Effects of thermal increase on aerobic capacity and ... - McGill Biology

Comparative Biochemistry and Physiology, Part A 199 (2016) 62–70

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Effects of thermal increase on aerobic capacity and swim performance in a tropical inland fish Laura H. McDonnell ⁎, Lauren J. Chapman Department of Biology, McGill University, Montreal H3A 1B1, Quebec, Canada

a r t i c l e

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Article history: Received 16 February 2016 Received in revised form 18 May 2016 Accepted 18 May 2016 Available online 20 May 2016 Keywords: Aerobic scope African cichlid Critical swim speed Standard metabolic rate Maximum metabolic rate Thermal window

a b s t r a c t Rising water temperature associated with climate change is increasingly recognized as a potential stressor for aquatic organisms, particularly for tropical ectotherms that are predicted to have narrow thermal windows relative to temperate ectotherms. We used intermittent flow resting and swimming respirometry to test for effects of temperature increase on aerobic capacity and swim performance in the widespread African cichlid Pseudocrenilabrus multicolor victoriae, acclimated for a week to a range of temperatures (2 °C increments) between 24 and 34 °C. Standard metabolic rate (SMR) increased between 24 and 32 °C, but fell sharply at 34 °C, suggesting either an acclimatory reorganization of metabolism or metabolic rate depression. Maximum metabolic rate (MMR) was elevated at 28 and 30 °C relative to 24 °C. Aerobic scope (AS) increased between 24 and 28 °C, then declined to a level comparable to 24 °C, but increased dramatically 34 °C, the latter driven by the drop in SMR in the warmest treatment. Critical swim speed (Ucrit) was highest at intermediate temperature treatments, and was positively related to AS between 24 and 32 °C; however, at 34 °C, the increase in AS did not correspond to an increase in Ucrit, suggesting a performance cost at the highest temperature. Crown Copyright © 2016 Published by Elsevier Inc. All rights reserved.

1. Introduction Over the past century global mean temperatures have risen significantly, as has the rate of global warming (IPCC, 2013). The persistence of species on a warming planet will depend on their capacity to shift their distributions to more favourable environments or adapt to their current environment through genetic change and/or phenotypic plasticity. An ectotherm's overall performance can be greatly influenced by environmental temperature (Tenv) and therefore it is likely that this group will be particularly sensitive to climate change (McNab, 2002). The ability of an ectotherm to carry out vital functions over a range of temperatures is defined as their thermal performance window (Brett, 1971; Huey and Stevenson, 1979) that results from temperaturedependent trade-offs at all levels of functioning (Pörtner, 2010). Beyond the limits of their thermal windows (upper and lower critical temperatures), ectotherms will start to exploit their passive range of tolerance but can only do so for limited time because important processes (e.g., feeding and reproductive behaviours), and thus their long-term fitness, are gradually reduced (Pörtner and Knust, 2007; Pörtner, 2010). Thus, with shifting thermal regimes due to climate change, ectotherms are at risk of being forced to perform at sub-optimal temperatures or at those beyond their thermal windows (Huey et al., 2009).

⁎ Corresponding author. E-mail address: [email protected] (L.H. McDonnell).

http://dx.doi.org/10.1016/j.cbpa.2016.05.018 1095-6433/Crown Copyright © 2016 Published by Elsevier Inc. All rights reserved.

Tropical ectotherms may be particularly sensitive to climate warming relative to temperate species because they have evolved in a relatively aseasonal thermal environment, which may select for narrow thermal windows (Janzen, 1967; Huey and Hertz, 1984; Hoegh-Guldberg et al., 2007; Tewksbury et al., 2008). In addition, they may have a more limited capacity to adjust their thermal sensitivities and upper thermal limit via acclimation than temperate ectotherms, a trend supported in salamanders (Feder, 1982), Sceloporus lizards (Tsuji, 1988), porcelain crabs (Stillman, 2003), Eucalyptus trees (Drake et al., 2014), and cleaner shrimps (Rosa et al., 2014). Studies on various tropical ectotherms also indicate a relatively small or negligible “thermal safety margin”, characterized as the difference between the thermal optimum for performance and the average environmental temperature (Deutsch et al., 2008), meaning they may need to rely on behaviour to avoid overheating under high temperatures (Sunday et al., 2014). Thus, small increases in environmental temperature may pose a significant risk. In aquatic systems, distributional shifts of ectothermic vertebrates associated with climate change suggest a strong pattern of poleward movement. Perry et al. (2005) determined that two thirds of 90 North Sea marine fish species, had undergone depth or latitudinal shifts in response to seawater temperature increases, specifically, deeper and/or northward. Additionally, in a recent study using distribution models to project effects of climate change, Jones and Cheung (2014) found that 93–97% of 802 species of harvested marine fish and invertebrate species were predicted to demonstrate poleward shifts to higher latitudes by

L.H. McDonnell, L.J. Chapman / Comparative Biochemistry and Physiology, Part A 199 (2016) 62–70

the year 2059. These results for marine systems are striking; however, freshwater species may have fewer options for northern distributional shifts, since they are often trapped within landlocked water bodies and therefore are more likely to need to adapt and/or acclimate in situ to warming, and may experience a faster rate of warming than surrounding ocean waters (Kintisch, 2015). These trends highlight the need for studies targeting effects of warming on freshwater fishes, particularly in tropical latitudes, where ectotherms may be particularly sensitive to warming waters. Here, we address this need by quantifying effects of ecologically-relevant thermal increases on aerobic performance of a widespread freshwater tropical fish. A key measure of metabolic performance in fishes is aerobic scope (AS), defined as the increase in fish's oxygen consumption from its standard (SMR) to maximal metabolic rate (MMR) (Fry, 1947, 1971). A large AS indicates that energy is available to fuel essential performancerelated activities (e.g., growth, reproduction), therefore changes in AS are predicted to influence overall fitness. AS is hypothesized to be tightly linked to the thermal window, described in a theoretical framework known as oxygen- and capacity-limited thermal tolerance (OCLTT) (Fry and Hart, 1948; Pörtner, 2001, 2010; Pörtner and Peck, 2010). OCLTT predicts that as temperature increases, oxygen delivery systems cannot keep pace with the rise in resting metabolism, and AS is expected to decline. The OCLTT concept has been successful in explaining changes in the distribution of eelpout Zoarces viviparus (Pörtner and Knust, 2007); in predicting success of spawning migrations in sockeye salmon Oncorhynchus nerka (Farrell et al., 2008); and in explaining variation in AS in coral reef fishes (Gardiner et al., 2010; Johansen and Jones, 2011; Rummer et al., 2014). However, some recent results are not in agreement with OCLTT predictions including recent studies on barramundi (Lates calcifer (Norin et al., 2014)), Atlantic halibut (Hippoglossus hippoglossus (Gräns et al., 2014)), python (Python regius (Fobian et al., 2014)), and a species of tropical freshwater shrimp (Macrobrachium rosenbergii, (Ern et al., 2014)), emphasizing the need for greater geographic and phylogenetic breadth in studies evaluating temperature effects on AS. According to OCLTT, SMR increases at temperatures above Topt in order to meet increasing aerobic energy requirements, which arise due to elevated cellular respiration, thereby driving the decline in aerobic scope (Pörtner, 2010). In support of this prediction, many studies using short-term acclimations have observed an increase in SMR in fish with increasing temperature (Ott et al., 1980; Claireaux and Lagardère, 1999; Lapointe et al., 2014). MMR represents the maximum metabolic rate (Fry, 1971; Beamish, 1978; Clark et al., 2011) and is often measured at the critical swimming speed of a fish during prolonged swimming (Schurmann and Steffensen, 1997; Korsmeyer and Dewar, 2001; Roche et al., 2013). OCLTT predicts that MMR will not increase significantly at temperatures beyond Topt; thus AS will decline if SMR is positively correlated with T env (Farrell, 2009). Studies testing this prediction across a wide temperature range are few, and patterns vary across studies and acclimation times. In their study of two cardinalfishes (Ostorhinchus cyanosoma and O. doederleini) and three damselfishes (Dascyllus anuarus, Chromis atripectoralis, and Acanthochromis polyacanthus) from the Great Barrier Reef, Australia, Nilsson et al. (2009) found that MMR did not increase or fell with 1-week acclimation to a range of temperatures between 29 and 33 °C. In contrast, Norin et al. (2014) reported an increase in MMR with temperature in barramundi (Lates calcifer) after short-term exposure, but found a 32% reduction in MMR after a 5-week acclimation. Interestingly, shorthorn sculpins (Myoxocephalus scorpius) also showed significant decrease in MMR after an 8-week acclimation period under high temperatures, which did not occur under shorter acclimation periods (1 or 4 weeks) (Sandblom et al., 2014). In this same study, however, an increased SMR measured after one-week acclimation to 16 °C was reduced after the 8-week acclimation, to levels similar to those measured at the pre-exposure temperature (10 °C).

