Aquatic Surface Respiration Research Articles

The effects of diel-cycling hypoxia acclimation on the hypoxia tolerance, swimming capacity and growth performance of southern catfish (Silurus meridionalis)

To investigate the effects of diel-cycling hypoxia acclimation on the hypoxia tolerance, swimming and growth performance of juvenile southern catfish, we initially measured the critical oxygen tension (Pcrit), oxygen thresholds of aquatic surface respiration (ASR) and loss of equilibrium (LOE) of diel-cycling hypoxia-acclimated (15d, 7:00–21:00, dissolved oxygen level (DO)=7.0±0.2mgL−1; 21:00–7:00, DO=3.0±0.2mgL−1) and non-acclimated (15d, DO=7.0±0.2mgL−1) southern catfish at 25°C. We then measured the critical swimming speed (Ucrit) and metabolic rate (MR) of hypoxia-acclimated and non-acclimated fish (under both hypoxic and normoxic conditions). The feeding rate (FR), feeding efficiency (FE) and specific growth rate (SGR) of fish in hypoxia-acclimated and non-acclimated groups were also measured. The Pcrit, ASR and LOE of hypoxia-acclimated fish were significantly lower than those of non-acclimated fish. Hypoxia acclimation resulted in a significantly higher Ucrit when the individuals swam in hypoxia. The Ucrit, maximum metabolic rate (MMR) and metabolic scope (MS) of both the hypoxia-acclimated and non-acclimated fish all decreased with the decrease of DO. However, the Ucrit, MMR and MS decreased by 31, 43 and 54%, respectively, in non-acclimated fish, whereas these values decreased by 15, 28 and 29%, respectively, in hypoxia-acclimated fish, which suggests that hypoxia-acclimated fish were less sensitive to the DO decrease. The FR, FE and SGR all decreased by 21, 20 and 45%, respectively, in the hypoxia-acclimated group compared to the non-acclimated group. This result suggests that diel-cycling hypoxia acclimation improved the hypoxia tolerance and aerobic swimming performance of southern catfish, whereas impaired the growth performance. The high hypoxia tolerance and physiological plasticity to hypoxia-acclimated southern catfish may be related to its lower maintenance energy expenditure, sit-and-wait lifestyle and bottom-dwelling living environment condition (usually facing oxygen fluctuation). The growth performance of so-called ‘visceral type’ fish species, such as southern catfish, are more sensitive to hypoxia compared to other fish species because of their high peak post-prandial metabolic rate, which may be restrained by the limited aerobic metabolic scope in hypoxia.

Implications of hypoxia tolerance for wetland refugia use in Lake Nabugabo, Uganda

AbstractAquatic hypoxia can affect predator‐prey interactions by altering the success rate of the predator and/or the vulnerability of prey. For example, in the Lake Victoria basin of East Africa, native prey exploit hypoxic wetlands as refugia from predation by introduced Nile perch (Lates niloticus). Here, it is predicted that species exploitation of wetlands depends on their hypoxia tolerance relative to the heterogeneity of wetland hypoxia. In this study, we compared the hypoxia tolerance of four fish taxa that differ in their use of hypoxic wetlands in Lake Nabugabo, Uganda: the cichlid Pseudocrenilabrus multicolor victoriae that inhabits the dense swamp interior; and three taxa that inhabit wetland ecotones including Nile tilapia (Oreochromis niloticus), L. niloticus and juvenile endemic haplochromine cichlids. We characterised hypoxia tolerance by exposing fish to progressive hypoxia and quantifying variation in aquatic surface respiration (ASR). The effect of body size on tolerance was explored in L. niloticus by quantifying ASR behaviour across a range of size classes. ASR behaviour was also compared between O. niloticus groups from wetland versus open‐water habitats to detect habitat‐associated intraspecific variation. The most tolerant taxon was the swamp specialist P. multicolor, indicated by its low ASR thresholds and small percentage of fish using ASR during the final sample interval. The other three taxa did not differ in ASR behaviour, and no differences were detected between O. niloticus groups. Body size effects were present for L. niloticus suggesting a lower tolerance to hypoxia in larger‐bodied individuals, thus limiting their ability to penetrate wetlands.

