Routine Metabolic Rate Research Articles

Aerobic scope predicts dominance during early life in a tropical damselfish

Summary A range of physiological traits are linked with aggression and dominance within social hierarchies, but the role of individual aerobic capacity in facilitating aggression has seldom been studied. Further, links previously observed between an individual's metabolic rate and aggression level may be context dependent and modulated by factors such as social stress and fcompetitor familiarity. We examined these issues in juvenile Ambon damselfish, Pomacentrus amboinensis, which display intraspecific competition for territories during settlement on coral reefs. Individuals were measured for routine metabolic rate, aerobic scope (AS) and anaerobic capacity using intermittent‐flow respirometry before dyadic dominance contests. Post‐contest, fish were measured for metabolic rate in isolation and while interacting with their previous competitor or a stranger in adjacent transparent respirometers. In arena contests, AS was correlated with aggression and dominance, while routine metabolic rate and anaerobic capacity were not related to dominance. Post‐contest, subordinates showed a rise in metabolic rate and decrease in available AS, presumably due to social stress. Dominants increased metabolic rate in the presence of a previous competitor, possibly due to the stresses of hierarchy maintenance. Metabolic rate during aggressive interactions did not approach that measured during exhaustive exercise, suggesting individuals do not fully utilise their AS during aggression. A greater AS may, however, allow faster post‐contest recovery. These results demonstrate a link between AS and dominance during intraspecific competition for territory. Selection on AS could therefore follow, either indirectly through correlations with other traits influencing resource‐holding potential, or directly if AS carries benefits important for territory acquisition or holding, such as an enhanced capacity to cope with socially induced stress.

Effect of temperature on hypoxia tolerance and its underlying biochemical mechanism in two juvenile cyprinids exhibiting distinct hypoxia sensitivities

It is increasingly important to investigate the effect of temperature on hypoxia tolerance in fish species, as worldwide hypoxia worsens with increases in global warming. We selected the hypoxia-tolerant crucian carp (Carassius carassius) and the hypoxia-sensitive Chinese bream (Parabramis pekinensis) as model fish and investigated their hypoxia tolerance based on the critical oxygen tension of the routine metabolic rate (M˙O2rout) (Pcrit), aquatic surface respiration (ASRcrit) and loss of equilibrium (LOEcrit) after two weeks of acclimation at either 10, 20 or 30°C. We also measured the tissue substrate (glycogen and glucose of muscle and liver) and lactate levels of both normoxia- and hypoxia-treated fish (post-LOE). Crucian carp exhibited significantly lower Pcrit and LOEcrit but not ASRcrit. Crucian carp possessed higher hypoxia tolerance, partially due to a higher tissue glycogen reserve, which provides cellular fuel under severe hypoxia, as well as higher lactate tolerance and clearance ability than Chinese bream. The hypoxia tolerance was maintained in crucian carp but was decreased in Chinese bream as the temperature increased. The difference between the two species is based on the greater recruitment of tissue glycogen, resulting in an increased level of cellular fuel during hypoxia in crucian carp than in Chinese bream. In addition, crucian carp possessed the greater liver lactate clearance capacity, and the smaller increase in the M˙O2rout at higher temperatures compared to Chinese bream. Furthermore, substrate shortage and decreased lactate tolerance at high temperatures in Chinese bream might also contribute to the difference in hypoxia tolerance between the two species.

