Enhanced oxygen unloading in two marine percomorph teleosts.

Teleost fishes are diverse and successful, comprising almost half of all extant vertebrate species. It has been suggested that their success as a group is related, in part, to their unique O2 transport system, which includes pH-sensitive hemoglobin, a red blood cell ß-adrenergic Na+/H+ exchanger (RBC ß-NHE) that protects red blood cell pH, and plasma accessible carbonic anhydrase which is absent at the gills but present in some tissues, that short-circuits the ß-NHE to enhance O2 unloading during periods of stress. However, direct support for this has only been examined in a few species of salmonids. Here, we expand the knowledge of this system to two warm-water, highly active marine percomorph fish, cobia (Rachycentron canadum) and mahi-mahi (Coryphaena hippurus). We show evidence for RBC ß-NHE activity in both species, and characterize the Hb-O2 transport system in one of those species, cobia. We found significant RBC swelling following ß-adrenergic stimulation in both species, providing evidence for the presence of a rapid, active RBC ß-NHE in both cobia and mahi-mahi, with a time-course similar to that of salmonids. We generated oxygen equilibrium curves (OECs) for cobia blood and determined the P50, Hill, and Bohr coefficients, and used these data to model the potential for enhanced O2 unloading. We determined that there was potential for up to a 61% increase in O2 unloading associated with RBC ß-NHE short-circuiting, assuming a - 0.2 ∆pHa-v in the blood. Thus, despite phylogenetic and life history differences between cobia and the salmonids, we found few differences between their Hb-O2 transport systems, suggesting conservation of this physiological trait across diverse teleost taxa.

Contrasting effects of constant and fluctuating pCO2 conditions on the exercise physiology of coral reef fishes.

Ocean acidification (OA) is predicted to affect the physiology of some fishes. To date, most studies have investigated this issue using stable pCO2 levels based on open ocean projections. Yet, most shallow, nearshore systems experience temporal and spatial pCO2 fluctuations. For example, pCO2 on coral reefs is highest at night and lowest during the day, but as OA progresses, both the average pCO2 and magnitude of fluctuations are expected to increase. We exposed four coral reef fishes - Lutjanus fulviflamma, Caesio cuning, Abudefduf whitleyi, and Cheilodipterus quinquelineatus - to ambient, stable elevated, or fluctuating elevated pCO2 conditions for 9-11 days. Then, we measured swimming performance, oxygen uptake rates, and haematological parameters during the day and at night. When compared to ambient pCO2 conditions, L. fulviflamma, C. cuning, and A. whitleyi exposed to fluctuating elevated pCO2 increased swimming performance, maximum oxygen uptake rates, and aerobic scope, regardless of time of day; whereas, the only nocturnal species studied, C. quinquelineatus, decreased maximum oxygen uptake rates and aerobic scope. Our findings suggest that exposure to fluctuating or stable elevated pCO2 can physiologically benefit some coral reef fishes; however, other species, such as the cardinalfish examined here, may be more sensitive to future OA conditions.

The effects of constant and fluctuating elevated pCO2 levels on oxygen uptake rates of coral reef fishes.

Ocean acidification, resulting from increasing atmospheric carbon dioxide (CO2) emissions, can affect the physiological performance of some fishes. Most studies investigating ocean acidification have used stable pCO2 treatments based on open ocean predictions. However, nearshore systems can experience substantial spatial and temporal variations in pCO2. Notably, coral reefs are known to experience diel fluctuations in pCO2, which are expected to increase on average and in magnitude in the future. Though we know these variations exist, relatively few studies have included fluctuating treatments when examining the effects of ocean acidification conditions on coral reef species. To address this, we exposed two species of damselfishes, Amblyglyphidodon curacao and Acanthochromis polyacanthus, to ambient pCO2, a stable elevated pCO2 treatment, and two fluctuating pCO2 treatments (increasing and decreasing) over an 8 h period. Oxygen uptake rates were measured both while fish were swimming and resting at low-speed. These 8 h periods were followed by an exhaustive swimming test (Ucrit) and blood draw examining swimming metrics and haematological parameters contributing to oxygen transport. When A. polyacanthus were exposed to stable pCO2 conditions (ambient or elevated), they required more energy during the 8 h trial regardless of swimming type than fish exposed to either of the fluctuating pCO2 treatments (increasing or decreasing). These results were reflected in the oxygen uptake rates during the Ucrit tests, where fish exposed to fluctuating pCO2 treatments had a higher factorial aerobic scope than fish exposed to stable pCO2 treatments. By contrast, A. curacao showed no effect of pCO2 treatment on swimming or oxygen uptake metrics. Our results show that responses to stable versus fluctuating pCO2 differ between species - what is stressful for one species many not be stressful for another. Such asymmetries may have population- and community-level impacts under higher more variable pCO2 conditions in the future.

Aquatic acidification: a mechanism underpinning maintained oxygen transport and performance in fish experiencing elevated carbon dioxide conditions.

