The aerobic capacity of tunas: Adaptation for multiple metabolic demands

Vertical energy seascapes and diving behaviour modulate metabolic scope in a pelagic predator

Pelagic fishes must obtain resources in prey-sparse habitats and may be considered energy speculators with maximization, gambling high energy costs (e.g. metabolism) for a high rate of return (prey capture). As such, they may have to carefully use their energy seascape to obtain the resources necessary for high growth rates. For diving animals, their energy seascape will also have a vertical component in addition to a horizontal one, which is rarely considered. Dolphinfish, Coryphaena hippurus, embody the maximization strategy as they have high metabolic rates and fast growth rates. We coupled biologging on wild individuals with lab-based respirometry to estimate dolphinfish swimming metabolic rates and vertical energy seascapes. Dolphinfish performed continuous yo-yo dives with deeper dives at night but higher activity during the day. Dive descents were approximately 27% less costly than the ascents. Fish modulated their behaviour so that metabolic costs during the descent/ascent phases of deeper dives were less than those for shallow dives. While temperature is likely the primary limit of dive depth, the vertical energy seascape may be secondary and limit maximum dive depths. Studies of pelagic animal energy seascapes should consider the vertical component that will help determine their ability to access and utilize prey.

Clinical efficacy of Yukgunja-tang combined with a proton pump inhibitor for refractory gastroesophageal reflux disease: study protocol for randomized, double-blind, double-dummy clinical trial

BACKGROUND

Yukgunja-tang (YGJ) is an herbal prescription used to treat the symptoms of gastroesophageal reflux disease (GERD). Although many preclinical and clinical studies on YGJ have been conducted on GERD, there is a lack of evidence from blinded studies to exclude placebo effects. Therefore, this protocol proposes a clinical trial that is single-centered, randomized, double-blinded, double-dummy to objectively evaluate the efficacy and safety of co-administered YGJ and rabeprazole (RPZ) in patients with GERD previously treated with proton pump inhibitors (PPIs) and still experiencing symptoms.

METHODS

A total of 86 participants with refractory GERD (rGERD) will be randomized in a 1:1 ratio to the treatment [YGJ and RPZ (10 mg/d)] and control groups [double-dose RPZ (20 mg/d)] for 4 weeks of treatment (weeks 0-4) followed by 4 weeks of follow-up (weeks 4-8). The Frequency Scale for the Symptoms of GERD will be analyzed for the primary endpoint. Reflux Disease Questionnaire, Reflux Symptom Score, GERD-Health Related Quality of Life, Overall Treatment Evaluation, Spleen Qi Deficiency Questionnaire, Damum Questionnaire, and dyspepsia Visual Analogue Scale will be used to evaluate treatment effects on GERD related symptoms and quality of life and to compare treatment effects by subgroups. Safety tests will be analyzed by investigating adverse events.

DISCUSSION

This clinical trial will be the first rigorous double-blind, double-dummy, placebo-controlled study to precisely evaluate the efficacy and safety of the combination of YGJ and PPIs in the treatment of rGERD. The results of this study will provide a reliable clinical basis for selecting botanical drug treatments for patients with rGERD.

TRIAL REGISTRATION

Clinical Research Information Service (registration number: KCT0008600, July 13, 2023, https://cris.nih.go.kr ).

Whole genome sequencing of the fast-swimming Southern bluefin tuna (Thunnus maccoyii)

The economically important Southern bluefin tuna (Thunnus maccoyii) is a world-famous fast-swimming fish, but its genomic information is limited. Here, we performed whole genome sequencing and assembled a draft genome for Southern bluefin tuna, aiming to generate useful genetic data for comparative functional prediction. The final genome assembly is 806.54 Mb, with scaffold and contig N50 values of 3.31 Mb and 67.38 kb, respectively. Genome completeness was evaluated to be 95.8%. The assembled genome contained 23,403 protein-coding genes and 236.1 Mb of repeat sequences (accounting for 29.27% of the entire assembly). Comparative genomics analyses of this fast-swimming tuna revealed that it had more than twice as many hemoglobin genes (18) as other relatively slow-moving fishes (such as seahorse, sunfish, and tongue sole). These hemoglobin genes are mainly localized in two big clusters (termed as "MNˮ and "LAˮ respectively), which is consistent with other reported fishes. However, Thr39 of beta-hemoglobin in the MN cluster, conserved in other fishes, was mutated as cysteine in tunas including the Southern bluefin tuna. Since hemoglobins are reported to transport oxygen efficiently for aerobic respiration, our genomic data suggest that both high copy numbers of hemoglobin genes and an adjusted function of the beta-hemoglobin may support the fast-swimming activity of tunas. In summary, we produced a primary genome assembly and predicted hemoglobin-related roles for the fast-swimming Southern bluefin tuna.

A general swimming response in exhausted obligate swimming fish

Marine organisms normally swim at elevated speeds relative to cruising speeds only during strenuous activity, such as predation or escape. We measured swimming speeds of 29 ram ventilating sharks from 10 species and of three Atlantic bluefin tunas immediately after exhaustive exercise (fighting a capture by hook-and-line) and unexpectedly found all individuals exhibited a uniform mechanical response, with swimming speed initially two times higher than the cruising speeds reached approximately 6 h later. We hypothesized that elevated swimming behaviour is a means to increase energetic demand and drive the removal of lactate accumulated during capture via oxidation. To explore this hypothesis, we estimated the mechanical work that must have been spent by an animal to elevate its swim speed and then showed that the amount of lactate that could have been oxidized to fuel it comprises a significant portion of the amount of lactate normally observed in fishes after exhaustive exercise. An estimate for the full energetic cost of the catch-and-release event ensued.