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Swimming is often a major component of the energy budget of active fishes; and high intensity swimming can comprise a very significant proportion of a fish's aerobic scope (Fry, 1971; Claireaux and Lefrançois, 2007). One of the most widely used swim performance metrics in fishes, critical swim speed (Ucrit), is a useful measure of prolonged aerobic swimming capacity (Farrell et al., 1998; Kolok, 1999; Reidy et al., 2000; Plaut, 2001). Ucrit is determined by forcing a fish to swim against a laminar flow in a swim tunnel while increasing the water velocity incrementally at regular time intervals until the fish fatigues (Brett, 1964; Beamish, 1978; Plaut, 2001). The effects of temperature on Ucrit have been widely studied, with variable results from no temperature effect on Ucrit (Jones et al., 1974; Kieffer et al., 1998), increased Ucrit with rising temperature acclimation (Beamish, 1978; Keen and Farrell, 1994; Adams and Parsons, 1998); and the highest Ucrit at intermediate water temperatures (Taylor et al., 1996). Measuring these metabolic and performance parameters in parallel across a wide thermal range provides an excellent opportunity to (1) test a series of predictions of OCLTT framework in a tropical freshwater fish and (2) determine thermal sensitivities of a suite of important physiological factors involved in whole-fish functioning. In this study we measured the effects of ecologically-relevant thermal increases on aerobic metabolism (AS, SMR, MMR) and Ucrit, in a widespread African cichlid, Pseudocrenilabrus multicolor victoriae Seegers. We predicted (a) AS would decrease with increased acclimation temperature, primarily driven by a rise in SMR and (b) Ucrit would peak at intermediate acclimation temperatures and decline at high temperatures due to the predicted decline in AS. 2. Methods 2.1. Study species and sites of origin Pseudocrenilabrus multicolor victoriae is a small African cichlid fish, found throughout the Lake Victoria basin of East Africa across habitats that encompass a broad thermal range from 18.1 °C in the dense interior of papyrus swamps to 30.8 °C (Chapman et al., 2002; Friesen et al., 2012) in warm ecotonal waters of lakes. This study measured the effect of temperature on aerobic performance in laboratory-acclimated stock P. multicolor. We used only male P. multicolor, because the metabolic rate is significantly elevated in brooding females relative to nonbrooders and males (Reardon and Chapman, 2010). To achieve a sample size of male P. multicolor adequate for our repeated measures study, we used six males caught from the Bwera site within the papyrus (Cyperus papyrus) dominated Kiaraguru Swamp of the Mpanga River system of western Uganda (0°0′34.55″S, 29°43′52.98″E) and three males from Lake Kayanja, where P. multicolor can be found in the ecotonal waters adjacent to Miscanthidium-dominated swamp (0°16′60.00″S, 31°52′ 0.00″E). Water temperature and dissolved oxygen (DO) at the Bwera site averages 21.7 °C (range = 17.3 to 25.7 °C) and 0.28 mg L−1, respectively (Crispo and Chapman, 2010; Friesen et al., 2012); while at the Lake Kayanja ecotone, water temperature and DO are both higher, 25.3 °C (range = 21.6 to 31.1 °C) and 6.97 mg L− 1, respectively (McDonnell and Chapman, 2015). P. multicolor were live-transferred to McGill University in either July 2011 (Bwera) or July 2013 (Kayanja). Fish were held under normoxic conditions (N 6.5 mg L− 1) at 23 °C (range = 22.2 to 23.6 °C), a temperature approximating the average temperature of the two sites for at least six months prior to initiating the thermal acclimation protocol described below, and they were held close to these levels between transfer from Uganda and initiation of the six-month common acclimation period. In common garden experiments, P. multicolor from both sites reared under high or low DO have demonstrated high levels of phenotypic plasticity in morphophysiological and biochemical traits to their environments, and therefore a six-month holding period should have reduced population effects related to their habitat of origin (Chapman et al., 2000, 2002, 2008; Martínez et al., 2009; Crispo and Chapman, 2010; Crocker et al.,

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2013). However, since fish were transferred to McGill as large immature fish or adults at different times, adaptive or developmental plastic responses may still contribute to interpopulational variation in metabolic performance. Therefore, the term ‘population’ was included in all statistical analyses (see below). 2.2. Temperature acclimation The predicted increase in temperature for the Lake Victoria region is ~2 °C by the mid-21st century and ~4 °C by the end of the 21st century under a Representative Concentration Pathway (RCP8.5) projection model (Niang et al., 2014). With this in mind, we selected a range of water temperatures for this experiment to include temperatures up to 3 °C beyond the current known thermal range of this species. Individual P. multicolor were acclimated for one week to and tested at all treatment temperatures in the same order (26, 28, 30, 32, 34 and 24 °C) (Clark et al., 2011). After a trial, each fish was given 48 h rest under its tested temperature, after which the temperature was raised by 1 °C h−1 and the next week-long acclimation began after the new temperature level remained stable for 12 h. After completion of trials, fish were again held at 26 °C and re-tested to determine if repeated temperature acclimation had any effects on the measures of metabolic performance (Clark et al., 2013). Temperatures were maintained by individual tank heaters and were monitored to ensure that fluctuations were kept within 0.3 °C of the target temperature. 2.3. Measuring oxygen uptake All metabolic rates and critical swim speeds were measured using intermittent flow-through respirometry equipment and AutoResp software (Loligo Systems, Tjele, Denmark). Individual fish were placed in a swim tunnel (1.5 L in volume, 5.5 cm diameter and 20 cm length) filled with NovAqua treated water (5 mL L−1). Temperatures in the experimental tank were monitored continuously by a temperature regulation instrument (TMP-REG) and maintained via relays connected to an adjacent water bath. The oxygen in the chamber was measured using a fiber-optic oxygen probe (Witrox 1, Loligo Systems), and calibrated using oxygen-free and air-saturated water (0% and 100% settings, respectively) prior to every trial. Flow speeds were controlled by a motor attached to one end of the swim tunnel, calibrated prior to the start of trials using a digital particle tracking velocimetry system and software (DPTV, Loligo Systems). For all fish, food was withheld for 24 h prior to commencing metabolic trials to ensure a post-absorptive state (Niimi and Beamish, 1974; Reardon and Chapman, 2010; Johansen and Jones, 2011; Roche et al., 2013). A modified Yeager and Ultsch (1989) program was used to convert the decline in oxygen (% saturation) over time, from the AutoResp software output to oxygen consumption rates, VO2 (mg O2 h−1), compensating for temperature and barometric pressure (Reardon and Chapman, 2010). 2.4. Standard metabolic rate We considered P. multicolor's behaviour both in its natural environment and while in the chamber, as well as observed activity levels overnight to determine the appropriate experimental design for this species (Roche et al., 2013; Chabot et al., 2016). SMR measures were obtained by monitoring an individual fish's oxygen consumption rate in the chamber for a minimum of 15 h spanning an overnight period. The fish was first allowed to acclimate to the chamber environment on a flush setting for 30 min prior to beginning metabolism monitoring (looping-period). Overnight, DO concentration in the chamber was recorded every second for 15 min before the system was opened for 10 min to be fully flushed, allowing oxygen levels in the chamber to return to near 100% levels, with a 3-min lag period before recordings for the next loop began. The total time of the looping-period prior to the swim trial varied, but was never b15 h (mean time spent in chamber