Ecophysiology of native and alien-invasive clams in an ocean warming context

Both climate change and biological invasions are among the most serious global environmental threats. Yet mechanisms underlying these eventual interactions remain unclear. The aim of this study was to undertake a comprehensive examination of the physiological and biochemical responses of native (Ruditapes decussatus) and alien-invasive (Ruditapes philippinarum) clams to environmental warming. We evaluated thermal tolerance limits (CTMax), routine metabolic rates (RMRs) and respective thermal sensitivity (Q10 values), critical oxygen partial pressure (Pcrit), heat shock response (HSP70/HSC70 levels), lipid peroxidation (MDA build-up) and antioxidant enzyme [glutathione-S-transferase (GST), catalase (CAT) and superoxide dismutase (SOD)] activities. Contrary to most studies that show that invasive species have a higher thermal tolerance than native congeners, here we revealed that the alien-invasive and native species had similar CTMax values. However, warming had a stronger effect on metabolism and oxidative status of the native R. decussatus, as indicated by the higher RMRs and HSP70/HSC70 and MDA levels, as well as GST, CAT and SOD activities. Moreover, we argue that the alien-invasive clams, instead of up-regulating energetically expensive cellular responses, have evolved a less demanding strategy to cope with short-term environmental (oxidative) stress-pervasive valve closure. Although efficient during stressful short-term periods to ensure isolation and guarantee longer survival, such adaptive behavioural strategy entails metabolic arrest (and the enhancement of anaerobic pathways), which to some extent will not be advantageous under the chronically warming conditions predicted in the future.

Temperature effects on metabolic rate and cardiorespiratory physiology of the spiny rock lobster (Jasus edwardsii) during rest, emersion and recovery

Although spiny rock lobster (Jasus edwardsii) is a wholly sub-littoral species, they show a considerable ability to survive prolonged emersion, a fact exploited during the commercial export of this species. Yet, despite this remarkable hardiness, basic information on how this species responds physiologically to emersion is somewhat lacking. Using flow-through respirometry and electrophysiological techniques, we identified that J. edwardsii undergoes marked physiological changes during rest, emersion and recovery over a broad range of temperatures (3.7-17.8°C). Under resting conditions, routine metabolic rates (RMR) were 22.57±2.39, 9.69±0.55 and 8.09±0.27mL O2 h(-1), average heart rates (Hr) were 54.72±4.46, 37.68±2.86 and 29.67±0.59 BPM, and ventilation frequencies were 83.71±5.86, 45.34±2.91 and 41.62±0.65 BPM at 15.0, 7.5 and 3.7°C, respectively. Notably, the surgical implantation of electrodes elevated RMR compared with non-surgical treatments. In surgery and non-surgery groups, Q 10 was calculated to be ca. 3.0. Upon emersion, rate of oxygen consumption and Hr decreased below resting rates in a temperature-dependent manner, but, along with rate of CO2 production, increased steadily during 24-h emersion. Ventilation frequencies upon emersion showed a contrasting response and increased significantly above resting rates. When returned to flow-through sea water for recovery, elevated respiration rates provided clear evidence of an O2 debt, and near-complete recovery was observed after 17h at both 15.0 and 7.5°C, but close to no debt was recovered at 3.7°C. In addition, J. edwardsii was observed to undergo marked diurnal and periodic ventilation cycles, characterised by synchronous changes in RMR, Hr and ventilation frequency.

Parental effects improve escape performance of juvenile reef fish in a high-CO2world

Rising CO2 levels in the oceans are predicted to have serious consequences for many marine taxa. Recent studies suggest that non-genetic parental effects may reduce the impact of high CO2 on the growth, survival and routine metabolic rate of marine fishes, but whether the parental environment mitigates behavioural and sensory impairment associated with high CO2 remains unknown. Here, we tested the acute effects of elevated CO2 on the escape responses of juvenile fish and whether such effects were altered by exposure of parents to increased CO2 (transgenerational acclimation). Elevated CO2 negatively affected the reactivity and locomotor performance of juvenile fish, but parental exposure to high CO2 reduced the effects in some traits, indicating the potential for acclimation of behavioural impairment across generations. However, acclimation was not complete in some traits, and absent in others, suggesting that transgenerational acclimation does not completely compensate the effects of high CO2 on escape responses.

