SUMMARY Ram ventilation and gill function in a lamnid shark, the shortfin mako, Isurus oxyrinchus , were studied to assess how gill structure may affect the lamnid–tuna convergence for high-performance swimming. Despite differences in mako and tuna gill morphology, mouth gape and basal swimming speeds, measurements of mako O 2 utilization at the gills (53.4±4.2%) and the pressure gradient driving branchial flow (96.8±26.1 Pa at a mean swimming speed of 38.8±5.8 cm s –1 ) are similar to values reported for tunas. Also comparable to tunas are estimates of the velocity (0.22±0.03 cm s –1 ) and residence time (0.79±0.14 s) of water though the interlamellar channels of the mako gill. However, mako and tuna gills differ in the sites of primary branchial resistance. In the mako, approximately 80% of the total branchial resistance resides in the septal channels, structures inherent to the elasmobranch gill that are not present in tunas. The added resistance at this location is compensated by a correspondingly lower resistance at the gill lamellae accomplished through wider interlamellar channels. Although greater interlamellar spacing minimizes branchial resistance, it also limits lamellar number and results in a lower total gill surface area for the mako relative to tunas. The morphology of the elasmobranch gill thus appears to constrain gill area and, consequently, limit mako aerobic performance to less than that of tunas.

SUMMARY Intertidal mudflats are highly productive ecosystems that impose severe environmental challenges on their occupants due to tidal oscillations and extreme shifts in habitat conditions. Reproduction on mudflats requires protection of developing eggs from thermal and salinity extremes,O 2 shortage, dislodgement by currents, siltation and predation. Mudskippers are air-breathing, amphibious fishes, and one of few vertebrates that reside on mudflats. They lay their eggs in mud burrows containing extremely hypoxic water, raising the question of how the eggs survive. We found that the Japanese mudskipper Periophthalmus modestus deposits its eggs on the walls of an air-filled chamber within its burrow. To ensure adequate O 2 for egg development, the burrow-guarding male mudskipper deposits mouthfuls of fresh air into the egg chamber during each low tide, a behaviour that can be upregulated by egg-chamber hypoxia. When egg development is complete the male, on a nocturnal rising tide, removes the egg-chamber air and releases it outside the burrow. This floods the egg chamber and induces egg hatching. Thus, P. modestus has developed a reproductive strategy that allows it to nurture eggs in this severe habitat rather than migrating away from the mudflat. This requires that mudskipper eggs be specialized to develop in air and that the air-breathing capacity of the egg-guarding male be integrated in a complex behavioural repertoire that includes egg guarding, ferrying air to and from the egg chamber, and sensing O 2 levels therein, all in concert with the tidal cycle.

SUMMARY Mitochondrial proton leak was assessed as a potential heat source in the slow, oxidative (red) locomotor muscle and liver of the shortfin mako shark( Isurus oxyrinchus ), a regional endotherm that maintains the temperature of both tissues elevated above ambient seawater temperature. We hypothesized that basal proton leak rates in red muscle and liver mitochondria of the endothermic shortfin mako shark would be greater than those of the ectothermic blue shark ( Prionace glauca ) and leopard shark( Triakis semifasciata ). Respiration rate and membrane potential in isolated mitochondria were measured simultaneously at 20°C using a Clark-type oxygen electrode and a lipophilic probe(triphenylmethylphosphonium, TPMP + ). Succinate-stimulated respiration was titrated with inhibitors of the electron transport chain, and the non-linear relationship between respiration rate and membrane potential was quantified. Mitochondrial densities of both tissues were measured by applying the point-contact method to electron micrographs so that proton leak activity of the entire tissue could be assessed. In all three shark species,proton leak occurred at a higher rate in red muscle mitochondria than in liver mitochondria. For each tissue, the proton leak curves of the three species overlapped and, at a membrane potential of 160 mV, mitochondrial proton leak rate (nmol H + min -1 mg -1 protein) did not differ significantly between the endothermic and ectothermic sharks. This finding indicates that red muscle and liver mitochondria of the shortfin mako shark are not specialized for thermogenesis by having a higher proton conductance. However, mako mitochondria did have higher succinate-stimulated respiration rates and membrane potentials than those of the two ectothermic sharks. This means that under in vivo conditions mitochondrial proton leak rates may be higher in the mako than in the ectothermic species, due to greater electron transport activity and a larger proton gradient driving proton leak. We also estimated each tissue's total proton leak by combining mitochondrial proton leak rates at 160 mV and tissue mitochondrial density data with published values of relative liver or red muscle mass for each of the three species. In red muscle, total proton leak was not elevated in the mako shark relative to the two ectothermic species. In the liver, total proton leak would be higher in the mako shark than in both ectothermic species, due to a lower proton conductance in the blue shark and a lower liver mitochondrial content in the leopard shark, and thus may contribute to endothermy.