21.8 h), of which the first 8 h were considered ‘acclimation’ and thus not used in SMR determinations. Any measures of MO2 that were outliers (±3 standard deviations from mean MO2) were eliminated from the data set used for calculating SMR. We averaged the five lowest MO2 measurements (~ lowest % 15) obtained after the first 8 h of the looping period, to calculate SMR for each fish (Fry and Hart, 1948; Crear and Forteath, 2000; McKenzie, 2001; Lefevre et al., 2012; Roche et al., 2013). Levels of background respiration were calculated by running trials with an empty respirometer before and after each trial. In cases where control rates differed between the post and pre-trials, we calculated the slope between them to determine the exact control rate at the time of SMR. 2.5. Critical swim speed The critical swim speed (Ucrit) trial was initiated immediately following the overnight acclimation period. Therefore, prior to beginning the swim performance experiment, each fish had already been acclimated to the chamber for at least 15 h, while exposed to minimal flow (high enough to maintain adequate water mixing but low enough to avoid forcing the fish to swim; ~ 1 cm s−1). Solid blocking effects of the fish in the swim tunnel were minimal, but nevertheless corrected by the respirometry software (AutoResp, Loligo Systems) following established methods from Bell and Terhune (1970). To measure Ucrit, fish were forced to swim for 20-min intervals, beginning at a flow rate of 5 cm s− 1 with an incremental increase in flow after each interval (5 cm s−1), until exhausted. Exhaustion was designated as the point when the fish became pinned parallel to the rear end of the swim tunnel and did not regain activity after a brief decrease in flow. At this point, flow was lowered to acclimation rates, and the fish was allowed to recover. Critical swim speed was calculated following the equation provided by Brett (1964): U crit ¼ ½Uf þ ðT=t ÞdU =cm; where Uf (cm s−1) is the highest swim velocity maintained for a full interval, T (s) is the time spent at the final velocity, t is the time interval (s), and dU is the increment in swim speed (cm s−1). Fish mass and length were determined post-swimming to minimize stress pretesting. Lengths were used to convert Ucrit to body lengths per second (BL s−1). 2.6. Maximal metabolic rate Maximum metabolic rate was determined using metabolic rates collected during the swim trial. As a fish's oxygen consumption is measured continuously until complete exhaustion, swimming respirometry has been shown to be a reliable method to obtain accurate estimates of MMR in fish (Farrell and Steffensen, 1987; Shultz et al., 2011; Roche et al., 2013). We chose to use this method, as critical swim trials with P. multicolor have been performed successfully in a previous study (Gotanda et al., 2012). Furthermore, during initial respirometry trials with this species it was noted that the chasing/handling stress that occurred as a result of placing the fish in the respirometer did not result in notably higher MO2, and the fish often would calm down relatively quickly once in the chamber (2nd MO2 reading is typically ~ 40–50% lower than the initial reading). We obtain much-increased MO2 rates during prolonged swimming in a respirometer with lab-acclimated P. multicolor, compared to those we have measured post-handling. For these reasons, we chose the U crit method for accurate MMR determination. For each speed interval, two measures of MO2 were obtained. We plotted MO 2 (mg O2 g− 1 h− 1) against swimming speed (BL s− 1) (both values were log-transformed) for each individual fish and a least-squares linear regression was performed (closely following protocol described in Mager et al. (2014)). Maximum metabolic rate was extrapolated at Ucrit by deriving it from the resulting


regression linear equations (Roche et al., 2013; Mager et al., 2014). In all cases, the regressions produced an R2 ≥ 0.74. 2.7. Statistical analysis Linear regressions of mass-specific SMR and MMR (MO2/mass) against mass (4.35–8.68 g) were not significant (data not shown) indicating no allometric influence of body mass on these rates. Therefore, metabolic rates were calculated as mass-specific and expressed as mg O2 g−1 h−1. Net aerobic scope measure (AS) was calculated as the difference between maximum and standard mass-specific metabolic rates (MMR - SMR). Factorial aerobic scope (FAS), the ratio MMR/ SMR, also provides a measure of aerobic performance that highlights effects of SMR on aerobic performance (Clark et al., 2005). Both FAS and AS have been used widely in thermal studies with fish and it has been recommended that studies report both measures to provide a more balanced metabolic data set (Clark et al., 2005, 2013; Donelson et al., 2012; Donelson et al., 2014; Norin et al., 2014). We therefore also calculated factorial aerobic scope MMR divided by SMR. To test for a significant effect of sequential acclimation of P. multicolor, a paired t-test was used to compare values for all metrics at 26 °C recorded during the sequential increase in temperature to the same metrics recorded when fish were re-acclimated to 26 °C after the full range of thermal acclimations was complete. To test for effect of temperature on our response variables, we used a linear mixed model with test temperature as the repeated measures variable, population as a fixed effect, and individual fish as the random factor. The main variable of interest was the repeated factor temperature; however, to control for any effects of population, we retained the population term and the population x temperature term in all models. The Levine's test was used to detect heteroscedasticity among population x temperature groups. SMR and MMR were log-transformed to normalize the data and stabilize the variance. We carried out a series of post hoc tests on the least squared means (derived from the full model) for the repeated factor temperature (combined across populations groups). Due to the large number of treatment groups and the observed patterns of response, we compared mean values for each temperature treatment to the lowest temperature level (24 °C). False detect rate was used to correct for Type I errors with FRD set to 0.05 (Benjamini and Hochberg, 1995; McDonald, 2014) Statistical analyses were performed using SPSS (v 23), and all data are presented as means ± S.E. For SMR and MMR, antilog values are presented in the figures. To determine the relationship between aerobic capacity (AS) and performance (Ucrit), we performed a non-parametric Spearman rank correlation between mean AS for each temperature treatment and mean Ucrit. Q10 values for SMR were calculated by applying the Van't Hoff equation to average mass-specific mean metabolic values (McNab, 2002) between the 24 °C and 34 °C treatment groups: Q 10 ¼ ðV2=V1Þ

10 =T2−T1

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Table 1 Summary of 2-tailed paired t-test results for standard metabolic rate (SMR), maximum metabolic rate (MMR), aerobic scope (AS) and critical swim speed (Ucrit) after repeated testing at 26 °C to detect effects of repeated acclimation to higher temperatures on metabolic and performance parameters in Pseudocrenilabrus multicolor. All rates are massspecific. Parameter

t

d.f.