A general swimming response in exhausted obligate swimming fish

Marine organisms normally swim at elevated speeds relative to cruising speeds only during strenuous activity, such as predation or escape. We measured swimming speeds of 29 ram ventilating sharks from 10 species and of three Atlantic bluefin tunas immediately after exhaustive exercise (fighting a capture by hook-and-line) and unexpectedly found all individuals exhibited a uniform mechanical response, with swimming speed initially two times higher than the cruising speeds reached approximately 6 h later. We hypothesized that elevated swimming behaviour is a means to increase energetic demand and drive the removal of lactate accumulated during capture via oxidation. To explore this hypothesis, we estimated the mechanical work that must have been spent by an animal to elevate its swim speed and then showed that the amount of lactate that could have been oxidized to fuel it comprises a significant portion of the amount of lactate normally observed in fishes after exhaustive exercise. An estimate for the full energetic cost of the catch-and-release event ensued.

Evaluating the Lipid Quality of Yellowfin Tuna (Thunnus albacares) Harvested from Different Oceans by Their Fatty Acid Signatures

The aim of this study was to evaluate the influence of the fishing location on yellowfin tuna’s (YFT; Thunnus albacares) white muscle total lipid (TL) content and fatty acid profile. The comparison included 45 YFT loins, equally divided between the Atlantic, Indian and Pacific Oceans. The ocean had no significant influence on YFT TL content, total PUFA and total n-3 PUFA (p > 0.05), averaging 1.09 g/100 g of muscle, 229.2 and 192.8 mg/100 g of muscle, respectively. On the other hand, the YFT harvested on the Indian Ocean displayed significantly higher contents of both SFA and MUFA totals than the Atlantic Ocean counterparts (p < 0.05), while the YFT harvested in the Pacific Ocean presented intermediate values, not differing significantly from the other two origins. The YFT from the Indian and Pacific oceans held twice the n-6 PUFA content recorded in the Atlantic YFT (44.1 versus 21.1 mg/100 g of muscle). Considering the recommended daily intake of EPA plus DHA is 250 mg, the consumption of 100 g of YFT from the Atlantic, Indian and Pacific Oceans would provide 149.2 mg, 191.8 mg or 229.6 mg of EPA plus DHA, representing 59.7%, 76.7% or 91.8% of the recommended daily intake, respectively.

Lipid and fatty acid dynamics in mature female albacore tuna (Thunnus alalunga) in the western Indian Ocean

Lipid composition in the reproductive and somatic tissues were investigated for female albacore tuna, Thunnus alalunga, in the western Indian Ocean, between latitude 18–21°S and longitude 56–60°E, from January 2014 to March 2015. Highest total lipids (TL) were found in the gonads of spawning-capable females (SCP) (mainly phospholipids, PL, triacylglycerols, TAG and wax esters, WE) and in the liver of females in the late regressing and regenerating ovary phases (mainly TAG, PL and sterols, ST). Muscle TL was low but exhibited high inter-individual variability. Correlations between gonadosomatic and hepatosomatic indices with TL and the lipid classes in albacore gonads and liver describes a pattern of reallocation of energy from the liver to the gonads during SCP. Female albacore were also observed to pursue foraging activities even during this period. Therefore, female albacore can be considered as a capital-income breeder relying mostly on stored lipids before the onset of reproduction and to a lesser extent on energy derived from concurrent feeding during the spawning season. Overall, the three examined tissues had similar general fatty acid profiles with the dominance of 22:6ω3 (docosahexaenoic acid, DHA), 16:0, 18:0 and 18:1ω9. The proportions of fatty acids varied with maturity stage and ovary lobe, with the smaller lobe having significantly higher proportions of essential fatty acids, as well as 16:0 and 18:1n9, compared to the larger one. Our results provide new information on the life-history and energy allocation strategy of albacore which will assist fisheries managers.

Mass Transport: Circulatory System with Emphasis on Nonendothermic Species

Mass transport can be generally defined as movement of material matter. The circulatory system then is a biological example given its role in the movement in transporting gases, nutrients, wastes, and chemical signals. Comparative physiology has a long history of providing new insights and advancing our understanding of circulatory mass transport across a wide array of circulatory systems. Here we focus on circulatory function of nonmodel species. Invertebrates possess diverse convection systems; that at the most complex generate pressures and perform at a level comparable to vertebrates. Many invertebrates actively modulate cardiovascular function using neuronal, neurohormonal, and skeletal muscle activity. In vertebrates, our understanding of cardiac morphology, cardiomyocyte function, and contractile protein regulation by Ca2+ highlights a high degree of conservation, but differences between species exist and are coupled to variable environments and body temperatures. Key regulators of vertebrate cardiac function and systemic blood pressure include the autonomic nervous system, hormones, and ventricular filling. Further chemical factors regulating cardiovascular function include adenosine, natriuretic peptides, arginine vasotocin, endothelin 1, bradykinin, histamine, nitric oxide, and hydrogen sulfide, to name but a few. Diverse vascular morphologies and the regulation of blood flow in the coronary and cerebral circulations are also apparent in nonmammalian species. Dynamic adjustments of cardiovascular function are associated with exercise on land, flying at high altitude, prolonged dives by marine mammals, and unique morphology, such as the giraffe. Future studies should address limits of gas exchange and convective transport, the evolution of high arterial pressure across diverse taxa, and the importance of the cardiovascular system adaptations to extreme environments. © 2017 American Physiological Society. Compr Physiol 7:17-66, 2017.