SUMMARY A laboratory burrow and mudflat system was used to examine aspects of burrow air-phase maintenance and utilization by the amphibious mudskipper Scartelaos histophorus. While confined to its burrow during simulated`high tide', this species respires both aquatically and aerially, in the latter case utilizing an air phase it had established by transporting air into the burrow during simulated `low tide'. Over the course of `high-tide'confinement, burrow-water P O 2 declines, making the air phase more important for respiration; the burrow-water O 2 tension eliciting air-phase respiration is 4.8±0.2 kPa. At `low tide',when the fish has access to air, it deposits new air in the air phase by transporting gulps into the burrow and releasing them. Observed air-deposition rates for both males and females were 12.3±4.5 trips h -1 . All of the fish tested ( N =8 individuals + 2 pairs) deposited air and responded to experimental air-phase withdrawal by replacing the air (72 of 74 tests, 97.3%). Also, repeated tests with one fish showed that experimental reduction of the air-phase P O 2 by mixing with N 2 elicited a gas-expelling behavior at O 2 levels less than 10.3 kPa. At O 2 levels greater than 10.3 kPa, the fish left the air phase intact and added to it by depositing surface air.

SUMMARY Thunniform swimming, the capacity to conserve metabolic heat in red muscle and other body regions (regional endothermy), an elevated metabolic rate and other physiological rate functions, and a frequency-modulated cardiac output distinguish tunas from most other fishes. These specializations support continuous, relatively fast swimming by tunas and minimize thermal barriers to habitat exploitation, permitting niche expansion into high latitudes and to ocean depths heretofore regarded as beyond their range.

SUMMARY The swimming kinematics of the eastern Pacific bonito Sarda chiliensis at a range of sustained speeds were analyzed to test the hypothesis that the bonito's swimming mode differs from the thunniform locomotor mode of tunas. Eight bonito (fork length FL 47.5±2.1 cm, mass 1.25±0.15 kg) (mean ± s.d. ) swam at speeds of 50–130 cm s -1 at 18±2°C in the same temperature-controlled water tunnel that was used in previous studies of tunas. Kinematics variables, quantified from 60 Hz video recordings and analyzed using a computerized, two-dimensional motion analysis system, were compared with published data for similar sized tunas at comparable speeds. Bonito tailbeat frequency, tailbeat amplitude and stride length all increased significantly with speed. Neither yaw (6.0±0.6% FL ) nor propulsive wavelength (120±65% fish total length) varied with speed,and there were no mass or body-length effects on the kinematics variables for the size range of bonitos used. Relative to similar sized yellowfin( Thunnus albacares ) and skipjack ( Katsuwonus pelamis ) tunas at similar speeds, the bonito has a lower tailbeat frequency, a higher yaw and a greater stride length. The lateral displacement and bending angle of each intervertebral joint during a complete tailbeat cycle were determined for the bonito at a swimming speed of 90 cm s -1 . The pattern of mean maximum lateral displacement ( z max ) and mean maximum bending angle (β max ) along the body in the bonito differed from that of both chub mackerel Scomber japonicus and kawakawa tuna Euthynnus affinis ; z max was highest in the bonito. This study verifies that S. chiliensis is a carangiform swimmer and supports the hypothesis that the thunniform locomotor mode is a derived tuna characteristic associated with changes in this group's myotomal architecture. The finding that yaw and z max were greater in the bonito than in both mackerels and tunas suggests that swimming kinematics in the bonito is not intermediate between that of tunas and mackerels, as would be predicted on the basis of morphological characteristics.