Interspecies variation in hypoxia tolerance, swimming performance and plasticity in cyprinids that prefer different habitats

This study quantified and compared hypoxia tolerance and swim performance among cyprinid fish species from rapid-, slow- and intermediate-flow habitats (four species per habitat) in China. In addition, we explored the effects of short-term acclimation on swim performance, maximum metabolic rate (M(O2,max)) and gill remodelling to detect habitat-associated patterns of plastic response to hypoxia. Indices of hypoxia tolerance included oxygen threshold for loss of equilibrium (LOE50) and aquatic surface respiration (ASR50), and critical oxygen tension for routine metabolic rate (Pcrit). Critical swimming speed (Ucrit) and M(O2,max) were measured under normoxic and hypoxic conditions after 48 h acclimation to normoxia and hypoxia, and gill remodelling was estimated after 48 h of hypoxia exposure. Both traditional ANCOVA and phylogenetically independent contrast (PDANOVA) analyses showed that fish species from rapid-flow habitats exhibited lower LOE50 compared with fish from intermediate- and slow-flow habitats. Habitat-specific differences in Pcrit and Ucrit were detected using PDANOVA but not traditional ANCOVA analyses, with fish species from rapid-flow habitats exhibiting lower Pcrit but higher Ucrit values compared with fish from intermediate- and slow-flow habitats. Fish species from rapid-flow habitats were also characterized by less plasticity in swim performance and gill morphology in response to hypoxia acclimation compared with species from slow-flow habitats, but a greater drop in swim performance in response to acute hypoxia exposure. The study detected a habitat-specific difference in hypoxia tolerance, swimming performance and its plasticity among fish from habitats with different flow conditions, possibly because of the long-term adaptation to the habitat caused by selection stress. The PDANOVA analyses were more powerful than traditional statistical analyses according to the habitat effects in both hypoxia tolerance and swimming performance in this study.

Shorter Gills in Mouth-brooding Females of the Cichlid Pseudocrenilabrus multicolor

In mouth-brooding fishes, there may be a trade-off between respiratory function (gill size and shape) and reproduction (space in the buccal cavity for offspring) during periods of parental care. This trade-off may become particularly apparent under low-oxygen conditions if a mouthful of eggs compromises respiratory pumping. In this study, we compare gill size and position among five males, ten brooding females, and eight non-brooding females of the African cichlid, Pseudocrenilabrus multicolor victoriae collected from a hypoxic swamp in Uganda. Brooding females exhibited a smaller total and average gill filament length than non-brooding females and males and a more elongated arch that may relate to spatial constraints imposed by the young. Importantly, brooding females also exhibited smaller overall hemibranch surface area than non-brooding females and males. In conjunction with earlier studies on this species that have reported a higher metabolic rate, higher aquatic surface respiration thresholds, and d...

Use of aquatic plants to create fluctuating hypoxia in an experimental environment

In freshwater systems, dissolved oxygen (DO) saturation frequently fluctuates, falling at night and rising during the day in response to respiration and photosynthesis, respectively, of aquatic biota. Low DO (hypoxia) is a common cause of fish kills in freshwater systems around the world. Laboratory studies on responses of fish to fluctuating DO are currently limited, and require techniques that produce a realistic cycle of DO depletion and replacement. Artificial DO-depletion mechanisms frequently used for hypoxia studies may underestimate the field effects of hypoxia on fish because of the lack of the naturally occurring synergistic effect of lower pH, and seldom allow fish to employ behavioural adaptations to hypoxia, such as aquatic surface respiration. We demonstrate proof-of-principle for an alternative method of creating fluctuating hypoxia in an experimental environment, using the natural rhythms of photosynthesis and respiration of aquatic plants to create realistic conditions. A range of volumes of aquatic macrophytes were used alone and in combination with fish to lower DO saturation in sealed freshwater aquaria, and achieved DO saturations as low as 1.3%. This cost-effective method can be deployed over long periods with minimal effort in comparison to traditional methods of DO reduction.