P

SMR MMR AS Ucrit

0.370 0.573 0.659 1.707

6 6 6 6

0.724 0.588 0.534 0.139

significant (Table 2); the overall mean SMR (anti-log values) for fish of lake origin was 0.153 mg O2 g−1 h−1 and 0.111 mg O2 g−1 h−1 for fish of swamp origin. The overall Q10 value for SMR over our entire thermal range was 1.52, but much higher between 24 and 32 °C (Q10 = 2.41). Acclimation temperature had a significant effect on maximum metabolic rate (MMR); the population effect was non-significant (Table 2). MMR was elevated at 28 and 30 °C relative to 24 °C; at higher temperatures (32 and 34 °C), MMR was marginally higher than 24 °C (Benjamini-Hochberg adjusted p-values = 0.059), suggesting a plateau in MMR after the peak (Fig. 1b). There was a significant effect of acclimation temperature on AS, but no effect of population (Table 2). AS increased between 24 and 28 °C, then declined to a level comparable to 24 °C, but increased dramatically 34 °C, the latter driven the drop in SMR in the warmest treatment. The effect of acclimation temperature on factorial aerobic scope was significant, with no effect of population (FAS, Table 2). The general pattern of FAS response was similar to AS, but post-hoc tests were not significant, reflecting high variance around the mean FAS values. Thermal acclimation also affected critical swim speed (Ucrit), which was significantly higher at 28 and 30 °C than at 24 °C, but then declined dramatically, with no significant difference between 24 °C and either 32 or 34 °C (Table 2, Fig. 2). The population effect was also significant (Table 2), with a higher mean Ucrit in fish of lake origin (mean = 9.18 BL s−1) than in fish of swamp origin (mean = 6.22 BL s−1). To explore the overall relationship between aerobic capacity (AS) and swim performance (Ucrit), we tested for a correlation between the mean AS values across the six temperature treatments and the mean Ucrit values. In general, there was a positive relationship between AS and Ucrit, with the exception of the 34 °C treatment, where Ucrit declined with increased AS (Fig. 3). When the 34 °C treatment was excluded, Ucrit was strongly and positively related to AS (rs = 0.900, p = 0.037. When Table 2 Results of linear mixed model analyses (repeated measures) to detect effects of population, acclimation temperature, and their interaction on standard metabolic rate (SMR), maximum metabolic rate (MMR), aerobic scope (AS), factorial aerobic scope (FAS) and critical swim speed (Ucrit) of P. multicolor. All rates are mass-specific. SMR and MMR were log-transformed. Parameter

Effect

F

d.f.

P

where V2 and V1 represent the metabolic rates at temperatures T2 and T1, respectively.

SMR

3. Results

MMR

Population Temperature Pop ∗ Temperature Population Temperature Pop ∗ Temperature Population Temperature Pop ∗ Temperature Population Temperature Pop ∗ Temperature Population Temperature Pop ∗ Temperature

4.725 4.181 0.808 3.711 1.694 0.826 0.085 6.218 0.639 0.777 6.218 0.639 13.948 2.659 2.484

1, 42.34 5, 14.36 5, 14.36 1, 42.26 5, 16.45 5, 16.45 1, 41.06 5, 20.97 5, 20.97 1, 35.87 5, 14.07 5, 14.07 1, 38.93 5, 14.097 5, 14.097

0.035 0.015 0.808 0.061 0.025 0.334 0.773 0.001 0.672 0.384 0.024 0.760 0.001 0.021 0.675

For all metrics, we detected no significant difference between acclimation and re-acclimation values at 26 °C (Table 1). For subsequent analyses we used data collected from the first exposure to 26 °C. Individuals reached SMR after an average of 11.3 h in the metabolic chamber, most often between 04:00–05:00 h; MO2 tended to increase shortly after 08:00 h. The effect of temperature on SMR was significant (Table 2). SMR increased between 24 and 32 °C, but then SMR fell dramatically, with no significant difference between 24 and 34 °C (Fig. 1a). The population effect on SMR was also

ASnet

FAS

Ucrit

Bold values indicates that this parameter was significantly (p b 0.05) affected by this term.

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Fig. 3. Relationship between mean aerobic scope and mean critical swim speed. Values were extracted from 1-week exposures of P. multicolor to a range of water temperatures between 24 and 34 °C.

all six temperatures were included, there was no significant relationship detected (rs = 0.371, p = 0.468). 4. Discussion

Fig. 1. Metabolic rates and aerobic scope of P. multicolor exposed to 1-week acclimations to a range of water temperatures. (A) Mean mass-specific standard (SMR), (B) mean mass-specific maximum (MMR) metabolic rates, and (C) mean mass-specific aerobic scope (AS) (±1 S.E.). Symbols that are starred differ significantly from the mean value at 24 °C.

The effects of a 7-day acclimation to ecologically-relevant thermal increases on aerobic capacity (AS, SMR, MMR), and performance (Ucrit) in P. multicolor victoriae allowed us to test some key predictions of OCLTT framework in a tropical freshwater fish and determine thermal sensitivities of a suite of important physiological factors involved in whole-fish functioning. Several lines of evidence suggest that metabolic and swim performance in P. multicolor are both affected by elevated temperatures. SMR increased between 24 and 32 °C, but fell sharply at 34 °C, suggesting either an acclimatory reorganization of metabolism or metabolic rate depression. AS increased between 24 and 28 °C, then declined to a level comparable to 24 °C, but increased dramatically 34 °C, driven largely by the drop in SMR. Critical swim speed (Ucrit) was highest at intermediate temperatures and was positively related to AS between 24 and 32 °C; however, at 34 °C, the increase in AS did not correspond to an increase in Ucrit, suggesting a performance cost at the highest temperature. Within the thermal range of exposure in this study, our results suggest a Topt of approximately 28 °C, as evidenced in a low SMR, a high MMR, and a high Ucrit at this temperature.

Fig. 2. Critical swim speeds (Ucrit) of P. multicolor exposed to 1-week acclimations to a range of water temperatures. Mean Ucrit values (±1 S.E.). Symbols that are starred differ significantly from the mean value at 24 °C.


Indeed, in another study on an African cichlid species (Tropheus dubiosi), the author predicts a Topt between 26 and 32 °C (Kim, 2014). Here we discuss our results in the context of the OCLTT framework and our current understanding of elevated water temperatures on fishes. 4.1. Standard metabolic rate SMR increased with temperature in P. multicolor across an 8-degree temperature range, as predicted by OCLTT, but dropped at the highest temperature exposure. An increase in SMR with temperature is supported by a range of fish taxa including: Carp (Cyprinus carpio) and rainbow trout (Salmo gairdneri) (Ott et al., 1980), Atlantic cod (Gadus morhua) (Schurmann and Steffensen, 1997), European sea bass (Dicentrarchus labrax) (Claireaux and Lagardère, 1999), and Striped bass (Morone saxatilis) (Lapointe et al., 2014). Further, these results support findings in an earlier study of P. multicolor tested shortly after capture from Lake Kayanja, where an increase in resting metabolic rate was detected after 3-day exposure to 32 °C relative to 23 °C; higher temperatures were not tested (McDonnell and Chapman, 2015). However, in some studies, the predicted increase in SMR based on Q10 effects is diminished or absent after sufficiently long acclimation to high temperatures. For example, in a study comparing the SMR of shorthorn sculpins (Myoxocephalus scorpius) exposed to a range of temperatures for 1, 4 or 8 weeks Sandblom et al. (2014) found evidence for metabolic compensation of SMR under high temperatures only after 8 weeks, while SMR remained elevated after both 1 and 4-week periods. Kim (2014) found no increase in SMR after a 6-week acclimation to 26, 29, and 32 °C in an African cichlid (T. duboisi), although an ~50% reduction in growth rate was measured in the highest temperature treatment group. A study on barramundi (Lates calcifer) also found that a 5-week acclimation time reduced the effect of temperature on SMR in comparison to acutely exposed fish (Norin et al., 2014). Interestingly, in P. multicolor, SMR dropped at 34 °C to a level that did not significantly differ from that measured at 24 °C. This decrease in SMR at 34 °C could reflect a reorganization of metabolism to reset the SMR at a lower level, a.k.a. acclimation/thermal compensation, though there was no evidence for this at 30 °C or 32 °C, where temperatures were elevated relative to the starting temperature of 24 °C. Alternatively, the drop in SMR at the highest temperature may reflect metabolic rate depression, a temporary suppression of energy turnover to below SMR in response to environmental stress (Guppy and Withers, 1999). SMR depression could potentially be an immediate stress response inducing hypometabolism (lowered metabolic rate) that could facilitate persistence under thermal stress on the short-term (Reipschläger and Pörtner, 1996). Hypometabolism has been shown to occur as an energy-saving mechanism under stress, particularly under anoxic conditions, and typically coincides with a lack of voluntary movement, as well as decreased heartbeat and respiration rates (Storey and Storey, 2004). Decreases in resting metabolic rates under stressful conditions have also been detected in spiny damselfish (Acanthochromis polyacanthus) (Rummer et al., 2013). After acclimation to high-levels of CO2 for over 2 weeks, the damselfish exhibited lower resting oxygen consumption rates than control fish. Furthermore, in their study of two sibling species coral reef gobies (Gobiodon histrio and G. erythrospilus), Sørensen et al. (2014) found that the more equatorial species had higher anaerobic scope (ability to persist under the critical oxygen level, Pcrit) at elevated water temperature. These findings suggest that anaerobic scope under thermal stress may be important to consider in hypoxia-tolerant, equatorial species such as P. multicolor, as they may possess additional coping mechanisms (i.e. lowering their SMR) under these stressful conditions, a strategy that may not be common in temperate or hypoxiaintolerant species. Although we did not measure anaerobic scope in this study, our findings suggest that this would be an important area for future investigation. This could be tested by comparing SMR at oxygen levels below their Pcrit between high- and low-DO populations or fish reared under high- and low-DO conditions.