Aquatic acidification, caused by elevating levels of atmospheric carbon dioxide (CO2), is increasing in both freshwater and marine ecosystems worldwide. However, few studies have examined how acidification will affect oxygen (O2) transport and, therefore, performance in fishes. Although data are generally lacking, the majority of fishes investigated in this meta-analysis exhibited no effect of elevated CO2 at the level of O2 uptake, suggesting that they are able to maintain metabolic performance during a period of acidosis. Notably, the mechanisms that fish employ to maintain performance and O2 uptake have yet to be verified. Here, we summarize current data related to one recently proposed mechanism underpinning the maintenance of O2 uptake during exposure to aquatic acidification, and reveal knowledge gaps that could be targeted for future research. Most studies have examined O2 uptake rates while fishes were resting and did not calculate aerobic scope, even though aerobic scope can aid in predicting changes to whole-animal metabolic performance. Furthermore, research is lacking on different age classes, freshwater species and elasmobranchs, all of which might be impacted by future acidification conditions. Finally, this Review further seeks to emphasize the importance of developing collaborative efforts between molecular, physiological and ecological approaches in order to provide more comprehensive predictions as to how future fish populations will be affected by climate change.

Hot and bothered: effects of elevated Pco2 and temperature on juvenile freshwater mussels.

Multiple environmental stressors may interact in complex ways to exceed or diminish the impacts of individual stressors. In the present study, the interactive effects of two ecologically relevant stressors [increased temperature and partial pressure of carbon dioxide (Pco2)] were assessed for freshwater mussels, a group of organisms that are among the most sensitive and rapidly declining worldwide. The individual and combined effects of elevated temperature (22°C-34°C) and Pco2 (~230, 58,000 µatm) on juvenile Lampsilis siliquoidea were quantified over a 5- or 14-day period, during which physiological and whole animal responses were measured. Exposure to elevated temperature induced a series of physiological responses, including an increase in oxygen consumption rates following 5 days of exposure at 31°C and an increase in carbonic anhydrase ( ca) and heat shock protein 70 mRNA levels following 14 days of exposure at 28°C and 34°C, respectively. Treatment with elevated Pco2 activated acid-base regulatory responses including increases in CA and Na+-K+-ATPase activity and a novel mechanism for acid-base regulation during Pco2 exposure in freshwater mussels was proposed. Thermal and CO2 stressors also interacted such that responses to the thermal stressor were diminished in mussels exposed to elevated Pco2, resulting in the greatest level of mortality. Additionally, larger mussels were more likely to survive treatment with elevated Pco2 and/or temperature. Together, exposure to elevated Pco2 may compromise the ability of juvenile freshwater mussels to respond to additional stressors, such as increased temperatures, highlighting the importance of considering not only the individual but also the interactive effects of multiple environmental stressors.

Chronic exposure of a freshwater mussel to elevated pCO2 : Effects on the control of biomineralization and ion-regulatory responses.

Freshwater mussels may be exposed to elevations in mean partial pressure of carbon dioxide (pCO2 ) caused by both natural and anthropogenic factors. The goal of the present study was to assess the effects of a 28-d elevation in pCO2 at 15 000 and 50 000 µatm on processes associated with biomineralization, ion regulation, and cellular stress in adult Lampsilis siliquoidea (Barnes, 1823). In addition, the capacity for mussels to compensate for acid-base disturbances experienced after exposure to elevated pCO2 was assessed over a 14-d recovery period. Overall, exposure to 50 000 µatm pCO2 had more pronounced physiological consequences compared with 15 000 µatm pCO2 . Over the first 7 d of exposure to 50 000 µatm pCO2 , the mRNA abundance of chitin synthase (cs), calmodulin (cam), and calmodulin-like protein (calp) were significantly affected, suggesting that shell formation and integrity may be altered during pCO2 exposure. After the removal of the pCO2 treatment, mussels may compensate for the acid-base and ion disturbances experienced during pCO2 exposure, and transcript levels of some regulators of biomineralization (carbonic anhydrase [ca], cs, cam, calp) as well as ion regulation (na+ -k+ -ATPase [nka]) were modulated. Effects of elevated pCO2 on heat shock protein 70 (hsp70) were limited in the present study. Overall, adult L. siliquoidea appeared to regulate factors associated with the control of biomineralization and ion regulation during and/or after the removal of pCO2 exposure. Environ Toxicol Chem 2018;37:538-550. © 2017 SETAC.

Valve movement of three species of North American freshwater mussels exposed to elevated carbon dioxide.