Energetic constraints on electric signalling in wave-type weakly electric fishes

Gymnotiform weakly electric fishes generate electric organ discharges (EODs) and sense perturbations of the resulting electric field for purposes of orientation, prey detection and communication. Some species produce oscillatory ('wave-type') EODs at very high frequencies (up to 2 kHz) that have been proposed to be energetically expensive. If high-frequency EODs are expensive, then fish may modulate their EOD frequency and/or amplitude in response to low-oxygen (hypoxic) stress and/or compensate for costs of signalling through other adaptations that maximize oxygen uptake efficiency. To test for evidence of an energetic cost of signalling, we recorded EOD in conjunction with metabolic rates, critical oxygen tension and aquatic surface respiration (ASR(90)) thresholds in Apteronotus leptorhynchus, a species found in high-oxygen habitats, and Eigenmannia virescens, a species more typically found in low-oxygen waters. Eigenmannia virescens had a lower mean ASR(90) threshold and critical oxygen tension compared with A. leptorhynchus, consistent with field distributions. Within each species, there was no evidence for a relationship between metabolic rate and either EOD frequency or amplitude under normoxia, suggesting that there is no significant direct metabolic cost associated with producing a higher frequency EOD. However, when exposed to progressive hypoxia, fish generally responded by reducing EOD amplitude, which may reduce energetic costs. The threshold at which fish reduced EOD amplitude tended to be lower in E. virescens, a pattern consistent with higher tolerance to hypoxic stress. The results of this study suggest that wave-type fish reduce their EOD amplitude to reduce direct energetic costs without reducing metabolic rate under hypoxia.

A relationship between metabolic rate and risk‐taking behaviour is revealed during hypoxia in juvenile European sea bass

Summary1. Inter‐individual variation in metabolic rate exists in a wide range of taxa. While this variation appears to be linked to numerous aspects of animal behaviour and personality, the ecological relevance of these relationships is not understood. The behavioural response of individual fish to acute aquatic hypoxia, for example, could be related to metabolic rate via influences on oxygen demand or the willingness to take risks. Individuals with higher metabolic rates could show greater hypoxia‐associated increases in activity that could allow them to locate areas with increased oxygen availability but that also make them susceptible to predation. Any relationship between metabolic rate and risk‐taking behaviour among individual fish could therefore be modulated by environmental oxygen levels, perhaps becoming stronger as oxygen availability declines.2. We measured spontaneous swimming activity as an index of risk‐taking by juvenile European sea bass in normoxia, moderate hypoxia (40% air saturation) and severe hypoxia (20% air saturation) after being startled by a predator model. All fish were also separately measured for routine metabolic rate by measuring oxygen uptake.3. In hypoxia, fish re‐emerged from cover sooner after a simulated attack and were generally more active than when the same fish were startled in normoxia. In addition, individual activity and risk‐taking in severe hypoxia were positively correlated with metabolic rate. Aquatic surface respiration was a major contributor towards increased activity in hypoxia and was positively related to metabolic rate during severe hypoxia. There were no relationships between risk‐taking and metabolic rate in moderate hypoxia or normoxia.4. Relative measures of risk‐taking among individual fish were not consistent among oxygen levels, further suggesting that individuals differ in sensitivity to hypoxia and the degree to which this environmental stressor affects risk‐taking behaviour.5. These results suggest that fish with relatively high metabolic rates become more active during acute hypoxia, possibly leading to increased susceptibility to predation in response to differences in metabolic demand. In addition, the relationship between metabolic rate and risk‐taking may only be observable during exposure to a physiological stressor or such a stressor may strengthen any relationships observable under more benign conditions.