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4.2. Maximal metabolic rate With respect to MMR, OCLTT predicts a rise in MMR with temperature after which it plateaus as PO2 in the mitochondria decreases at higher temperatures, restricting the ability for MMR to be pushed higher (Topt for MMR) (Pörtner, 2002). In P. multicolor, MMR was elevated at 28 and 30 °C relative to 24 °C; at higher temperatures (32 and 34 °C), MMR was marginally higher than 24 °C, providing some support for the OCLTT. Norin et al. (2014) also reported an increase in MMR with temperature in barramundi (Lates calcifer) after short-term exposure, but measured a reduced MMR after a 5-week acclimation. The same trend in MMR over varying acclimation periods was observed in shorthorn sculpins (Myoxocephalus scorpius) (Sandblom et al., 2014). Therefore, it is possible that the significant changes that we measured after a 1-week acclimation may be diminished or less pronounced after prolonged exposure to high temperatures. However, it is interesting to note that in both studies, a decreased SMR was measured after the longer acclimation periods, possibly suggesting that this compensation may come at the cost of reduced MMR at high temperatures (Norin et al., 2014; Sandblom et al., 2014). 4.3. Aerobic scope The OCLTT framework proposes that aerobic scope declines at high temperatures when MMR reaches its physiological limit and SMR continues to increase (Fry, 1947; Fry and Hart, 1948; Pörtner, 2010). In P. multicolor AS increased between 24 and 28 °C, then declined to a mean AS comparable to 24 °C, However, at 34 °C, AS declined dramatically, driven largely by the drop in SMR. Thus, our results support the pattern predicted by the OCLTT framework but only up until 32 °C. Our results for FAS were similar, though post-hoc tests were not significant, reflecting the high variance in this trait within treatments. Clark et al. (2013) argue that FAS can vary quite dramatically with only minor changes in the denominator (SMR), and thus suggest that AS is a more informative and robust measure. Effects of elevated temperature on aerobic scope in freshwater fish are variable among studies. In the killifish (Fundulus heteroclitus) a 4week acclimation to temperatures ranging between 5 and 33 °C showed a Topt range for AS between 25 and 30 °C; however, AS was relatively constant across the entire thermal range with acute exposure (Healy and Schulte, 2012). In their study of a coral reef damselfish (A. polyacanthus) Donelson et al. (2012) found that FAS was reduced at acute exposure to temperatures 3 °C beyond present-day temperatures, but both AS and FAS were maintained in groups of fish that were reared under and then tested at this temperature. The mean AS values the authors measured in acclimated coral reef fish ranged from ~ 0.21–0.4 mg O2 g−1 h− 1 over a 27–31.5 °C range. However, in fish that were only acutely exposed to temperature treatments, minimum mean AS was lower (~0.16 mg O2 g−1 h−1). Our mean AS values over our higher temperatures (28–34 °C) range from 0.13– 0.21 mg O2 g−1 h−1, therefore it's possible that these values may also increase with longer acclimation periods. In their study of the barramundi (Lates calcifer), Norin et al. (2014) found that AS increased with temperatures, in acutely exposed fish, even close to the upper critical temperature (38 °C), while FAS decreased, suggesting that aerobic scope in this species is driven by changes in MMR. However, they also found that after sufficient acclimation, AS at 38 °C no longer significantly differed from that at the control temperature. Interestingly, we found no studies on tropical fishes that detected a pattern similar to that observed in P. multicolor (a decrease in AS after Topt, followed by a rapid increase at the highest acclimation temperature). In P. multicolor, the drop in SMR at the highest temperature, if reflecting stress-induced metabolic depression, may result in a high AS, but not necessarily an increase that is realized in high performance.

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4.4. Critical swim speed OCLTT predicts that decreased AS will limit aerobic performance, such as sustained exercise, growth, and reproduction (Pörtner, 2002). While we did not measure growth or reproduction in this study, we did measure Ucrit, a measure of sustained exercise. Ucrit was highest at intermediate temperature treatments, and was positively related to AS between 24 and 32 °C; however, at 34 °C, the increase in AS did not correspond to an increase in Ucrit, suggesting a performance cost at the highest temperature. This may suggest that oxygen consumption allocation was shifted from Ucrit to another energetically expensive activity such as high gill ventilation at 34 °C. Indeed, even if metabolic capacity is maintained over a thermal gradient, elevated temperature can have negative consequences for fitness related traits, performance and population dynamics. For example, in his study of the Eastern mosquitofish (Gambusia holbrooki) Meffe (1992) reported reduced growth, condition, and smaller size and age at maturity in fish reared at 32 °C compared to those reared at 25 °C. Barramundi have shown reduced growth and protein and feed efficiency ratios at temperatures only 3 °C above the higher range of their growth optimum (Katersky and Carter, 2005). In the aforementioned study by Norin et al. (2014), thermal compensation in barramundi was detected in SMR and AS, but not in MMR after a 5-week acclimation to near-lethal temperatures. Integrating the results of these two studies, it becomes apparent that perhaps compensation of metabolic parameters under high temperatures in barramundi has come at the cost of a reduction of growth, maximal metabolic output and other important processes. 4.5. Population effects To obtain an adequate sample size of adult male P. multicolor for this study, we needed to use lab-acclimated fish originating from two wild populations. Our main variable of interest was the repeated factor temperature; however, to control for any effects of population, we retained the population term and the population x temperature term in all models. Population effects were significant for SMR and Ucrit, both of which were higher in the fish of lake origin, though it should be noted that our sample size is very low for the lake system. In Lake Kayanja, P. multicolor persists in an open-water ecotone characterized by higher DO, higher water temperature, and more open waters than in the densely structured, hypoxic Bwera swamp. Hypoxia in the swamp system may select for a lower SMR to reduce the oxygen requirements via stress-induced hypometabolism or compensation/acclimation (Storey and Storey, 2004). In earlier studies, we have found that P. multicolor from hypoxic swamps exhibits traits related to hypoxia tolerance, including a larger gill size than conspecifics from high-DO river and lake systems (Chapman et al., 2000, 2008; Wiens et al., 2014). Although much of the variation in gill size is due to developmental plasticity (Chapman et al., 2008; Crispo and Chapman, 2010), it is possible that our stock fish from Bwera maintained a higher level of hypoxia tolerance than Kayanja fish. The low Ucrit in P. multicolor of swamp origin may reflect the need to minimize oxygen requirements in hypoxic swamp waters, and/or attain a higher level of sustained swimming in more open waters of Lake Kayanja. 4.6. Conclusions Overall, the results of this study highlight the value and the need for integrative studies that link aerobic capacity to performance metrics, and the need for studies of varying thermal exposure periods. Predictions concerning potential effects of climate warming on critical physiological parameters can be informed by changes observed in shortterm acclimations, such as the present study (Stillman, 2003; Magozzi and Calosi, 2015). However, the longer-term response to increasing water temperature via developmental plasticity and/or genetic change