Morphology, metabolism and behaviour: responses of three fishes with different lifestyles to acute hypoxia

Summary Behavioural responses to changing environments affect community composition, so the identification of associations between environmental gradients, behavioural traits and physiological traits makes a significant contribution to the quest for trait‐based rules of community change. We tested the hypothesis that fish morphology and lifestyle are associated with metabolic rate, hence oxygen demand, and behavioural response to gradual hypoxia [low dissolved oxygen (DO)], using respirometry and behavioural experiments. Three species fell along different points of the fast–slow lifestyle continuum: Melanotaenia fluviatilis, a pelagic fish adapted to endurance swimming, lies at the fast end of the lifestyle continuum, while Mogurnda adspersa, a benthic fish adapted to burst swimming, lies at the other end. The benthopelagic Hypseleotris sp. has an intermediate lifestyle. Standard and routine metabolic rates varied strongly among the species and were associated with lifestyle according to the inequality M. fluviatilis > Hypseleotris > M. adspersa. As DO declined, aquatic surface respiration behaviour also varied significantly among the species and indicated a sensitivity to hypoxia described by the same inequality. As hypoxia ensued, changes in habitat were also linked to lifestyle, but changes in activity level among species were not neatly correlated with lifestyle. Overall, our experiments imply that there are significant links between morphology, lifestyle, metabolism and behavioural response to hypoxia in these three species of fish.

Open-top static respirometry is a reliable method to determine the routine metabolic rate of barramundi, Lates calcarifer

Closed-system respirometry is a standard technique used to determine aerobic metabolism of aquatic organisms. Open-top systems are rarely used due to concerns of gas exchange across the air–water interface. Here, we evaluated an open-top respirometry system by comparing the mass-specific routine metabolic rate (RMR) of the tropical diadromous finfish barramundi, Lates calcarifer, in both closed-top and open-top respirometers. The RMR of 190 g barramundi was determined across broad temperatures ranging from 18 to 38 °C. There was no significant difference in RMR between barramundi in either closed- or open-top respirometers at any temperature (p > 0.05). To ensure RMR measurements were not an artifact of the respirometry system, barramundi were reciprocally transplanted into either respective closed-top or open-top respirometer and oxygen consumption re-measured at each temperature treatment. The RMR of transplanted barramundi was found to be virtually identical in either respirometer. RMR increased linearly with increasing temperature; the relationship between RMR and temperature (T; 18–38 °C) can be described as 3.658T−36.294 mg O2 kg−0.8 h−1. The daily energetic cost of RMR was 1.193T−11.838 kJ kg−0.8 day−1. Q10 for barramundi increased significantly with increasing temperature (p < 0.0001). Q10(18–28) was the lowest at 1.7 and Q10(28–38) the highest at 1.9, over the whole experiment temp range Q10(18–28) was 1.8. The current study demonstrates that open-top respirometry is a reliable and practical alternative to closed-top respirometry for accurate determination of the aerobic metabolism of barramundi and has potential application for a number of different aquatic organisms.

Behavioural and physiological adaptations to low-temperature environments in the common frog, Rana temporaria

BackgroundExtreme environments can impose strong ecological and evolutionary pressures at a local level. Ectotherms are particularly sensitive to low-temperature environments, which can result in a reduced activity period, slowed physiological processes and increased exposure to sub-zero temperatures. The aim of this study was to assess the behavioural and physiological responses that facilitate survival in low-temperature environments. In particular, we asked: 1) do high-altitude common frog (Rana temporaria) adults extend the time available for larval growth by breeding at lower temperatures than low-altitude individuals?; and 2) do tadpoles sampled from high-altitude sites differ physiologically from those from low-altitude sites, in terms of routine metabolic rate (RMR) and freeze tolerance? Breeding date was assessed as the first day of spawn observation and local temperature recorded for five, paired high- and low-altitude R. temporaria breeding sites in Scotland. Spawn was collected and tadpoles raised in a common laboratory environment, where RMR was measured as oxygen consumed using a closed respiratory tube system. Freeze tolerance was measured as survival following slow cooling to the point when all container water had frozen.ResultsWe found that breeding did not occur below 5°C at any site and there was no significant relationship between breeding temperature and altitude, leading to a delay in spawning of five days for every 100 m increase in altitude. The relationship between altitude and RMR varied by mountain but was lower for individuals sampled from high- than low-altitude sites within the three mountains with the highest high-altitude sites (≥900 m). In contrast, individuals sampled from low-altitudes survived freezing significantly better than those from high-altitudes, across all mountains.ConclusionsOur results suggest that adults at high-altitude do not show behavioural adaptations in terms of breeding at lower temperatures. However, tadpoles appear to have the potential to adapt physiologically to surviving at high-altitude via reduced RMR but without an increase in freeze tolerance. Therefore, survival at high-altitude may be facilitated by physiological mechanisms that permit faster growth rates, allowing completion of larval development within a shorter time period, alleviating the need for adaptations that extend the time available for larval growth.