Morphological plasticity associated with environmental hypoxia in characiform fishes from neotropical floodplain lakes

The ability to develop reversible dermal extensions on the lower jaw in some South American characiform fishes has been proposed as a way to optimize the performance of aquatic surface respiration (ASR) during hypoxic conditions. These structures are formed by edema of the hypodermal tissues and can develop in a large proportion of individuals inhabiting a lake within a few hours following daily changes in dissolved oxygen. Our study report the development of dermal lip protuberances in elevenspeciesof characiform fishes associated with periods of strong environmental hypoxia in floodplain lakes of Salado River, Argentina. Protuberances occurred in different basic forms in fishes with divergent head morphology (genera Astyanax, Ctenobrycon, Aphyocharax, Brycon, Mylossoma, Tri- portheus, Oligosarcus and Acestrorhynchus). The dis- covery of dermal projections on the anterior border of maxillary bone extends the known range of structures affected by lip protuberances. Dermal structures began to develop simultaneously in both jaws below dissolved oxygen (DO) concentrations of 1.20-1.75 mgl �1 and rose in a steep manner as oxygen level decreased. The degree of morphological plasticity differed among traits and species. The shape of response of morphology to DO was similar to that previously reported on ASR, providing additional evidence about the functional link between these traits. Our results suggests that dermal lip protuberances are widely spread among characiform fishes, affecting several mouth structures. The different types of protuberances would make up for the limi- tations imposed by body size and mouth shape and position on the performance of ASR in fishes with divergent morphology.

Hypoxia tolerance of three native and three alien species of bitterling inhabiting Lake Kasumigaura, Japan

Differences in hypoxia tolerance among three native and three alien bitterling species were examined by means of field surveys and aquarium experiments. I caught fish in minnow traps and measured environmental factors such as dissolved oxygen and current velocity once a month between June and November 2006 at 30 points around Lake Kasumigaura, Japan. In addition, aquarium experiments were carried out to compare aquatic surface respiration thresholds among the bitterling species. Much more individuals of an alien species, Rhodeus ocellatus ocellatus, was caught at the least oxygen range than expected in the field, and showed the highest hypoxia tolerance in the laboratory experiments. Another alien bitterling, Acheilognathus rhombeus, also showed higher hypoxia tolerance than the three native bitterling species. Lake Kasumigaura is well known for eutrophication and water masses with low dissolved oxygen have often been observed there. Differences in hypoxia tolerance among bitterling species might have affected bitterling assemblage structure in Lake Kasumigaura, which is currently characterized by the dominance of R. o. ocellatus.

Leap of faith: Voluntary emersion behaviour and physiological adaptations to aerial exposure in a non-aestivating freshwater fish in response to aquatic hypoxia

Lowland stream fauna in areas of intensive agriculture are increasingly under threat from anthropogenic activities leading to eutrophication and subsequent hypoxia. Survival of hypoxic episodes depends upon a combination of behavioural and physiological adaptations. Responses of inanga (Galaxias maculatus: Galaxiidae) to aquatic hypoxia were investigated in the laboratory. Contrary to expectation inanga did not display behaviour that might reduce energy expenditure during oxygen limitation, with swimming activity slightly, but significantly elevated relative to normoxia. Instead, as dissolved oxygen concentrations decreased, the fish moved higher in the water column, increased their swimming speed and exhibited aquatic surface respiration. Physiological changes such as enhanced opercular frequency were also noted. As hypoxia deepened inanga started to leap out of the water, emersing themselves on a floating platform. Once emersed, fish exhibited an enhanced oxygen consumption rate compared to hypoxic fish. Thus inanga appear better adapted to escape hypoxia (a behavioural adaptation) rather than tolerate it (physiological adaptation). The emersion strategy used for inanga in response to severe hypoxia is in agreement with their ability to take up more oxygen from the air than from hypoxic water and therefore may justify the potentially increased risks of desiccation and predation associated with leaving the water.