Substitutions in the Glycogenin-1 Gene Are Associated with the Evolution of Endothermy in Sharks and Tunas

Despite 400–450 million years of independent evolution, a strong phenotypic convergence has occurred between two groups of fish: tunas and lamnid sharks. This convergence is characterized by centralization of red muscle, a distinctive swimming style (stiffened body powered through tail movements) and elevated body temperature (endothermy). Furthermore, both groups demonstrate elevated white muscle metabolic capacities. All these traits are unusual in fish and more likely evolved to support their fast-swimming, pelagic, predatory behavior. Here, we tested the hypothesis that their convergent evolution was driven by selection on a set of metabolic genes. We sequenced white muscle transcriptomes of six tuna, one mackerel, and three shark species, and supplemented this data set with previously published RNA-seq data. Using 26 species in total (including 7,032 tuna genes plus 1,719 shark genes), we constructed phylogenetic trees and carried out maximum-likelihood analyses of gene selection. We inferred several genes relating to metabolism to be under selection. We also found that the same one gene, glycogenin-1, evolved under positive selection independently in tunas and lamnid sharks, providing evidence of convergent selective pressures at gene level possibly underlying shared physiology.

Pragmatic perspective on aerobic scope: peaking, plummeting, pejus and apportioning

A major challenge for fish biologists in the 21st century is to predict the biotic effects of global climate change. With marked changes in biogeographic distribution already in evidence for a variety of aquatic animals, mechanistic explanations for these shifts are being sought, ones that then can be used as a foundation for predictive models of future climatic scenarios. One mechanistic explanation for the thermal performance of fishes that has gained some traction is the oxygen and capacity-limited thermal tolerance (OCLTT) hypothesis, which suggests that an aquatic organism's capacity to supply oxygen to tissues becomes limited when body temperature reaches extremes. Central to this hypothesis is an optimum temperature for absolute aerobic scope (AAS, loosely defined as the capacity to deliver oxygen to tissues beyond a basic need). On either side of this peak for AAS are pejus temperatures that define when AAS falls off and thereby reduces an animal's absolute capacity for activity. This article provides a brief perspective on the potential uses and limitations of some of the key physiological indicators related to aerobic scope in fishes. The intent is that practitioners who attempt predictive ecological applications can better recognize limitations and make better use of the OCLTT hypothesis and its underlying physiology.

Emerging pharmacological therapy for functional dyspepsia

Functional dyspepsia (FD) is a multifactorial disease with complex underlying pathophysiology. To date, there is no established treatment for FD. This review summarizes recent progress in pharmacological therapy for the disease. A newly developed drug, acotiamide, is expected to improve symptoms of postprandial distress syndrome. Herbal medicines are also expected to become options for FD treatment.

Integrating dynamic energy budget (DEB) theory with traditional bioenergetic models

Dynamic energy budget (DEB) theory offers a systematic, though abstract, way to describe how an organism acquires and uses energy and essential elements for physiological processes, in addition to how physiological performance is influenced by environmental variables such as food density and temperature. A ‘standard’ DEB model describes the performance (growth, development, reproduction, respiration, etc.) of all life stages of an animal (embryo to adult), and predicts both intraspecific and interspecific variation in physiological rates. This approach contrasts with a long tradition of more phenomenological and parameter-rich bioenergetic models that are used to make predictions from species-specific rate measurements. These less abstract models are widely used in fisheries studies; they are more readily interpretable than DEB models, but lack the generality of DEB models. We review the interconnections between the two approaches and present formulae relating the state variables and fluxes in the standard DEB model to measured bioenergetic rate processes. We illustrate this synthesis for two large fishes: Pacific bluefin tuna (Thunnus orientalis) and Pacific salmon (Oncorhynchus spp.). For each, we have a parameter-sparse, full-life-cycle DEB model that requires adding only a few species-specific features to the standard model. Both models allow powerful integration of knowledge derived from data restricted to certain life stages, processes and environments.

The physiological response to anthropogenic stressors in marine elasmobranch fishes: a review with a focus on the secondary response

Elasmobranchs (sharks, rays, and skates) are currently facing substantial anthropogenic threats, which expose them to acute and chronic stressors that may exceed in severity and/or duration those typically imposed by natural events. To date, the number of directed studies on the response of elasmobranch fishes to acute and chronic stress are greatly exceeded by those related to teleosts. Of the limited number of studies conducted to date, most have centered on sharks; batoids are poorly represented. Like teleosts, sharks exhibit primary and secondary responses to stress that are manifested in their blood biochemistry. The former is characterized by immediate and profound increases in circulating catecholamines and corticosteroids, which are thought to mobilize energy reserves and maintain oxygen supply and osmotic balance. Mediated by these primary responses, the secondary effects of stress in elasmobranchs include hyperglycemia, acidemia resulting from metabolic and respiratory acidoses, and profound disturbances to ionic, osmotic, and fluid volume homeostasis. The nature and magnitude of these secondary effects are species-specific and may be tightly linked to metabolic scope and thermal physiology as well as the type and duration of the stressor. In fishes, acute and chronic stressors can incite a tertiary response, which involves physiological changes at the organismal level, thereby impacting growth rates, reproductive outputs or investments, and disease resistance. Virtually no studies to date have been conducted on the tertiary stress response in elasmobranchs. Given the diversity of elasmobranchs, additional studies that characterize the nature, magnitude, and consequences of physiological stress over a broad spectrum of stressors are essential for the development of conservation measures. Additional studies on the primary, secondary, and tertiary stress response in elasmobranchs are warranted, with particular emphasis on expanding the range of species and stressors examined. Future studies should move beyond simply studying the effects of known stressors and focus on the underlying physiological mechanisms. Such studies should include the coupling of stress indicators with quantifiable aspects of the stressor, which will allow researchers to test hypotheses on survivorship and, ultimately, derive models that effectively link physiology to mortality. Studies of this nature are essential for decision-making that will result in the effective management and conservation of these species.