may certainly alter thermal sensitivities is critical to predicting persistence under changing climatic conditions. Such studies are limited at present (but see Donelson et al., 2011, 2012, Munday, 2014), but offer a fruitful area for future investigation. Some of our findings supported predictions consistent with the OCLTT concept: MMR does not increase after Topt, AS was driven primarily by changes in SMR; and Topt (estimated as the temperature where SMR was low and MMR and Ucrit were both high) corresponded with the first peak in AS (28 °C). However, we also observed patterns that were inconsistent with predictions derived from the OLCTT framework. Of particular note was the dramatic drop in SMR at the highest temperature, suggesting either an acclimatory reorganization of metabolism or metabolic rate depression; and a corresponding increase in the AS, that was not reflected in an increase in Ucrit, suggesting a performance cost at the highest temperature. Acknowledgements We thank Dr. Twinomugisha for his logistical and field assistance in Uganda, in addition to our field assistants Mutebi, Sseguya and Kiberu, who provided fish collection. K. Ackerly assisted with care and maintenance of experimental fish. We also wish to thank J. Herskin of Loligo® Systems for his assistance in setting up the respirometry system and technical support. Funding for this project was provided by the Natural Sciences and Engineering Research Council of Canada (grant no. 420078-2012 to L.H.M and Discovery Grant 312201-2010 to L.J.C) and McGill University (grant no. 00286 to L.H.M), as well as by funds to Lauren Chapman (NSERC Discovery Grant, Canada Research Chairs program). References Adams, S.R., Parsons, G.R., 1998. Laboratory-based measurements of swimming performance and related metabolic rates of field-sampled smallmouth buffalo (Ictiobus bubalus): a study of seasonal changes. Physiol. Biochem. Zool. 71, 350–358. Beamish, F.W.H., 1978. Swimming capacity. In: Hoar, W.S., Randall, D.J. (Eds.), Fish Physiolgy. Academic Press, New York, NY, USA. Bell, W.H., Terhune, L.D.B., 1970. Water tunnel design for fisheries research. In: Bell, W.H., Terhune, L.D.B. (Eds.), Tech. Rep. Fish. Res. Board Can., Biological Station, p. 69. Benjamini, Y., Hochberg, Y., 1995. Controlling the false discovery rate: a practical and powerful approach to multiple testing. J. R. Stat. Soc. 289-300. Brett, J.R., 1964. The respiratory metabolism and swimming performance of young sockeye salmon. J. Fish. Res. Board Can. 21, 1183–1226. Brett, J.R., 1971. Energetic responses of salmon to temperature. A study of some thermal relations in the physiology and freshwater ecology of sockeye salmon (Oncorhynchus nerka). Am. Zool. 11, 99–113. Chabot, D., Steffensen, J., Farrell, A., 2016. The determination of standard metabolic rate in fishes. J. Fish Biol. 88, 81–121. Chapman, L.J., Galis, F., Shinn, J., 2000. Phenotypic plasticity and the possible role of genetic assimilation: hypoxia-induced trade-offs in the morphological traits of an African cichlid. Ecol. Lett. 3, 387–393. Chapman, L.J., Chapman, C.A., Nordlie, F.G., Rosenberger, A.E., 2002. Physiological refugia: swamps, hypoxia tolerance and maintenance of fish diversity in the Lake Victoria region. Comp. Biochem. Physiol. A 133, 421–437. Chapman, L.J., Albert, J., Galis, F., 2008. Developmental plasticity, genetic differentiation, and hypoxia-induced trade-offs in an African cichlid fish. Open Evol. J. 2, 75–88. Claireaux, G., Lagardère, J.-P., 1999. Influence of temperature, oxygen and salinity on the metabolism of the European sea bass. J. Sea Res. 42, 157–168. Claireaux, G., Lefrançois, C., 2007. Linking environmental variability and fish performance: integration through the concept of scope for activity. Philos. Trans. R. Soc. Lond. Ser. B Biol. Sci. 362, 2031–2041. Clark, T.D., Ryan, T., Ingram, B.A., Woakes, A.J., Butler, P., Frappell, P.B., 2005. Factorial aerobic scope is independent of temperature and primarily modulated by heart rate in exercising Murray cod (Maccullochella peelii peelii). Physiol. Biochem. Zool. 78, 347–355. Clark, T.D., Jeffries, K.M., Hinch, S.G., Farrell, A.P., 2011. Exceptional aerobic scope and cardiovascular performance of pink salmon (Oncorhynchus gorbuscha) may underlie resilience in a warming climate. J. Exp. Biol. 214, 3074–3081. Clark, T.D., Sandblom, E., Jutfelt, F., 2013. Aerobic scope measurements of fishes in an era of climate change: respirometry, relevance and recommendations. J. Exp. Biol. 216, 2771–2782. Crear, B., Forteath, G., 2000. The effect of extrinsic and intrinsic factors on oxygen consumption by the southern rock lobster, Jasus edwardsii. J. Exp. Mar. Biol. Ecol. 252, 129–147. Crispo, E., Chapman, L.J., 2010. Geographic variation in phenotypic plasticity in response to dissolved oxygen in an African cichlid fish. J. Exp. Biol. 23, 2091–2103.