Physiological, behavioral and biochemical adaptations of intertidal fishes to hypoxia

Hypoxia survival in fish requires a well-coordinated response to either secure more O(2) from the hypoxic environment or to limit the metabolic consequences of an O(2) restriction at the mitochondria. Although there is a considerable amount of information available on the physiological, behavioral, biochemical and molecular responses of fish to hypoxia, very little research has attempted to determine the adaptive value of these responses. This article will review current attempts to use the phylogenetically corrected comparative method to define physiological and behavioral adaptations to hypoxia in intertidal fish and further identify putatively adaptive biochemical traits that should be investigated in the future. In a group of marine fishes known as sculpins, from the family Cottidae, variation in hypoxia tolerance, measured as a critical O(2) tension (P(crit)), is primarily explained by variation in mass-specific gill surface area, red blood cell hemoglobin-O(2) binding affinity, and to a lesser extent variation in routine O(2) consumption rate (M(O(2))). The most hypoxia-tolerant sculpins consistently show aquatic surface respiration (ASR) and aerial emergence behavior during hypoxia exposure, but no phylogenetically independent relationship has been found between the thresholds for initiating these behaviors and P(crit). At O(2) levels below P(crit), hypoxia survival requires a rapid reorganization of cellular metabolism to suppress ATP consumption to match the limited capacity for O(2)-independent ATP production. Thus, it is reasonable to speculate that the degree of metabolic rate suppression and the quantity of stored fermentable fuel is strongly selected for in hypoxia-tolerant fishes; however, these assertions have not been tested in a phylogenetic comparative model.

Extreme habitats are not refuges: poeciliids suffer from increased aerial predation risk in sulphidic southern Mexican habitats

Extreme environments are often considered a predation refuge for organisms living in them. In southern Mexico several species of poeciliid fishes are undergoing incipient speciation in a variety of extreme (i.e. permanently dark and/or sulphidic) freshwater systems, and previous research has demonstrated reproductive isolation between populations from sulphidic and adjacent benign habitats. In the present study, we investigated bird predation rates (measured as successful captures per minute) in two sulphidic surface and several benign surface habitats, to test the hypothesis that extreme habitats are predation refuges. We found capture rates to be approximately 20 times higher in sulphidic environments: probably facilitated by extremophile poeciliids spending most of their time at the water surface, where they engage in aquatic surface respiration as a direct response to hypoxia. Even birds that are usually not considered major fish predators regularly engage in fish predation in the toxic habitats of southern Mexico. Our results demonstrate that extreme environments do not necessarily represent a refuge from predation, and we discuss the general importance of predation in driving incipient speciation in these systems. Finally, we hypothesize that natural selection via avian predation may play an important role in maintaining reproductive isolation between divergent poeciliid populations. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101, 417–426.

Control of respiration in fish, amphibians and reptiles

Fish and amphibians utilise a suction/force pump to ventilate gills or lungs, with the respiratory muscles innervated by cranial nerves, while reptiles have a thoracic, aspiratory pump innervated by spinal nerves. However, fish can recruit a hypobranchial pump for active jaw occlusion during hypoxia, using feeding muscles innervated by anterior spinal nerves. This same pump is used to ventilate the air-breathing organ in air-breathing fishes. Some reptiles retain a buccal force pump for use during hypoxia or exercise. All vertebrates have respiratory rhythm generators (RRG) located in the brainstem. In cyclostomes and possibly jawed fishes, this may comprise elements of the trigeminal nucleus, though in the latter group RRG neurons have been located in the reticular formation. In air-breathing fishes and amphibians, there may be separate RRG for gill and lung ventilation. There is some evidence for multiple RRG in reptiles. Both amphibians and reptiles show episodic breathing patterns that may be centrally generated, though they do respond to changes in oxygen supply. Fish and larval amphibians have chemoreceptors sensitive to oxygen partial pressure located on the gills. Hypoxia induces increased ventilation and a reflex bradycardia and may trigger aquatic surface respiration or air-breathing, though these latter activities also respond to behavioural cues. Adult amphibians and reptiles have peripheral chemoreceptors located on the carotid arteries and central chemoreceptors sensitive to blood carbon dioxide levels. Lung perfusion may be regulated by cardiac shunting and lung ventilation stimulates lung stretch receptors.