A full lifecycle bioenergetic model for bluefin tuna

We formulated a full lifecycle bioenergetic model for bluefin tuna relying on the principles of Dynamic Energy Budget theory. Traditional bioenergetic models in fish research deduce energy input and utilization from observed growth and reproduction. In contrast, our model predicts growth and reproduction from food availability and temperature in the environment. We calibrated the model to emulate physiological characteristics of Pacific bluefin tuna (Thunnus orientalis, hereafter PBT), a species which has received considerable scientific attention due to its high economic value. Computer simulations suggest that (i) the main cause of different growth rates between cultivated and wild PBT is the difference in average body temperature of approximately 6.5°C, (ii) a well-fed PBT individual can spawn an average number of 9 batches per spawning season, (iii) food abundance experienced by wild PBT is rather constant and sufficiently high to provide energy for yearly reproductive cycle, (iv) energy in reserve is exceptionally small, causing the weight-length relationship of cultivated and wild PBT to be practically indistinguishable and suggesting that these fish are poorly equipped to deal with starvation, (v) accelerated growth rate of PBT larvae is connected to morphological changes prior to metamorphosis, while (vi) deceleration of growth rate in the early juvenile stage is related to efficiency of internal heat production. Based on these results, we discuss a number of physiological and ecological traits of PBT, including the reasons for high Feed Conversion Ratio recorded in bluefin tuna aquaculture.

Comparative metabolic rates of common western North Atlantic Ocean sciaenid fishes

The resting metabolic rates (R(R)) of western North Atlantic Ocean sciaenids, such as Atlantic croaker Micropogonias undulatus, spot Leiostomus xanthurus and kingfishes Menticirrhus spp., as well as the active metabolic rates (R(A)) of M. undulatus and L. xanthurus were investigated to facilitate inter and intraspecific comparisons of their energetic ecology. The R(R) of M. undulatus and L. xanthurus were typical for fishes with similar lifestyles. The R(R) of Menticirrhus spp. were elevated relative to those of M. undulatus and L. xanthurus, but below those of high-energy-demand species such as tunas Thunnus spp. and dolphinfish Coryphaena hippurus. Repeated-measures non-linear mixed-effects models were applied to account for within-individual autocorrelation and corrected for non-constant variance typical of noisy R(A) data sets. Repeated-measures models incorporating autoregressive first-order [AR(1)] and autoregressive moving average (ARMA) covariances provided significantly superior fits, more precise parameter estimates (i.e. reduced s.e.) and y-intercept estimates that more closely approximated measured R(R) for M. undulatus and L. xanthurus than standard least-squares regression procedures.

The energetics of embryonic growth

Embryos typically operate under much tighter energy constraints than older animals. This has had a profound impact on how energy is stored, mobilized and partitioned. The result is sometimes quite different ways of doing things. Growth, in particular, is a much more important activity during development. Compared with adults, specific growth rates (g) are extremely high (≥150%day(-1) for some fish). Production efficiencies are also much higher, particularly for early stages where values of 80-90% are not uncommon. Higher production efficiencies are possible, in part, because of lower unit costs at high g. Unlike in adults, the unit cost of growth does not appear to be fixed during early life. Energy also tends to be partitioned in a different manner, with compensatory partitioning being much more important during early life. Other differences include much higher routine metabolic intensities, smaller aerobic scopes and approximately isometric scaling of routine metabolism. The implications for ontogenetic growth models are discussed.

Properties of Rikkunshi-to (TJ-43)-induced relaxation of rat gastric fundus smooth muscles

The relaxant effects of Rikkunshi-to (TJ-43), a gastroprotective herbal medicine, on rat gastric fundus were investigated. Experiments were carried out using standard tension and intracellular microelectrode recording techniques. During contraction induced by enprostil (0.5 microM), a prostaglandin E(2) analog, TJ-43, produced relaxation dose dependently (0.1-5.0 mg/ml) in the rat fundic circular smooth muscle (CSM) strips. The relaxant effects of TJ-43 were not affected by tetrodotoxin or 1 H[1, 2, 4] oxadiazolo [4, 3-a] quinoxalin-1-one (10 microM), an inhibitor of soluble guanylate cyclase. TJ-43 inhibited enprostil-induced membrane depolarization. Apamin (1 microM), a blocker of small-conductance Ca(2+)-activated K(+) (SK) channel, inhibited T-43-induced membrane repolarization. TJ-43-induced relaxation was biphasic, comprising of an initial fast followed by a second slow relaxation. The fast relaxation was abolished by apamin. Application of high K(+) (29.4 mM [K(+)](o)) also abolished the fast relaxation induced by TJ-43. In diabetic Goto-Kakizaki (GK) rat fundic CSM strips, the relaxant responses of TJ-43 during enprostil-induced contraction were increased compared with control rat strips. These results indicate that TJ-43 elicited fast muscle relaxation through membrane hyperpolarization induced by the activation of SK channels; the time-dependent slow relaxation reflects an additional direct of TJ-43 on CSM in the rat gastric fundus. Because TJ-43-evoked relaxation of fundic CSM strips was more potent in diabetic GK rat than in control rat, further analysis of this herb could lead to better treatments of diabetic gastroparesis.