SUMMARY The mako shark ( Isurus oxyrinchus ) has specialized vascular networks (retia mirabilia) forming counter-current heat exchangers that allow metabolic heat retention in certain regions of the body, including the aerobic, locomotor red muscle and the viscera. Red muscle, white muscle and stomach temperatures were measured in juvenile (5–13.6 kg) makos swimming steadily in a water tunnel and exposed to stepwise square-wave changes in ambient temperature ( T a ) to estimate the rates of heat transfer and to determine their capacity for the activity-independent control of heat balance. The rates of heat gain of red muscle during warming were significantly higher than the rates of heat loss during cooling, and neither the magnitude of the change in T a nor the direction of change in T a had a significant effect on red muscle latency time. Our findings for mako red muscle are similar to those recorded for tunas and suggest modulation of retial heat-exchange efficiency as the underlying mechanism controlling heat balance. However, the red muscle temperatures measured in swimming makos (0.3–3°C above T a ) are cooler than those measured previously in larger decked makos. Also, the finding of non-stable stomach temperatures contrasts with the predicted independence from T a recorded in telemetry studies of mako and white sharks. Our studies on live makos provide new evidence that, in addition to the unique convergent morphological properties between makos and tunas, there is a strong functional similarity in the mechanisms used to regulate heat transfer.

A large, sea-going water tunnel was used in various studies of shark swimming performance. The critical swimming velocity (U crit , an index of aerobically sustainable swimming speed) of a 70 cm long lemon shark ( Negaprion brevirostris Poey) was determined to be 1.1 Ls −1 , where L is body length. The U crit of the leopard shark ( Triakis semifasciata Girard) was found to vary inversely with body size; from about 1.6Ls −1 in 30–50cm sharks to 0.6LS −1 in 120cm sharks. Large Triakis adopt ram gill ventilation at swimming speeds between 27 and 60cms −1 , which is similar to the speed at which this transition occurs in teleosts. Analyses of tail-beat frequency (TBF) in relation to velocity and body size show that smaller Triakis have a higher TBF and can swim at higher relative speeds. TBF, however, approaches a maximal value at speeds approaching U crit , suggesting that red muscle contraction velocity may limit sustained swimming speed. The TBF of both Triakis and Negaprion rises at a faster rate with swimming velocity than does that of the more thunniform mako shark ( Isurus oxyrinchus Rafinesque). This is consistent with the expectation that, at comparable relative speeds, sharks adapted for efficient swimming should have a lower TBF. The rates of O 2 consumption of swimming lemon and mako sharks are among the highest yet measured for elasmobranchs and are comparable to those of cruise-adapted teleosts.

Changes in pericardial pressure, pericardial fluid volume, cardiac stroke volume and heart rate induced by swimming were monitored for Triakis semifasciata (Girard). Maximum pericardial pressure (P max , 0.07±0.03 kPa) in resting sharks was typically above ambient, whereas minimum pressure (Pmin, −0.08±0.03 kPa) was slightly subambient. During swimming, both P max (0.23±0.03 kPa) and P min (−0.02±0.03 kPa) became elevated, as did heart rate (51±2 to 55±2 beats min −1 ) and fractional cardiac stroke volume (0.49±0.03 to 0.65±0.04ml). After swimming, all variables fell, except fractional cardiac stroke volume. Estimates of total cardiac output from fractional cardiac stroke volume data during rest, exercise and recovery were 33.1, 56.2 and 60.4 ml kg ‘1 min’ 1, respectively. The occurrence of both elevated pericardial pressure and cardiac output during swimming argues against a primary role for pericardial-induced vis a fronte filling as the principal mechanism responsible for increasing cardiac output with exercise. Pericardial fluid loss via the pericardioperitoneal canal (PPC) occurs during swimming as a result of steady-state elevation of pericardial pressure, a series of transient high pericardial pressures, or both. Good general agreement seen for net pericardial fluid loss (0.6 ml kg − ) and the net increase in cardiac stroke volume (0.45 ml kg − ) during swimming establishes fluid displacement as a mechanism for increasing cardiac stroke volume and suggests that this is the primary function of the PPC.