Branchial NH

BackgroundCephalopods have evolved strong acid–base regulatory abilities to cope with CO2 induced pH fluctuations in their extracellular compartments to protect gas transport via highly pH sensitive hemocyanins. To date, the mechanistic basis of branchial acid–base regulation in cephalopods is still poorly understood, and associated energetic limitations may represent a critical factor in high power squids during prolonged exposure to seawater acidification.ResultsThe present work used adult squid Sepioteuthis lessoniana to investigate the effects of short-term (few hours) to medium-term (up to 168 h) seawater acidification on pelagic squids. Routine metabolic rates, NH4+ excretion, extracellular acid–base balance were monitored during exposure to control (pH 8.1) and acidified conditions of pH 7.7 and 7.3 along a period of 168 h. Metabolic rates were significantly depressed by 40% after exposure to pH 7.3 conditions for 168 h. Animals fully restored extracellular pH accompanied by an increase in blood HCO3− levels within 20 hours. This compensation reaction was accompanied by increased transcript abundance of branchial acid–base transporters including V-type H+-ATPase (VHA), Rhesus protein (RhP), Na+/HCO3− cotransporter (NBC) and cytosolic carbonic anhydrase (CAc). Immunocytochemistry demonstrated the sub-cellular localization of Na+/K+-ATPase (NKA), VHA in basolateral and Na+/H+-exchanger 3 (NHE3) and RhP in apical membranes of the ion-transporting branchial epithelium. Branchial VHA and RhP responded with increased mRNA and protein levels in response to acidified conditions indicating the importance of active NH4+ transport to mediate acid–base balance in cephalopods.ConclusionThe present work demonstrated that cephalopods have a well developed branchial acid–base regulatory machinery. However, pelagic squids that evolved a lifestyle at the edge of energetic limits are probably more sensitive to prolonged exposure to acidified conditions compared to their more sluggish relatives including cuttlefish and octopods.

Predator-driven intra-species variation in locomotion, metabolism and water velocity preference in pale chub (Zacco platypus) along a river

Fish inhabit environments that vary greatly in terms of predation intensity, and these predation regimes are generally expected to be a major driver of divergent natural selection. To test whether there is predator-driven intra-species variation in the locomotion, metabolism and water velocity preference of pale chub (Zacco platypus) along a river, we measured unsteady and steady swimming and water velocity preference among fish collected from both high- and low-predation habitats in the Wujiang River. We also measured the routine metabolic rate (RMR), maximum metabolic rate (MMR) and cost of transport (COT) and calculated the optimal swimming speed (Uopt). The fish from the high-predation populations showed a shorter response latency, elevated routine metabolism, lower swimming efficiency at low swimming speed and lower water velocity preference compared with those from the low-predation populations. Neither of the kinematic parameters fast-start and critical swimming speed (Ucrit) showed a significant difference between the high- and low-predation populations. The fish from the high-predation populations may improve their predator avoidance capacity primarily through an elevated routine metabolism and shorter response latency to achieve advanced warning and escape, rather than an improved fast-start swimming speed or acceleration. Thus, the cost of this strategy is an elevated RMR, and no trade-off between unsteady and steady swimming performance was observed in the pale chub population under various predation stresses. It was interesting to find that the high-predation fish showed an unexpected lower velocity preference, which might represent a compromise between predation avoidance, foraging and energy saving.