Postprandial metabolic increment of southern bluefin tuna Thunnus maccoyii ingesting high or low-lipid sardines Sardinops sagax

This study examined the postprandial metabolism and swimming speed of southern bluefin tuna Thunnus maccoyii when fed sardines Sardinops sagax of either high-lipid and high-energy content or low-lipid and low-energy content. Five groups of two or three T. maccoyii (mean+/-s.e. mass=19.8+/-0.5 kg, n=14) were fed either low [2.2% lipid, 5.5 MJ kg(-1) gross energy (GE)] or high-lipid (12.9%, 9.2 MJ kg(-1) GE) S. sagax. Before feeding, T. maccoyii swam at 0.74+/-0.03 body lengths s(-1) (n=5) and their routine metabolic rate was 305+/-15 mg kg(-1) h(-1). Swimming speed and metabolic rate of T. maccoyii increased following feeding. Thunnus maccoyii swam 1.3 and 1.8 times faster during digestion of low and high-lipid S. sagax, respectively. Postprandial peak metabolic rate, duration of elevated metabolism and total postprandial metabolic increment were all greater for T. maccoyii that ingested high-lipid S. sagax. When total postprandial increment is represented as a proportion of ingested energy, there was no difference between high and low-lipid meals, equating to between 30 and 35% of ingested energy. It was estimated that increased postprandial swimming costs account for 25 and 46% of the total postprandial metabolic response for low and high-lipid S. sagax meals, respectively. Specific dynamic action (SDA) accounts for c. 20% of ingested energy regardless of S. sagax lipid level. These results confirm that the postprandial metabolic increment of T. maccoyii is greater than most other fish species. Much of the high cost of postprandial metabolic increment can be attributed to increased postprandial swimming costs. For T. maccoyii, it appears that activity and SDA are not independent, which complicates bioenergetic evaluation. High postprandial metabolic costs accentuate the great energetic requirements of T. maccoyii.

Heterologous hybridization to a complementary DNA microarray reveals the effect of thermal acclimation in the endothermic bluefin tuna (Thunnus orientalis)

The temperature stress that pelagic fishes experience can induce physiological and behavioural changes that leave a signature in gene expression profiles. We used a functional genomics approach to identify genes that were up- or down-regulated following thermal stress in the Pacific bluefin tuna. Following the acclimation period, 113, 81 and 196 genes were found to be differentially expressed between the control (20 degrees C) and cold (15 degrees) treatment groups, in ventricle, red muscle and white muscle, respectively. The genes whose expression levels were responsive to thermal acclimation varied according to muscle fibre type, perhaps reflecting the tissue-specific degrees of endothermy characteristic of this species.

Routine metabolic rate of southern bluefin tuna (Thunnus maccoyii)

Routine metabolic rate (RMR) was measured in fasting southern bluefin tuna, Thunnus maccoyii, the largest tuna species studied so far (body mass=19.6 kg (+/-1.9 SE)). Mean mass-specific RMR was 460 mg kg(-1) h(-1) (+/-34.9) at a mean water temperature of 19 degrees C. When evaluated southern bluefin tuna standard metabolic rate (SMR) is added to published values of other tuna species, there is a strong allometeric relationship with body mass (423 M(0.86), R(2)=0.97). This demonstrates that tuna interspecific SMR scale with respect to body mass similar to that of other active teleosts, but is approximately 4-fold higher. However, RMR (not SMR) is most appropriate in ram-ventilating species that are physiologically unable to achieve complete rest. Respiration was measured in a large (250,000 l) flexible polypropylene respirometer (mesocosm respirometer) that was deployed within a marine-farm sea cage for 29 days. Fasted fish were maintained within the respirometer up to 42 h while dissolved oxygen dropped by 0.056 (+/-0.004) mg l(-1) h(-1). Fish showed no obvious signs of stress. They swam at 1.1 (+/-0.1) fork lengths per second and several fed within the respirometer immediately after measurements.

The energetic consequence of specific dynamic action in southern bluefin tuna Thunnus maccoyii

The effect of feeding on the rate of oxygen consumption (M(O2)) of four groups of three southern bluefin tuna Thunnus maccoyii (SBT) was examined in a large static respirometer at water temperatures of 18.2-20.3 degrees C. Six feeding events of rations between 2.1-8.5% body mass (%M(b)) of Australian sardines (Sardinops neopilchardus) were recorded (two of the groups were fed twice). Before feeding, fish swam between 0.71 and 1.4 body lengths s(-1) (BL s(-1)) and the routine metabolic rate (RMR) was 366+/-32.5 mg kg(-1) h(-1) (mean +/- s.e.m.). For all trials, M(O2) was elevated post feeding, presumably as a result of specific dynamic action (SDA). Swimming velocity was also elevated post feeding for periods similar to that of M(O2) (between 20-45 h, longest for the largest rations). Post feeding swimming velocity increased to between 0.87-2.6 BL s(-1) and was also dependent on ration consumed. It is suggested that the purpose of increased post-feeding swimming velocity was to increase ventilation volume as a response to the enhanced metabolic demand associated with SDA. Peak post-prandial M(O2) increased linearly with ration size to a maximum of 1290 mg kg(-1) h(-1), corresponding to 2.8 times the RMR. When converted to its energy equivalent, total magnitude of SDA was linearly correlated with ration size to a maximum of 192 kJ kg(-1) h(-1), and as a proportion of gross energy ingested (SDA coefficient), it averaged 35+/-2.2%. These results demonstrate that, although the factorial increase of SDA in SBT is similar to that of other fish species, the absolute energetic cost of SDA is much higher. These results support the contention that tuna are energy speculators, gambling high rates of energy expenditure for potentially higher rates of energy returns. The ration that southern bluefin tuna require to equal the combined metabolic costs of SDA and RMR is estimated in this study to be 3.5%M(b) of Australian sardines per day.