The effect of progressive hypoxia on spontaneous activity in single and shoaling golden grey mulletLiza aurata

The hypothesis of a differential effect of hypoxia on activity in shoaling v. solitary fish was tested in golden grey mullet Liza aurata. In both solitary and shoaling fish, (1) swimming activity increased significantly at <or=60% air saturation compared with normoxia, (2) aquatic surface respiration increased significantly below 15% air saturation and (3) a significant increase in ventilation frequency was observed between 40 and 10% air saturation. Swimming activity was higher in shoaling than in solitary fish, possibly due to social interactions in shoaling fish. Despite showing a higher swimming activity, shoaling individuals showed similar ventilation frequency to the less active solitary fish. This suggests that shoaling exerts a calming effect on L. aurata, probably related to increased safety while in numbers. In addition, shoaling fish spent a higher proportion of time performing aquatic surface respiration at 15% air saturation (i.e. the highest oxygen level at which aquatic surface respiration was observed) than solitary fish, possibly because of (1) synchronization of aquatic surface respiration in shoaling fish and (2) lower risk of predation perceived by shoaling fish.

Escaping to the Surface: A Phylogenetically Independent Analysis of Hypoxia‐Induced Respiratory Behaviors in Sculpins

Behavioral responses to progressive hypoxia exposure were assessed in several species of fish from the family Cottidae (sculpins), which are distributed along the near-shore marine environment and differ in their hypoxia tolerance. The use of aquatic surface respiration (ASR) and aerial emergence as a response to progressive decreases in environmental O(2) differed between intertidal and subtidal sculpins. Intertidal sculpins consistently displayed ASR followed by emergence behaviors, while the subtidal species performed these behaviors at low frequency or not at all. There was a significant negative correlation between the O(2) thresholds for the onset of ASR and critical O(2) tensions (P(crit), a measure of hypoxia tolerance), such that the hypoxia-tolerant species performed ASR at higher O(2) tensions than the more hypoxia-sensitive species. There was no relationship between the O(2) thresholds for emergence and P(crit). When restricted from accessing the water surface during progressive decreases in O(2), all species of sculpin displayed the same sequence of behaviors including agitation, attempts to escape, quiescence, and unresponsiveness. The only behavior from this suite that correlated with P(crit) was the first sign of agitation, which occurred at a lower O(2) tension in the most hypoxia-tolerant species. With the application of phylogenetically independent contrasts, all the relationships between behavior and P(crit) were nonsignificant, which limits our capacity to separate selection-driven changes in behavior from the phylogenetic signal.

Dermal lip protuberances associated with aquatic surface respiration in juveniles of the piscivorous characid Salminus brasiliensis (Actinopterygii: Characidae)

Some South American freshwater fishes can improve their capability of aquatic surface respiration (ASR) by developing dermal lip protuberances in the lower jaw. This adaptation was thought to be limited to omnivorous or herbivorous fishes. The present work provides the first evidence that juveniles of a piscivorous characid, Salminus brasiliensis, develop dermal lip protuberances during periods of hypoxia in floodplain ponds of the Salado River, in Argentina. The protuberance of S. brasiliensis involves dermal portions of both jaws exhibiting lateral lobes on the sides of the mouth arranged in the vertical plane. Water dissolved oxygen concentrations less than or equal to 1.05 mgl-1 were associated with a remarkable increase in lip protuberance. The lateral lobes of the protuberance in this species may limit the access of water to the anterior portion of the mouth which is positioned nearer to the oxygenated surface water during ASR. Finally, ASR, complemented with the development of dermal lip protuberances, can be considered a valuable strategy to survive in hypoxic environments, even for carnivorous fishes with elevated oxygen requirements.