L.H. McDonnell, L.J. Chapman / Comparative Biochemistry and Physiology, Part A 199 (2016) 62–70 Crocker, C.D., Chapman, L.J., Martínez, M.L., 2013. Natural variation in enzyme activity of the African cichlid Pseudocrenilabrus multicolor victoriae. Comp. Biochem. Physiol. B 164, 53–60. Deutsch, C.A., Tewksbury, J.J., Huey, R.B., Sheldon, K.S., Ghalambor, C.K., Haak, D.C., Martin, P.R., 2008. Impacts of climate warming on terrestrial ectotherms across latitude. Proc. Natl. Acad. Sci. U. S. A. 105, 6668–6672. Donelson, J.M., Munday, P.L., McCormick, M.I., Nilsson, G.E., 2011. Acclimation to predicted ocean warming through developmental plasticity in a tropical reef fish. Glob. Chang. Biol. 17 (4), 1712–1719. Donelson, J.M., Munday, P.L., McCormick, M.I., Pitcher, C.R., 2012. Rapid transgenerational acclimation of a tropical reef fish to climate change. Nat. Clim. Chang. 2, 30–32. Donelson, J.M., McCormick, M.I., Booth, D.J., Munday, P.L., 2014. Reproductive acclimation to increased water temperature in a tropical reef fish. PLoS ONE 9, e97223. Drake, J.E., Aspinwall, M.J., Pfautsch, S., Rymer, P.D., Reich, P.B., Smith, R.A., Crous, K.Y., Tissue, D.T., Ghannoum, O., Tjoelker, M.G., 2014. The capacity to cope with climate warming declines from temperate to tropical latitudes in two widely distributed Eucalyptus species. Glob. Chang. Biol. http://dx.doi.org/10.1111/gcb.12729. Ern, R., Phuong, N.T., Wang, T., Bayley, M., 2014. Oxygen delivery does not limit thermal tolerance in a tropical eurythermal crustacean. J. Exp. Biol. 217, 809–814. Farrell, A.P., 2009. Environment, antecedents and climate change: lessons from the study of temperature physiology and river migration of salmonids. J. Exp. Biol. 212, 3771–3780. Farrell, A.P., Steffensen, J.F., 1987. An analysis of the energetic cost of the branchial and cardiac pumps during sustained swimming in trout. Fish Physiol. Biochem. 4, 73–79. Farrell, A.P., Gamperl, A.K., Birtwell, I.K., 1998. Prolonged swimming, recovery and repeat swimming performance of mature sockeye salmon Oncorhynchus nerka exposed to moderate hypoxia and pentachlorophenol. J. Exp. Biol. 201, 2183–2193. Farrell, A.P., Hinch, S.G., Cooke, S.J., Patterson, D.A., Crossin, G.T., Lapointe, M., Mathes, M.T., 2008. Pacific salmon in hot water: applying aerobic scope models and biotelemetry to predict the success of spawning migrations. Physiol. Biochem. Zool. 81, 697–709. Feder, M.E., 1982. Thermal ecology of neotropical lungless salamanders (Amphibia: Plethodontidae): environmental temperatures and behavioral responses. Ecology 63, 1665–1674. Fobian, D., Overgaard, J., Wang, T., 2014. Oxygen transport is not compromised at high temperature in pythons. J. Exp. Biol. 217, 3958–3961. Friesen, C.N., Aubin-Horth, N., Chapman, L.J., 2012. The effect of hypoxia on sex hormones in an African cichlid Pseudocrenilabrus multicolor victoriae. Comp. Biochem. Physiol. A 162, 22–30. Fry, F.E.J., 1947. Effects of the Environment on Animal Activity. Publication of the Ontario Fisheries Research Laboratory 68. University of Toronto Press, Toronto (62 pp.). Fry, F.E.J., 1971. The effect of environmental factors on the physiology of fish. In: Hoar, W.S., Randall, D.J. (Eds.), Fish Physiol. Academic Press, New York, pp. 1–98. Fry, F.E.J., Hart, J.S., 1948. The relation of temperature to oxygen consumption in the goldfish. Biol. Bull. 94, 66–77. Gardiner, N.M., Munday, P.L., Nilsson, G.E., 2010. Counter-gradient variation in respiratory performance of coral reef fishes at elevated temperatures. PLoS One 5, e13299. Gotanda, K.M., Reardon, E.E., Murphy, S.M.C., Chapman, L.J., 2012. Critical swim speed and fast-start response in the African cichlid Pseudocrenilabrus multicolor victoriae: convergent performance in divergent oxygen regimes. Can. J. Zool. 90, 545–554. Gräns, A., Jutfelt, F., Sandblom, E., Jönsson, E., Wiklander, K., Seth, H., Olsson, C., Dupont, S., Ortega-Martinez, O., Einarsdottir, I., 2014. Aerobic scope fails to explain the detrimental effects on growth resulting from warming and elevated CO2 in Atlantic halibut. J. Exp. Biol. 217, 711–717. Guppy, M., Withers, P., 1999. Metabolic depression in animals: physiological perspectives and biochemical generalizations. Biol. Rev. 74, 1–40. Healy, T.M., Schulte, P.M., 2012. Thermal acclimation is not necessary to maintain a wide thermal breadth of aerobic scope in the common killifish (Fundulus heteroclitus). Physiol. Biochem. Zool. 85, 107–119. Hoegh-Guldberg, O., Mumby, P.J., Hooten, A.J., Steneck, R.S., Greenfield, P., Gomez, E., Harvell, C.D., Sale, P.F., Edwards, A.J., Caldeira, K., 2007. Coral reefs under rapid climate change and ocean acidification. Science 318, 1737–1742. Huey, R.B., Hertz, P.E., 1984. Is a jack-of-all-temperatures a master of none? Evolution 441–444. Huey, R.B., Stevenson, R.D., 1979. Integrating thermal physiology and ecology of ectotherms: a discussion of approaches. Am. Zool. 19, 357–366. Huey, R.B., Deutsch, C.A., Tewksbury, J.J., Vitt, L.J., Hertz, P.E., Pérez, H.J.Á., Garland, T., 2009. Why tropical forest lizards are vulnerable to climate warming. Proc. R. Soc. Lond. B Biol. Sci. 276, 1939–1948. IPCC, 2013. Climate change 2013: the physical science basis. Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Janzen, D.H., 1967. Why mountain passes are higher in the tropics. Am. Nat. 101, 233–249. Johansen, J.L., Jones, G.P., 2011. Increasing ocean temperature reduces the metabolic performance and swimming ability of coral reef damselfishes. Glob. Chang. Biol. 17, 2971–2979. Jones, M.C., Cheung, W.W.L., 2014. Multi-model ensemble projections of climate change effects on global marine biodiversity. ICES J. Mar. Sci. J. Conseil http://dx.doi.org/10. 1093/icesjms/fsu172. Jones, D.R., Kiceniuk, J.W., Bamford, O.S., 1974. Evaluation of the swimming performance of several fish species from the Mackenzie River. J. Fish. Board Can. 31, 1641–1647. Katersky, R.S., Carter, C.G., 2005. Growth efficiency of juvenile barramundi, Lates calcarifer, at high temperatures. Aquaculture 250, 775–780.

