conditions. Such changes could impact oxygen transport, which is key to critical life history activities essential to support biological fitness. We also found a shift from 'healthy' to pathogenic bacterial communities on the gills, which could increase disease susceptibility. The impact of suspended sediments at the gill may represent an underlying mechanism contributing to the negative effects that suspended sediments have on fish assemblages. Our findings underscore the necessity for future coastal development plans to consider the adverse effects of suspended sediments on fish recruitment, and consequently fish populations, and ecosystem health

Nonlethally assessing elasmobranch ontogenetic shifts in energetics

Body condition is an important proxy for the overall health and energetic status of fishes. The classically used Fulton's condition factor requires length and mass measurements, but mass can be difficult to obtain in large species. Girth measurements can replace mass for wild pelagic sharks. However, girth-calculated condition has not been validated against Fulton's condition factor intraspecifically, across ontogeny or reproduction, or in a controlled setting. We used the epaulette shark (Hemiscyllium ocellatum), because they are amenable to captive reproduction, to track fine-scale body condition changes across life stages, oviparous reproduction and between condition indices. We measured four girths, total length and mass of 16 captive epaulette sharks across 1 year and tracked female reproduction daily. We also collected length and mass data from an additional 72 wild-caught sharks and 155 sharks from five previous studies and two public aquaria to examine the relationship between length and mass for this species. Even though data were derived from a variety of sources, a predictable length–mass relationship (R2 = 0.990) was achievable, indicating that combining data from a variety of sources could help overcome knowledge gaps regarding basic life history characteristics. We also found that condition factor decreased during early life stages, then increased again into adulthood, with predictable changes across the female reproductive cycle. Finally, we determined that both Fulton's and girth condition analyses were comparable. Outcomes from this study uniquely provide body condition changes across the complete life history, including fine-scale female reproductive stages, and validate the use of girths as a nonlethal whole-organism energetic assessment for fishes

The upper thermal limit of epaulette sharks (Hemiscyllium ocellatum) is conserved across three life history stages, sex and body size

Owing to climate change, most notably the increasing frequency of marine heatwaves and long-term ocean warming, better elucidating the upper thermal limits of marine fishes is important for predicting the future of species and populations. The critical thermal maximum (CTmax), or the highest temperature a species can tolerate, is a physiological metric that is used to establish upper thermal limits. Among marine organisms, this metric is commonly assessed in bony fishes but less so in other taxonomic groups, such as elasmobranchs (subclass of sharks, rays and skates), where only thermal acclimation effects on CTmax have been assessed. Herein, we tested whether three life history stages, sex and body size affected CTmax in a tropical elasmobranch, the epaulette shark (Hemiscyllium ocellatum), collected from the reef flats surrounding Heron Island, Australia. Overall, we found no difference in CTmax between life history stages, sexes or across a range of body sizes. Findings from this research suggest that the energetically costly processes (i.e. growth, maturation and reproduction) associated with the life history stages occupying these tropical reef flats do not change overall acute thermal tolerance. However, it is important to note that neither embryos developing in ovo, neonates, nor females actively encapsulating egg cases were observed in or collected from the reef flats. Overall, our findings provide the first evidence in an elasmobranch that upper thermal tolerance is not impacted by life history stage or size. This information will help to improve our understanding of how anthropogenic climate change may (or may not) disproportionally affect particular life stages and, as such, where additional conservation and management actions may be required

A negative correlation between behavioural and physiological performance under ocean acidification and warming

Many studies have examined the average effects of ocean acidification and warming on phenotypic traits of reef fishes, finding variable, but often negative effects on behavioural and physiological performance.Yet the presence and nature of a relationship between these traits is unknown. A negative relationship between phenotypic traits could limit individual performance and even the capacity of populations to adapt to climate change. Here, we examined the relationship between behavioural and physiological performance of a juvenile reef fish under elevated CO2 and temperature in a full factorial design. Behaviourally, the response to an alarm odour was negatively affected by elevated CO2, but not elevated temperature. Physiologically, aerobic scope was significantly diminished under elevated temperature, but not under elevated CO2. At the individual level, there was no relationship between behavioural and physiological traits in the control and sing le-stressor treatments. However, a statistically significant negative relationship was detected between the traits in the combined elevated CO(2 )and temperature treatment. Our results demonstrate that trade-offs in performance between behavioural and physiological traits may only be evident when multiple climate change stressors are considered, and suggest that this negative relationship could limit adaptive potential to climate change