Growth, Metabolic and Physiological Response of Juvenile Cheraxquadricarinatus Fed Different Available Nutritional Substrates

There are many factors in intensive culture conditions which vary the physiological response of organisms in culture affecting performance such as growth, metabolic levels in blood, respiratory metabolism and the inflection point of dissolved oxygen concentration. The study was conducted under high density culture conditions with multitrophic systems. Results show that the species can tolerate intensive culture conditions without affecting their growth and survival and has the ability to leverage multiple food sources. This work also shows that Cheraxquadricarinatus has the ability to maintain a low routine metabolic rate, is highly tolerant to limited dissolved oxygen conditions and thus uses energy efficiently. The metabolic rate in juvenile C. quadricarinatus was 0.07 ± 0.003 mg O2/g/h while the critical level of oxygenconcentration, when the organism passes from regulator to conformer is 0.483 ± 0.002 mg O2/L. This is significantly lower than previously reported for cultured decapod species.

Metabolic and locomotor responses of juvenile paddlefish Polyodon spathula to hypoxia and temperature

Hypoxia is an increasing problem in the natural habitats that the paddlefish (Polyodon spathula) has historically inhabited, and a potential problem in managed culture conditions. However, the effects of hypoxia on paddlefish are not well understood. In order to understand the effects of hypoxia on juvenile paddlefish, acute hypoxia tolerance, aerobic metabolic rates and swimming capabilities were measured under normoxic (PO2=140–155mm Hg) and hypoxic (PO2=62–70mm Hg) conditions at 18°C and 26°C. The results showed that paddlefish acclimated to 18°C and 26°C had routine metabolic rates of 211mg/kg/h and 294mg/kg/h, respectively, with a corresponding Q10 of 1.5. At 18°C and 26°C, paddlefish had a critical partial pressure of oxygen (PO2crit) of 74mm Hg and 89mm Hg, respectively. Paddlefish had a lethal oxygen threshold of 31.0mm Hg and 37.0mm Hg at 18°C and 26°C, respectively. Further, paddlefish exhibited a reduction in swimming capability when exposed to hypoxia with a 24% and 41% decrease in Ucrit at 18°C and 26°C, respectively. Therefore, paddlefish are relatively sensitive to hypoxia, and at temperatures from 18 to 26°C require a dissolved oxygen concentration ≥4.7mg/L to maintain basal aerobic metabolism and >2.0mg/L to survive under acute hypoxia.

Aerobic scope and its optimum temperature: clarifying their usefulness and limitations – correspondence on J. Exp. Biol. 216, 2771-2782

Clark and colleagues ([Clark et al., 2013][1]) have set people straight on how to make high quality measurements of maximum and minimum oxygen uptake rates in fishes. They correctly state that great care and understanding are needed to properly measure a fish's aerobic scope (=maximum metabolic rate

Influences of body mass, temperature, oxygen tension, and salinity on respiratory oxygen consumption of cyprinodontoid fishes of three families

This review evaluates the current state of knowledge of influences of body mass, ambient temperature, PO2, and salinity on routine metabolic rates of members of three families, Fundulidae, Cyprinodontidae, and Poeciliidae, belonging to the order Cyprinodontiformes. The study was motivated by Winberg’s (Fisheries Research Board of Canada, Translation Series No. 194. Distributed by the Fisheries Research Board of Canada Biological Station, Nanaimo, BC, 1960) conclusion that the Cyprinodontiformes (Winberg included only live-bearing poeciliids) generally have lower metabolic rates that do other fishes. Based on available information Winberg’s conclusion was borne out that live-bearing freshwater poeciliids show lower than average routine metabolic rates compared to other freshwater fish groups. This is also true of poeciliids from saline waters, and of both freshwater and saline-water members of the related families Fundulidae and Cyprinodontidae. However, considerable variation in metabolic patterns was noted within and among these three families. There were geographic variations between subspecies of some species. Some island groups/species showed lower routine metabolic rates than did allied mainland groups/species. Thermal responses (Q10 values) in routine metabolic rates of these fishes showed variations with geographic location, PO2, salinity and size. Values of PO2crit were altered among species by temperature, body size/age, and possibly salinity. Influences of ambient salinity on routine metabolic rates of these cyprinodontoid fishes also showed variations with temperature and size/age of individuals. The patterns of metabolic responses in these cyprinodontoid fishes to environmental conditions were generally similar to those of other species. Unfortunately, the available information on these fishes lacks the uniformity that would allow for critical and quantitative comparisons between and among the cyprinodontoids and with other species.