Influence of Swimming Speed on Metabolic Rates of Juvenile Pacific Bluefin Tuna and Yellowfin Tuna

Bluefin tuna are endothermic and have higher temperatures, heart rates, and cardiac outputs than tropical tuna. We hypothesized that the increased cardiovascular capacity to deliver oxygen in bluefin may be associated with the evolution of higher metabolic rates. This study measured the oxygen consumption of juvenile Pacific bluefin Thunnus orientalis and yellowfin tuna Thunnus albacares swimming in a swim-tunnel respirometer at 20 degrees C. Oxygen consumption (Mo2) of bluefin (7.1-9.4 kg) ranged from 235+/-38 mg kg(-1) h(-1) at 0.85 body length (BL) s(-1) to 498+/-55 mg kg(-1) h(-1) at 1.80 BL s(-1). Minimal metabolic rates of swimming bluefin were 222+/-24 mg O(2) kg(-1) h(-1) at speeds of 0.75 to 1.0 BL s(-1). Mo2 of T. albacares (3.7-7.4 kg) ranged from 164+/-18 mg kg(-1) h(-1) at 0.65 BL s(-1) to 405+/-105 mg kg(-1) h(-1) at 1.8 BL s(-1). Bluefin tuna had higher metabolic rates than yellowfin tuna at all swimming speeds tested. At a given speed, bluefin had higher metabolic rates and swam with higher tailbeat frequencies and shorter stride lengths than yellowfin. The higher M dot o2 recorded in Pacific bluefin tuna is consistent with the elevated cardiac performance and enhanced capacity for excitation-contraction coupling in cardiac myocytes of these fish. These physiological traits may underlie thermal-niche expansion of bluefin tuna relative to tropical tuna species.

Cardiorespiratory modifications, and limitations, in post-smolt growth hormone transgenic Atlantic salmon Salmo salar

In recent years, there has been a great deal of interest in how growth hormone (GH) transgenesis affects fish physiology. However, the results of these studies are often difficult to interpret because the transgenic and non-transgenic fish had very different environmental/rearing histories. This study used a stable line of size-matched GH Atlantic salmon (Salmo salar) that were reared in a shared tank with controls (at 10 degrees C, for approximately 9 months) to perform a comprehensive examination of the cardiorespiratory physiology of GH transgenic salmon, and serves as a novel test of the theory of symmorphosis. The GH transgenic salmon had a 3.6x faster growth rate, and 21 and 25% higher values for mass-specific routine and standard oxygen consumption (M(O(2))), respectively. However, there was no concurrent increase in their maximum M(O(2)), which resulted in them having an 18% lower metabolic scope and a 9% reduction in critical swimming speed. This decreased metabolic capacity/performance was surprising given that the transgenics had a 29% larger heart with an 18% greater mass-specific maximum in situ cardiac output, a 14% greater post-stress blood haemoglobin concentration, 5-10% higher red muscle and heart aerobic enzyme (citrate synthase or cytochrome oxidase) activities, and twofold higher resting and 1.7x higher post-stress, catecholamine levels. However, gill surface area was the only cardiorespiratory parameter that was not enhanced, and our data suggest that gill oxygen transfer may have been limiting. Overall, this research: (1) shows that there are significant metabolic costs associated with GH transgenesis in this line of Atlantic salmon; (2) provides the first direct evidence that cardiac function is enhanced by GH transgenesis; (3) shows that a universal upregulation of post-smolt (adult) GH transgenic salmon cardiorespiratory physiology, as suggested by symmorphosis, does not occur; and (4) supports the idea that whereas differences in arterial oxygen transport (i.e. cardiac output and blood oxygen carrying capacity) are important determinants of inter-specific differences in aerobicity, diffusion-limited processes must be enhanced to achieve substantial intra-specific improvements in metabolic and swimming performance.

Evolution and consequences of endothermy in fishes

Regional endothermy, the conservation of metabolic heat by vascular countercurrent heat exchangers to elevate the temperature of the slow-twitch locomotor muscle, eyes and brain, or viscera, has evolved independently among several fish lineages, including lamnid sharks, billfishes, and tunas. All are large, active, pelagic species with high energy demands that undertake long-distance migrations and move vertically within the water column, thereby encountering a range of water temperatures. After summarizing the occurrence of endothermy among fishes, the evidence for two hypothesized advantages of endothermy in fishes, thermal niche expansion and enhancement of aerobic swimming performance, is analyzed using phylogenetic comparisons between endothermic fishes and their ectothermic relatives. Thermal niche expansion is supported by mapping endothermic characters onto phylogenies and by combining information about the thermal niche of extant species, the fossil record, and paleoceanographic conditions during the time that endothermic fishes radiated. However, it is difficult to show that endothermy was required for niche expansion, and adaptations other than endothermy are necessary for repeated diving below the thermocline. Although the convergent evolution of the ability to elevate slow-twitch, oxidative locomotor muscle temperatures suggests a selective advantage for that trait, comparisons of tunas and their ectothermic sister species (mackerels and bonitos) provide no direct support of the hypothesis that endothermy results in increased aerobic swimming speeds, slow-oxidative muscle power, or energetic efficiency. Endothermy is associated with higher standard metabolic rates, which may result from high aerobic capacities required by these high-performance fishes to conduct many aerobic activities simultaneously. A high standard metabolic rate indicates that the benefits of endothermy may be offset by significant energetic costs.