The gills, adjacent buccopharyngeal epithelium, and skin of the swamp eel Synbranchus marmoratus (Bloch) function for both aerial and aquatic respiration. Aquatic cutaneous O 2 uptake occurs continuously at rates that, while dependent upon aquatic 2 tension (Pw O O2 ), are in direct proportion to body surface area. Branchial aquatic O 2 uptake takes place during intermittent ventilation which occurs in proportion to body mass. Because of reductions in the body surface area to volume ratio that occur with growth, cutaneous oxygen uptake comprises a larger percentage of the total oxygen uptake of small fish and, to compensate, large fish ventilate more. The mass exponent for total rate of oxygen uptake (V o o2 ) (0.894 ± 0.145) is within the range predicted from the contributions of cutaneous V o o2 (mass exponent 0.651 ± 0.167) and the number of minutes each hour that branchial ventilation occurs (0.378 ± 0.105). Hyperoxia increases cutaneous V O O2 and reduces branchial ventilation. Total V o o2 was also reduced in hyperoxia and calculations relating this to the reduction in ventilation time yield ventilatory cost estimates that increase with body size and that are high compared to those of other fish when the large component of cutaneous respiration in this species is considered. Large ventilatory costs reflect gill and branchial apparatus specialization for aerial respiration. Accessory cutaneous respiration and intermittent aquatic ventilation reduce these costs, and intermittent gill use in aquatic breathing, which is the exact analogue of the pattern for branchial respiratory use during air breathing, seems to optimize aquatic O 2 uptake with minimal ventilatory cost.

During anguilliform swimming at the surface, four half waves are present along the body of the yellow-bellied sea snake Pelamis platurus (Hydrophiidae). As in other anguilliform swimmers, wave amplitude increases towards the tail; however, the relative caudal amplitude of P. platurus is less than that of the aquatic snakes Natrix and Nerodia and the eel Anguilla . Kinematic analyses of near-surface swimming at 15 and 32cms −1 were made from high-speed ciné films, and Lighthill's bulk momentum hydromechanical model was used to calculate swimming thrust power at these two velocities. The total thrust power generated by a 51 cm snake is 3–641×10 −4 Js −1 at 15cms −1 and 29.877×10 −4 Js −1 at 32cms −1 , with respective Froude efficiencies of 79% and 81%. Subsurface swimming velocities are low (2–4cms −1 ), and snakes usually assume a posture in which the tail is elevated and the posterior part of the body assumes a nearly vertical orientation. Undulatory movements by snakes swimming in this posture involve torsional and rolling motions of the body which, through changes in the camber of the keel and body, may contribute to thrust.

Samples of unbuffered, whole blood from freshly-caught albacore ( Thunnus alalunga Bonnaterre) were equilibrated at 5, 10, 15, 20, 25, 30 and 35 °C and at 0 and 1 % CO 2 for construction of oxygen dissociation curves. A strong Bohr effect (−1.17), a negligible Root effect, and a reverse temperature effect (ΔH = +1.72 for 0% CO 2 and +0.26 for 1% CO 2 ) characterized these hyperbolic (Hill's n = 1.1) curves. The unusual reverse temperature effect was especially pronounced when blood was quickly warmed or cooled, simulating passage through the heat exchanging, countercurrent vascular rete system of this warm-bodied fish. A diagrammatic model of blood gas dynamics in the rete incorporating these in vitro data illustrates protection of arterial oxygen from premature haemoglobin dissociation and consequent loss to the venous circulation as blood warms in the rete. More conventional temperature effects on the carbon dioxide equilibria of albacore blood lower the P co co2 of venous blood being cooled in the rete. This reduces the venous-arterial P co co2 gradient, thereby minimizing the diffusion of CO 2 to arterial blood with resulting haemoglobin-oxygen dissociation via the strong Bohr effect. The temperature range (10–30 °C) over which the albacore haemoglobin-oxygen binding exhibits the reversed thermal effect closely matches the maximum thermal gradient (ambient water-core body temperature) typically present in this fish, suggesting that its highly specialized haemoglobin-oxygen dissociation characteristics evolved within-and now establishes thermal limits upon-the existing geographic distribution of this species.