69

Keen, J.E., Farrell, A.P., 1994. Maximum prolonged swimming speed and maximum cardiac performance of rainbow trout, Oncorhynchus mykiss, acclimated to two different water temperatures. Comp. Biochem. Phys. A 108, 287–295. Kieffer, J.D., Alsop, D., Wood, C.M., 1998. A respirometric analysis of fuel use during aerobic swimming at different temperatures in rainbow trout (Oncorhynchus mykiss). J. Exp. Biol. 201, 3123–3133. Kim, L.Y.-J., 2014. Effects of Increased Temperature and Decreased Food Quality on Metabolism and Growth of an Algivorous Cichlid, Tropheus duboisi, and Effect of Food Habit on the Field Metaoblilc of Afircan Cichlids, Department of Biological Sciences. Wright State University, Dayton, Ohio. Kintisch, E., 2015. Earth's lakes are warming faster than its air. Science 350, 1449-1449. Kolok, A.S., 1999. Interindividual variation in the prolonged locomotor performance of ectothermic vertebrates: a comparison of fish and herpetofaunal methodologies and a brief review of the recent fish literature. Can. J. Fish. Aquat. Sci. 56, 700–710. Korsmeyer, K.E., Dewar, H., 2001. Tuna metabolism and energetics. Fish Physiol. 19, 35–78. Lapointe, D., Vogelbein, W.K., Fabrizio, M.C., Gauthier, D.T., Brill, R.W., 2014. Temperature, hypoxia, and mycobacteriosis: effects on adult striped bass Morone saxatilis metabolic performance. Dis. Aquat. Org. 108, 113–127. Lefevre, S., Phuong, N.T., Wang, T., Bayley, M., 2012. Effects of hypoxia on the partitioning of oxygen uptake and the rise in metabolism during digestion in the air-breathing fish Channa striata. Aquaculture 364, 137–142. Mager, E.M., Esbaugh, A., Stieglitz, J., Hoenig, R., Bodinier, C., Incardona, J., Scholz, N.L., Benetti, D., Grosell, M., 2014. Acute embryonic or juvenile exposure to deepwater horizon crude oil impairs the swimming performance of mahi-mahi (Coryphaena hippurus). Environ. Sci. Technol. 48, 7053–7061. Magozzi, S., Calosi, P., 2015. Integrating metabolic performance, thermal tolerance, and plasticity enables for more accurate predictions on species vulnerability to acute and chronic effects of global warming. Glob. Change Biol. 21, 181–194. Martínez, M.L., Chapman, L.J., Rees, B.B., 2009. Population variation in hypoxic responses of the cichlid Pseudocrenilabrus multicolor victoriae. Can. J. Zool. 87, 188–194. McDonald, J.H., 2014. Handbook of Biological Statistics. third ed Sparky House Publishing, Baltimore, Maryland. McDonnell, L.H., Chapman, L.J., 2015. At the edge of the thermal window: effects of elevated temperature on the resting metabolism, hypoxia tolerance and upper critical thermal limit of a widespread African cichlid. Conserv. Physiol. 3. http://dx.doi.org/ 10.1093/conphys/cov050. McKenzie, D.J., 2001. Effects of dietary fatty acids on the respiratory and cardiovascular physiology of fish. Comp. Biochem. Physiol. A 128, 605–619. McNab, B.K., 2002. The Physiological Ecology of Vertebrates: A View from Energetics. Cornell University Press. Meffe, G.K., 1992. Plasticity of life-history characters in eastern mosquitofish (Gambusia holbrooki: Poeciliidae) in response to thermal stress. Copeia 94–102. Munday, P.L., 2014. Transgenerational acclimation of fishes to climate change and ocean acidification. F1000 Prime Rep. 6 (99). Niang, I., Ruppel, O.C., Abdrabo, M.A., Essel, A., Lennard, C., Padgham, J., Urquhart, P., 2014. Africa. In: Barros, V.R., Field, C.B., Dokken, D.J., Mastrandrea, M.D., Mach, K.J., Bilir, T.E., Chatterjee, M., Ebi, K.L., Estrada, Y.O., Genova, R.C., Girma, B., Kissel, E.S., Levy, A.N., MacCracken, S., Mastrandrea, P.R., White, L.L. (Eds.), Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge,UK and New York, USA, pp. 1199–1265. Niimi, A.J., Beamish, F.W.H., 1974. Bioenergetics and growth of largemouth bass (Micropterus salmoides) in relation to body weight and temperature. Can. J. Zool. 52 (4), 447–456. Nilsson, G.E., Crawley, N., Lunde, I.G., Munday, P.L., 2009. Elevated temperature reduces the respiratory scope of coral reef fishes. Glob. Chang. Biol. 15, 1405–1412. Norin, T., Malte, H., Clark, T.D., 2014. Aerobic scope does not predict the performance of a tropical eurythermal fish at elevated temperatures. J. Exp. Biol. 217, 244–251. Ott, M.E., Heisler, N., Ultsch, G.R., 1980. A re-evaluation of the relationship between temperature and the critical oxygen tension in freshwater fishes. Comp. Biochem. Physiol. A 67, 337–340. Perry, A.L., Low, P.J., Ellis, J.R., Reynolds, J.D., 2005. Climate change and distribution shifts in marine fishes. Science 308, 1912–1915. Plaut, I., 2001. Critical swimming speed: its ecological relevance. Comp. Biochem. Physiol. A 131, 41–50. Pörtner, H.-O., 2001. Climate change and temperature-dependent biogeography: oxygen limitation of thermal tolerance in animals. Naturwissenschaften 88, 137–146. Pörtner, H.-O., 2002. Climate variations and the physiological basis of temperature dependent biogeography: systemic to molecular hierarchy of thermal tolerance in animals. Comp. Biochem. Physiol. A 132, 739–761. Pörtner, H.-O., 2010. Oxygen- and capacity-limitation of thermal tolerance: a matrix for integrating climate-related stressor effects in marine ecosystems. J. Exp. Biol. 213, 881–893. Pörtner, H.-O., Knust, R., 2007. Climate change affects marine fishes through the oxygen limitation of thermal tolerance. Science 315, 95–97. Pörtner, H.-O., Peck, M.A., 2010. Climate change effects on fishes and fisheries: towards a cause-and-effect understanding. J. Fish Biol. 77, 1745–1779. Reardon, E.E., Chapman, L.J., 2010. Energetics of hypoxia in a mouth-brooding cichlid: evidence for interdemic and developmental effects. Physiol. Biochem. Zool. 83, 414–423. Reidy, S.P., Kerr, S.R., Nelson, J.A., 2000. Aerobic and anaerobic swimming performance of individual Atlantic cod. J. Exp. Biol. 203, 347–357. Reipschläger, A., Pörtner, H.-O., 1996. Metabolic depression during environmental stress: the role of extracellular versus intracellular pH in Sipunculus nudus. J. Exp. Biol. 199, 1801–1807.

70


Roche, D.G., Binning, S.A., Bosiger, Y., Johansen, J.L., Rummer, J.L., 2013. Finding the best estimates of metabolic rates in a coral reef fish. J. Exp. Biol. 216, 2103–2110. Rosa, R., Lopes, A.R., Pimentel, M., Faleiro, F., Baptista, M., Trübenbach, K., Narciso, L., Dionísio, G., Pegado, M.R., Repolho, T., 2014. Ocean cleaning stations under a changing climate: biological responses of tropical and temperate fish-cleaner shrimp to global warming. Glob. Chang. Biol. 20, 3068–3079. Rummer, J.L., Stecyk, J.A.W., Couturier, C.S., Watson, S.-A., Nilsson, G.E., Munday, P.L., 2013. Elevated CO2 enhances aerobic scope of a coral reef fish. Conserv. Physiol. 1. http:// dx.doi.org/10.1093/conphys/cot023. Rummer, J.L., Couturier, C.S., Stecyk, J.A.W., Gardiner, N.M., Kinch, J.P., Nilsson, G.E., Munday, P.L., 2014. Life on the edge: thermal optima for aerobic scope of equatorial reef fishes are close to current day temperatures. Glob. Chang. Biol. 20, 1055–1066. Sandblom, E., Gräns, A., Axelsson, M., Seth, H., 2014. Temperature acclimation rate of aerobic scope and feeding metabolism in fishes: implications in a thermally extreme future. Proc. R. Soc. Lond. B Biol. Sci. 281, 20141490. Schurmann, H., Steffensen, J.F., 1997. Effects of temperature, hypoxia and activity on the metabolism of juvenile Atlantic cod. J. Fish Biol. 50, 1166–1180. Shultz, A.D., Murchie, K.J., Griffith, C., Cooke, S.J., Danylchuk, A.J., Goldberg, T.L., Suski, C.D., 2011. Impacts of dissolved oxygen on the behavior and physiology of bonefish: implications for live-release angling tournaments. J. Exp. Mar. Biol. Ecol. 402, 19–26. Sørensen, C., Munday, P.L., Nilsson, G.E., 2014. Aerobic vs. anaerobic scope: sibling species of fish indicate that temperature dependence of hypoxia tolerance can predict future survival. Glob. Chang. Biol. 20, 724–729.

Stillman, J.H., 2003. Acclimation capacity underlies susceptibility to climate change. Science 301, 65-65. Storey, K.B., Storey, J.M., 2004. Oxygen limitation and metabolic rate depression. In: Storey, K.B. (Ed.), Functional Metabolism: Regulation and Adaptation. Wiley-Liss, Hobochen, New Jersey, pp. 415–442. Sunday, J.M., Bates, A.E., Kearney, M.R., Colwell, R.K., Dulvy, N.K., Longino, J.T., Huey, R.B., 2014. Thermal-safety margins and the necessity of thermoregulatory behavior across latitude and elevation. Proc. Natl. Acad. Sci. 111, 5610–5615. Taylor, S., Egginton, S., Taylor, E., 1996. Seasonal temperature acclimatisation of rainbow trout: cardiovascular and morphometric influences on maximal sustainable exercise level. J. Exp. Biol. 199, 835–845. Tewksbury, J.J., Huey, R.B., Deutsch, C.A., 2008. Putting the heat on tropical animals. Science 320, 1296. Tsuji, J.S., 1988. Thermal acclimation of metabolism in Sceloporus lizards from different latitudes. Physiol. Zool. 241–253. Wiens, K.E., Crispo, E., Chapman, L.J., 2014. Phenotypic plasticity is maintained despite geographical isolation in an African cichlid fish, Pseudocrenilabrus multicolor. Integr. Zool. 9, 85–96. Yeager, D.P., Ultsch, G.R., 1989. Physiological regulation and conformation: a BASIC program for the determination of critical points. Physiol. Zool. 888-907.