Electrophysiological properties of the L-type Ca2+current in cardiomyocytes from bluefin tuna and Pacific mackerel

Tunas are capable of exceptionally high maximum metabolic rates; such capability requires rapid delivery of oxygen and metabolic substrate to the tissues. This requirement is met, in part, by exceptionally high maximum cardiac outputs, opening the possibility that myocardial Ca2+delivery is enhanced in myocytes from tuna compared with those from other fish. In this study, we investigated the electrophysiological properties of the cardiac L-type Ca2+channel current ( ICa) to test the hypothesis that Ca2+influx would be larger and have faster kinetics in cardiomyocytes from Pacific bluefin tuna ( Thunnus orientalis) than in those from its sister taxon, the Pacific mackerel ( Scomber japonicus). In accordance with this hypothesis, ICain atrial myocytes from bluefin tuna had significantly greater peak current amplitudes and faster fast inactivation kinetics (-4.4 ± 0.2 pA/pF and 25.9 ± 1.6 ms, respectively) than those from mackerel (-2.7 ± 0.5 pA/pF and 32.3 ± 3.8 ms, respectively). Steady-state activation, inactivation, and recovery from inactivation were also faster in atrial myocytes from tuna than from mackerel. In ventricular myocytes, current amplitude and activation and inactivation rates were similar in both species but elevated compared with those of other teleosts (Vornanen M. Am J Physiol Regul Integr Comp Physiol 272: R1432-R1440, 1997). These results indicate enhanced ICain atrial myocytes from bluefin tuna compared with Pacific mackerel; this enhanced ICamay be associated with elevated cardiac performance, because ICadelivers the majority of Ca2+involved in excitation-contraction coupling in most fish hearts. Similarly, ICais enhanced in the ventricle of both species compared with other teleosts and may play a role in the robust cardiac performance of fishes of the family Scombridae.

Scaling effects on metabolism of a teleost

Scaling effects on citrate synthase (CS), glucose-6-phosphate dehydrogenase (G6-PDH), RNA. RNA/DNA ratio and protein contents of brain, liver and skeletal muscle were studied in a teleost, Clarias batrachus. The activity of white skeletal muscle CS decreased significantly as a function of increasing body mass of the fish. It shows that the fulfilment of energy demand in white skeletal muscle is not dependent on aerobic metabolism. The activity of liver G6-PDH decreased with the increasing body mass showing reduction in NADPH generation for lipogenic activity. However, increase in G6-PDH activity showed enhancement in reductive synthesis in skeletal muscle of the larger-sized individuals. A positive scaling of RNA, RNA/DNA ratio and protein contents reflects changes in macromolecular turnover for ATP-supplying enzymes and proteins.

Diseases of tunas, Thunnus spp

Much is known about those aspects of tuna health which can be studied in wild populations, e.g. helminth parasites. However, because aquaculture of these species is in its infancy, knowledge of microbial, nutritional and environmental diseases is limited. This review is an attempt to bring together the available information on those diseases of Thunnus spp. which cause significant morbidity, mortality or economic loss. In doing so it has become clear that much more research needs to be undertaken on the physiology of the species (southern, northern and Pacific bluefin tuna) currently used in aquaculture in order for the pathogenesis of some conditions to be properly understood. Attempts at hatchery culture of Pacific bluefin tuna has indicated that Thunnus spp. will be problematic to hatch and propagate.

Environmental and functional limits to muscular exercise and body size in marine invertebrate athletes

Many similarities exist between the key characteristics of muscular metabolism in marine invertebrates and those found in vertebrate striated muscle, even though there are important phosphagens and glycolytic end products that differ between groups. Lifestyles and modes of locomotion also vary extremely among invertebrates thereby shaping the pattern of exercise metabolism. In accordance with the limited availability of integrated ecological and physiological information the present paper reports recent progress in the exercise physiology of cephalopods, which are characterized by high rates of aerobic and anaerobic energy turnover during high velocity hunts or escapes in their pelagic environment, and a sipunculid worm, which mostly uses anaerobic resources during extended marathon-like digging excursions in the hypoxic marine sediment. Particular attention is paid to how lifestyle and oxygen availability in various marine environments shapes the use and rates of aerobic and anaerobic metabolism and acidosis as they depend on activity levels and energy saving strategies. Whereas aerobic scope and, accordingly, use of ambient oxygen by blood oxygen transport and skin respiration is maximized in some squids, aerobic scope is very small in the worm and anaerobic metabolism readily used upon muscular activity. Until recently, it was widely accepted that the glycolytic end product octopine, produced in the musculature of these invertebrates, acted as a weak acid and so did not compromise acid-base balance. However, it has now been demonstrated that octopine does cause acidosis. Concomitant study of tissue energy and acid-base status allows to evaluate the contribution of glycolysis, pH and free ADP accumulation to the use of the phosphagen and to the delayed drop in the Gibb's free energy change of ATP hydrolysis. The analysis reveals species specific capacities of these mechanisms to support exercise beyond the anaerobic threshold. During high intensity anaerobic exercise observed in squid, the levels of ATP free energy change finally fall to critical minimum levels contributing to fatigue. Maintenance of sufficiently high energy levels is found at low but extended rates of anaerobic metabolism as observed in the long term digging sipunculid worm. The greatest aerobic and anaerobic performance levels are seen in squid inhabiting the open ocean and appear to be made possible by the uniform and stable physicochemical parameters (esp. high O(2) and low CO(2) levels) that characterize such an environment. It is suggested that at least some squid operate at their functional and environmental limits. In extremely different environments, both the worm and the squids display a tradeoff between oxygen availability, temperature, performance level and also, body size.