Salinity effects on behavioural response to hypoxia in the non‐native Mayan cichlid Cichlasoma urophthalmus from Florida Everglades wetlands

This study quantified the hypoxia tolerance of the Mayan cichlid Cichlasoma urophthalmus over a range of salinities. The species was very tolerant of hypoxia, using aquatic surface respiration (ASR) and buccal bubble holding when oxygen tensions dropped to <20 mmHg (c. 1.0 mg l(-1)) and 6 mmHg, respectively. Salinity had little effect on the hypoxia tolerance of C. urophthalmus, except that bubble holding was more frequent at the higher salinities tested. Levels of aggression were greatest at the highest salinity. The ASR thresholds of C. urophthalmus were similar to native centrarchid sunfishes from the Everglades, however, aggression levels for C. uropthalmus were markedly higher.

Male-biased predation of a cave fish by a giant water bug

Male-biased predation has been described from several epigean species, and in many cases, intrinsic differences between the sexes (such as male ornaments) have been suggested as an explanation. Here we report on male-biased predation of a cave fish (Poecilia mexicana) by an aquatic insect (Belostoma sp.) in a Mexican sulfur cave. P. mexicana use aquatic surface respiration (ASR) to survive in their sulfidic, hypoxic habitat. We found that males typically exhibit more ASR activity than females, which leads to increased exposure to the sit-and-wait predator that catches fish near the water surface. Our finding is novel, because male vulnerability to predation is not directly related to male traits involved in courtship, but rather due to other sexual differences in behavior and ultimately, oxygen demands.

The response of the tidepool sculpin, Oligocottus maculosus, to hypoxia in laboratory, mesocosm and field environments

Animals living in the intertidal zone experience regular, predictable fluctuations in physical parameters including temperature, oxygen and salinity and rely on behavioural, physiological and biochemical mechanisms to cope with environmental variation. In the present study, behavioural strategies induced by aquatic hypoxia (e.g. emergence) were performed at similar oxygen tensions across laboratory, mesocosm and field environments; the number of individuals performing these behaviours at any one time was similar in mesocosms and the field. The use of aquatic surface respiration (ASR) was more plastic than emergence behaviour, occurring at a lower oxygen tension in juveniles than adults and being influenced by the addition of alarm substance. Oxygen uptake was lower in air than in water in adults but, in contrast, oxygen uptake was not influenced by the respiratory medium in juveniles. In the laboratory, 72 h of forced emergence did not affect whole body concentrations of lactate but when ASR and emergence were prevented within mesocosm environments there was a significant elevation of lactate. The present study highlights the benefits of transcending traditional laboratory/field boundaries allowing the responses of laboratory-held animals to environmental fluctuation to be integrated with how these animals perform in their natural environment.

Hypoxia tolerance of two centrarchid sunfishes and an introduced cichlid from karstic Everglades wetlands of southern Florida, U.S.A.

In this study, the hypoxia tolerance of three Everglades fishes, two native centrarchids (Lepomis gulosus and Lepomis marginatus) and a recently introduced cichlid (Hemichromis letourneuxi), were documented. Aquatic surface respiration (ASR) thresholds were lowest for H. letourneuxi, followed by L. gulosus, then L. marginatus. The ASR thresholds for L. marginatus were within ranges reported for small, freshwater tropical fishes, while those for L. gulosus were similar to swamp‐adapted fishes. For H. letourneuxi, ASR thresholds were some of the lowest reported. All three species showed excellent tolerance of low dissolved oxygen levels when allowed access to the surface. When denied surface access, L. marginatus lost equilibrium at a higher oxygen tension than the other species. Overall, although all species easily tolerated hypoxia, H. letourneuxi appeared to be best equipped to deal with hypoxia, followed by L. gulosus, then L. marginatus. Hemichromis letourneuxi also exhibited more aggressive behaviours than the centrarchids. These results suggest that hypoxia is not likely to prevent H. letourneuxi from exploiting the seasonally inundated wetlands of south Florida while expanding its range there.