Freshwater mussels are at-risk taxa and may be exposed to high levels of carbon dioxide (CO2) because of the potential use of CO2 to control the movement of invasive aquatic fish species. One potential behavioral response to a change in the partial pressure of CO2 (pCO2) may be altered valve movement. In this study, three species of mussels were fitted with modified sensors and exposed to two regimes of pCO2 to define thresholds of impaired valve movement. The first experiment demonstrated that Pyganodon grandis were much more tolerant to rising pCO2 relative to Lampsilis siliquoidea (acute closure at ∼200,000 µatm in comparison to ∼80,000 µatm). The second experiment consisted of monitoring mussels for 6 days and exposing them to elevated pCO2 (∼70,000 µatm) over a 2-day period. During exposure to high pCO2, Lampsilis cardium were open for nearly the entire high pCO2 period. Conversely, P. grandis were closed for most of the period following exposure to high pCO2. For L. siliquoidea, the number of closures decreased nearly 40-fold during high pCO2. The valve movement responses observed suggest species differences, and exposure to elevated pCO2 requires a reactive response.

Responses to elevated CO2 exposure in a freshwater mussel, Fusconaia flava.

Freshwater mussels are some of the most imperiled species in North America and are particularly susceptible to environmental change. One environmental disturbance that mussels may encounter that remains understudied is an increase in the partial pressure of CO2 (pCO2). The present study quantified the impacts of acute (6 h) and chronic (up to 32 days) exposures to elevated pCO2 on genes associated with shell formation (chitin synthase; cs) and the stress response (heat shock protein 70; hsp70) in Fusconaia flava. Oxygen consumption (MO2) was also assessed over the chronic CO2 exposure period. Although mussels exhibited an increase in cs following an acute exposure to elevated pCO2, long-term exposure resulted in a decrease in cs mRNA abundance, suggesting that mussels may invest less in shell formation during chronic exposure to elevated pCO2. In response to an acute elevation in pCO2, mussels increased hsp70 mRNA abundance in mantle and adductor muscle and a similar increase was observed in the gill and adductor muscle in response to a chronic elevation in pCO2. A chronic elevation in pCO2 also increased mussel MO2. This overall increase in hsp70 mRNA levels and MO2 in F. flava indicates that exposure to elevated pCO2 initiates activation of the general stress response and an increased energy demand. Together, the results of the present study suggest that freshwater mussels respond to elevated pCO2 by increasing processes necessary to 'deal with' the stressor and, over the long-term, may reduce their investment in non-essential processes such as shell growth.

The response of two species of unionid mussels to extended exposure to elevated carbon dioxide.

Changes in environmental conditions can act as stressors, with potential consequences for the health and fitness of organisms. Rising levels of carbon dioxide (CO2) is one potential environmental stressor that is occurring more frequently in the environment and can be a stressor for aquatic organisms. In this study, the physiological responses of two species of unionid mussel, Lampsilis siliquoidea and Amblema plicata, were assessed in response to exposure to two levels of elevated partial pressure of CO2 (pCO2) (~20,000 and ~55,000µatm) over a 28d period, followed by a subsequent 14d recovery period. Observations were consistent with responses associated with respiratory acidosis, as demonstrated by changes in hemolymph HCO3(-), Ca(2+), Cl(-), and Na(2+). Both species exposed to elevated pCO2 had elevated hemolymph HCO3(-) during the pCO2 treatment period compared to control mussels, but recovered once pCO2 was removed. Similarly, both species had elevated hemolymph Na(+) during exposure to elevated pCO2, and this returned to control levels for A. plicata but remained elevated for L. siliquoidea once the pCO2 stimuli was removed. Changes in hemolymph Ca(2+) and Cl(-) in response to elevated pCO2 were also observed, but these changes were species-specific. Additional physiological responses to elevated pCO2 (e.g., changes in hemolymph glucose and Mg(2+)) were consistent with a stress response in both species. This study demonstrates the importance of considering inter-specific differences in the response of organisms to stress, and also that responses to elevated pCO2 may be transient and can recover once the stress is removed.

Physiological responses of three species of unionid mussels to intermittent exposure to elevated carbon dioxide.

Freshwater systems are at risk owing to increasing carbon dioxide (CO2) levels, and one of the possible reasons for these elevations is the deployment of non-physical fish barriers to prevent invasive fish movements. Carbon dioxide barriers have the potential to create short, chronic and intermittent exposures of CO2 for surrounding freshwater biota. Although intermittent exposures to a stressor may be more ecologically relevant, the majority of laboratory tests use chronic or short-term time periods to determine how organisms will respond to an environmental stressor. Measurements of the physiological responses of three species of unionid mussel, giant floaters (Pyganodon grandis), threeridge (Amblema plicata) and plain pocketbook (Lampsilis cardium), exposed to control pCO2 (~1000 µatm) or intermittent conditions of pCO2 (ranging from ~1000 to ~55 000 µatm) 12 times per day over a 28 day period were gathered. There was no indication of recovery in the physiological responses of mussels between applications of CO2, suggesting that the recovery time between CO2 pulses (1.5 h) was not sufficient for recovery from the CO2 exposure period (0.5 h). Observations of acid-base and stress responses were consistent with what has been observed in chronic studies of freshwater mussels exposed to elevated pCO2 (i.e. elevations in HCO3-, Ca2+, Na+ and glucose, and decreases in Mg2+ and Cl-). However, species differences were observed across almost all variables measured, which emphasizes the need for multispecies studies.