A response-surface analysis of the relative importance of the temperature, salinity and body weight on the respiratory metabolism of the white shrimp Litopenaeus vannamei (Boone, 1931)

We used a response-surface analysis to determine the importance of different factors affecting the resting routine metabolic rate (QO2) of the white shrimp Litopenaeus vannamei. The oxygen consumption rates were estimated using a multi-factorial design with 28 combinations of different salinities (15, 20, 25, 30, 35, 40 and 45 psu) and temperature (20, 25, 30 and 35 °C) values. The response-surface analysis produced a quadratic function showing that temperature more profoundly affects the oxygen consumption rate. Response-surface curves were generated to predict the optimal conditions resulting in oxygen consumption to better understand the successful growth of this species.

Lower hypoxia thresholds of cuttlefish early life stages living in a warm acidified ocean

The combined effects of future ocean acidification and global warming on the hypoxia thresholds of marine biota are, to date, poorly known. Here, we show that the future warming and acidification scenario led to shorter embryonic periods, lower survival rates and the enhancement of premature hatching in the cuttlefish Sepia officinalis. Routine metabolic rates increased during the embryonic period, but environmental hypercapnia significantly depressed pre-hatchling's energy expenditures rates (independently of temperature). During embryogenesis, there was also a significant rise in the carbon dioxide partial pressure in the perivitelline fluid (PVF), bicarbonate levels, as well as a drop in pH and oxygen partial pressure (pO₂). The critical partial pressure (i.e. hypoxic threshold) of the pre-hatchlings was significantly higher than the PVF oxygen partial pressure at the warmer and hypercapnic condition. Thus, the record of oxygen tensions below critical pO₂ in such climate scenario indicates that the already harsh conditions inside the egg capsules are expected to be magnified in the years to come, especially in populations at the border of their thermal envelope. Such a scenario promotes untimely hatching and smaller post-hatching body sizes, thus challenging the survival and fitness of early life stages.

Temperature has a reduced effect on routine metabolic rates of juvenile shortnose sturgeon (Acipenser brevirostrum)

This study examined the effects of acclimation temperature (10, 15, 20, or 25 °C) and an acute exposure to various temperatures on the routine metabolism of juvenile (~11 g) shortnose sturgeon (Acipenser brevirostrum). For the acclimation experiment, the minimum, mean, and maximum routine metabolic rates were established for sturgeon at each temperature. Mean routine metabolic rates for 10, 15, 20, and 25 °C were 134, 277, 313, and 309 mg O2 kg(-1) h(-1), respectively, with significant differences occurring between 10 and 15, 10 and 20, and 10 and 25 °C. For the acute exposure, similar patterns and significant differences were observed. Temperature quotient (Q 10) values indicate that the greatest effect of temperature occurred between 10 and 15 °C for both the acclimation and acute temperature experiments. In addition, the effect of temperature on the metabolic rate of sturgeon was nearly negligible between 15 and 25 °C. These results suggest that juvenile shortnose sturgeon are sensitive to temperature changes at the lower end of the range, and less sensitive in the mid-to-upper temperature range.