Synbranchus marmoratus (Bloch) breathes air during terrestrial excursions and while dwelling in hypoxic water and utilizes its gills and adjacent buccopharyngeal epithelium as an air-breathing organ (ABO). This fish uses gills and skin for aquatic respiration in normoxic (air-saturated) water but when exposed to progressive aquatic hypoxia it becomes a metabolic O 2 conformer until facultative air breathing is initiated. The threshold Pw O O2 (aquatic O 2 tension or partial pressure in mmHg) that elicits air breathing in S. marmoratus is higher in larger fish. However, neither air-breathing threshold nor the blood haemoglobin (Hb) concentration of this species were changed following hypoxia (Pw O O2 < 20 mmHg) acclimation. In hypoxic water S. marmoratus supplies all of its metabolic O 2 requirement through air breathing. ABO volume scales with body weight raised to the power of 0.737 and the amount of O 2 that is removed from each air breath depends upon the length of time it is held in the ABO. Ambient Pw O O2 directly affects the air-breath duration of this fish, but the effect is smaller than in other species. Also, average air-breath duration (15.7 min at Pw O O2 0–20 mmHg) and the average inter-air-breath interval (15.1 min) of S. marmoratus are both longer than those of other air-breathing fishes. Although the gills of S. marmoratus are involved in aerial O 2 uptake, expelled air-breath CO 2 levels are not high and always closely correspond to ambient Pw CO CO2 , indicating that virtually no respiratory CO 2 is released to air by this fish. CO 2 extrusion therefore must occur aquatically either continuously across another exchange surface or intermittently across the gills during intervals between air breaths. This study with S. marmoratus from Panama reveals physiological differences between this population and populations in South America. The greater Hb content of South American S. marmoratus may be the result of different environmental selection pressures.

The armoured catfish, Ancistrus chagresi , is a facultative air breather and uses its stomach as an air-breathing organ (ABO). Comparisons of control fish and fish that had become acclimated to hypoxia and air breathing for 14–21 days were carried out to assess the effects of this treatment on bimodal (aerial and aquatic) gas exchange capacity. Hypoxia acclimation elicits physiological and biochemical changes that enable A. chagresi to increase O 2 utilization both by its gills in hypoxic water and by its ABO. Compared with control fish, hypoxia-acclimated Ancistrus have a higher blood--O 2 affinity and more haemoglobin (Hb) and can maintain a higher aquatic oxygen consumption rate (VO 2 ) in hypoxic (P w, O w, O2 , = 5–20 mmHg) water. They also have a 25% larger ABO volume, are able to hold each air breath longer, and can reduce ABO O 2 partial pressure to a lower level. In both groups, respiratory CO 2 -release occurs primarily through the gills. An air breath instantly causes tachycardia and a reduction in the frequency and amplitude of branchial ventilation. Their lower cardiac and gill ventilation rates in hypoxia and during air breathing suggest that hypoxia-acclimated fish are more adapted for hypoxia than are control fish. During the period an air breath is held in the ABO, hypoxia-acclimated fish exhibit more coordinated phase shifts in gill ventilation and cardiac rates. These may favour an initial phase of efficient aerial O 2 uptake from the ABO and transport through the body followed by a period of aquatic CO 2 release from the gills.

In response to progressive aquatic hypoxia, the armoured loricariid catfishes Ancistrus chagresi and Hypostomus plecostomus become facultative air-breathers and utilize their stomachs as accessory air-breathing organs. Hypostomus initiates air breathing at a higher aquatic O 2 tension ( P w, O w, O2 ) than does Ancistrus (60 v . 33 mmHg). Once begun, the air-breathing frequencies of both species increase with decreasing P w, O w, O2 ; the frequency of Ancistrus , however, is greater than and increases more with hypoxia than does that of Hypostomus , which appears to be a more efficient air breather. Hypoxia acclimation reduces the air-breathing rate of both species. A larger rate reduction occurs in Ancistrus , which, however, continues to require more frequent breaths than Hypostomus . Hypoxia acclimation does not affect the air-breathing threshold of either species, suggesting that external O 2 receptors initiate facultative air breathing. In progressive aquatic hypercapnia Ancistrus has a lower air-breathing CO 2 threshold (8.7 mmHg) than Hypostomus (12.8 mmHg). However, in some tests, individual fish of both species did not initiate air breathing even at P w, CO w, CO2 as high as 21 mmHg. Also, air breathing evoked by hypercapnia was short-lived; both species quickly compensated for this gas and resumed exclusively aquatic respiration within a few hours of exposure. Thus, CO 2 is not an important regulator of air breathing in these species. Between 25 and 35 °C, the P w, O w, O2 air breathing threshold of Ancistrus is temperature-independent, but air-breathing frequency increases with temperature. Ancistrus and Hypostomus do not breathe air in normoxic (air-saturated) water; their air-breathing responses are evoked by environmental hypoxia. This is fundamentally different from other fish species that breathe air in normoxia in order to meet heightened metabolic demands. Also, the facultative air-breathing adaptations of Ancistrus and Hypostomus differ in scope and magnitude from those utilized by species that breathe air in nor-moxia and adapt to hypoxia by increasing air-breathing rate.