Energy demand of larval and juvenile Cape horse mackerels, Trachurus capensis, and indications of hypoxia tolerance as benefit in a changing environment

Trachurus capensis is an important fisheries resource in the degraded Namibian upwelling ecosystem. Food supply and shoaling of hypoxic zones are hypothesised to influence the species’ recruitment success. This paper is the first to quantify energy requirements and hypoxia tolerance of larval and juvenile stages of a Trachurus species. We measured normoxic respiration rates of T. capensis with a size range from 0.001 to 20.8 g wet mass (WM) collected off Cape Town (33.9°S, 18.5°E, 12/2009) and in the northern Benguela (17–24°S, 11–15°E, 02/2011). Routine metabolic rate (RMR) and standard routine rate (SRR) (mg O2 h−1) followed the allometric functions RMR = 0.418 WM0.774 and SRR = 0.275 WM0.855, respectively. Larvae and juveniles had comparatively high metabolic rates, and the energy demand of juveniles at the upper end of the size range appeared too high to be fuelled by a copepod diet alone. T. capensis’ early life stages showed a high tolerance to hypoxic conditions. RMR in larvae did not change until 30 % O2sat at 18 °C. In juveniles, critical oxygen saturation levels were low (PC for SRR = 11.2 ± 1.7 % O2sat and PC for RMR = 13.2 ± 1.6 % O2sat at 20 °C) and oxy-regulation effective (regulation index = 0.78 ± 0.09). A high hypoxia tolerance may facilitate the retention of larvae in near-shore waters providing favourable feeding conditions and allowing juveniles to exploit food resources in the oxygen minimum zone. These mechanisms seem to well adapt T. capensis to a habitat affected by spreading hypoxic zones and probably enhance its recruitment success.

Metabolic divergence between sibling species of cichlidsPundamilia nyerereiandPundamilia pundamilia

This study compared Pundamilia nyererei and Pundamilia pundamilia males in routine metabolic rate (R(R)) and in the metabolic costs males pay during territorial interactions (active metabolic rate, R(A)). Pundamilia nyererei and P. pundamilia males housed in social isolation did not differ in RR . In contrast to expectation, however, P. nyererei males used less oxygen than P. pundamilia males, for a given mass and level of agonistic activity. This increased metabolic efficiency may be an adaptation to limit the metabolic cost that P. nyererei males pay for their higher rate of aggressiveness compared to P. pundamilia males. Thus, the divergence between the species in agonistic behaviour is correlated with metabolic differentiation. Such concerted divergence in physiology and behaviour might be widespread in the dramatically diverse cichlid radiations in East African lakes and may be an important factor in the remarkably rapid speciation of these fishes. The results did not support the hypothesis that higher metabolic rates caused a physiological cost to P. nyererei males that would offset their dominance advantage.

Thermal dependence of clearance and metabolic rates in slow- and fast-growing spats of manila clam Ruditapes philippinarum

Thermal dependence of clearance rate (CR: lh(-1)), standard (SMR: Jh(-1)) and routine metabolic rates (RMR: Jh(-1)), were analyzed in fast (F)- and slow (S)-growing juveniles of the clam Ruditapes philippinarum. Physiological rates were measured at the maintenance temperature (17°C), and compared with measurements performed at 10 and 24°C after 16h and 14days to analyze acute and acclimated responses, respectively. Metabolic rates (both RMR and SMR) differed significantly between F and S seeds, irrespective of temperature. Mass-specific CRs were not different for F and S seeds but were significantly higher in F clams for rates standardized according to allometric size-scaling rules. Acute thermal dependency of CR was equal for F and S clams: mean Q 10 were ≈3 and 2 in temperature ranges of 10-17 and 17-24°C, respectively. CR did not change after 2weeks of acclimation to temperatures. Acute thermal effects on SMR were similar in both groups (Q 10≈1 and 1.6 in temperature ranges of 10-17 and 17-24°C, respectively). Large differences between groups were found in the acute thermal dependence of RMR: Q 10 in F clams (≈1.2 and 1.9 at temperature ranges of 10-17 and 17-24°C, respectively) were similar to those found for SMR (Q 10=1.0 and 1.7). In contrast, RMR of S clams exhibited maximum thermal dependence (Q 10=3.1) at 10-17°C and become depressed at higher temperatures (Q 10=0.9 at 17-24°C). A recovery of RMR in S clams was recorded upon acclimation to 24°C. Contrasting metabolic patterns between fast and slow growers are interpreted as a consequence of differential thermal sensitivity of the fraction of metabolism associated to food processing and assimilation.