Physiological basis of temperature-dependent biogeography: trade-offs in muscle design and performance in polar ectotherms

Polar, especially Antarctic, oceans host ectothermic fish and invertebrates characterized by low-to-moderate levels of motor activity; maximum performance is reduced compared with that in warmer habitats. The present review attempts to identify the trade-offs involved in adaptation to cold in the light of progress in the physiology of thermal tolerance. Recent evidence suggests that oxygen limitations and a decrease in aerobic scope are the first indications of tolerance limits at both low and high temperature extremes. The cold-induced reduction in aerobic capacity is compensated for at the cellular level by elevated mitochondrial densities, accompanied by molecular and membrane adjustments for the maintenance of muscle function. Particularly in the muscle of pelagic Antarctic fish, among notothenioids, the mitochondrial volume densities are among the highest known for vertebrates and are associated with cold compensation of aerobic metabolic pathways, a reduction in anaerobic scope, rapid recovery from exhaustive exercise and enhanced lipid stores as well as a preference for lipid catabolism characterized by high energy efficiency at high levels of ambient oxygen supply. Significant anaerobic capacity is still found at the very low end of the activity spectrum, e.g. among benthic eelpout (Zoarcideae).

In contrast to the cold-adapted eurytherms of the Arctic, polar (especially Antarctic) stenotherms minimize standard metabolic rate and, as a precondition, the aerobic capacity per milligram of mitochondrial protein,thereby minimizing oxygen demand. Cost reductions are supported by the downregulation of the cost and flexibility of acid—base regulation. At maintained factorial scopes, the reduction in standard metabolic rate will cause net aerobic scope to be lower than in temperate species. Loss of contractile myofilaments and, thereby, force results from space constraints due to excessive mitochondrial proliferation. On a continuum between low and moderately high levels of muscular activity, polar fish have developed characteristics of aerobic metabolism equivalent to those of high-performance swimmers in warmer waters. However, they only reach low performance levels despite taking aerobic design to an extreme.

EFFECTS OF HIGH INTENSITY EXERCISE TRAINING ON CARDIOVASCULAR FUNCTION,OXYGEN UPTAKE, INTERNAL OXYGEN TRANSPORT AND OSMOTIC BALANCE IN CHINOOK SALMON(ONCORHYNCHUS TSHAWYTSCHA) DURING CRITICAL SPEED SWIMMING

To examine cardiorespiratory plasticity, cardiovascular function, oxygen consumption, oxygen delivery and osmotic balance were measured at velocities up to critical swimming speed (Ucrit) in seawater-adapted chinook salmon. We used two groups of fish. The control group had swum continuously for 4 months at a low intensity (0.5 BL s-1)and the other was given a high-intensity training regimen (a Ucrit swim test on alternate days) over the same period of time. Compared with available data for other salmonid species, the control group had a higher maximum oxygen consumption(Ṁo2max; 244μmol O2 min-1 kg-1), cardiac output(Q̇max; 65 ml min-1 kg-1) and blood oxygen content(CaO2; 15 ml O2 dl-1). Exercise training caused a 50% increase in Ṁo2max without changing either Ucrit or CaO2, even though there were small but significant increases in hematocrit, hemoglobin concentration and relative ventricular mass. During swimming, however,exercise-trained fish experienced a smaller decrease in body mass and muscle moisture, a smaller increase in plasma osmolality, and reduced venous oxygen stores compared with control fish. Consequently, exercise training apparently diminished the osmo—respiratory compromise, but improved oxygen extraction at the tissues. We conclude that the training-induced increase in Ṁo2max provided benefits to systems other than the locomotory system, such as osmoregulation,enabling trained fish to better multitask physiological functions while swimming. Furthermore, because a good interspecific correlation exists between Ṁo2max and arterial oxygen supply(Ṫo2max; r2=0.99) among temperate fish species, it is likely that CaO2 and Q̇max are principal loci for cardiorespiratory evolutionary adaptation but not for intraspecific cardiorepiratory plasticity as revealed by high intensity exercise training.

Review: Analysis of the evolutionary convergence for high performance swimming in lamnid sharks and tunas

Elasmobranchs and bony fishes have evolved independently for more than 400 million years. However, two Recent groups, the lamnid sharks (Family Lamnidae) and tunas (Family Scombridae), display remarkable similarities in features related to swimming performance. Traits separating these two groups from other fishes include a higher degree of body streamlining, a shift in the position of the aerobic, red, locomotor muscle that powers sustained swimming to a more anterior location in the body and nearer to the vertebral column, the capacity to conserve metabolic heat (i.e. regional endothermy), an increased gill surface area with a decreased blood-water barrier thickness, a higher maximum blood oxygen carrying capacity, and greater muscle aerobic and anaerobic enzyme activities at in vivo temperatures. The suite of morphological, physiological, and biochemical specializations that define "high-performance fishes" have been extensively characterized in the tunas. This review examines the convergent features of lamnid sharks and tunas in order to gain insight into the extent that comparable environmental selection pressures have led to the independent origin of similar suites of functional characteristics in these two distinctly different taxa. We propose that, despite differences between teleost and elasmobranch fishes, lamnid sharks and tunas have evolved morphological and physiological specializations that enhance their swimming performance relative to other sharks and most other high performance pelagic fishes.

Hydrogen ion binding properties of tuna haemoglobins

Tunas are very active fish with a high aerobic capacity, but they also regularly perform burst swimming with massive production of lactic acid. The present study examines whether H(+) buffering by tuna haemoglobin (Hb) is elevated to cope with metabolic acidoses (by analogy with the high buffer capacity of tuna white musculature) or whether the Hb-H(+) binding properties resemble other teleosts that have low buffer values and high Haldane effects. H(+) titration of oxygenated and deoxygenated composite Hb from yellowfin tuna, skipjack tuna and bigeye tuna in 0.1 M KCl revealed low Hb-specific buffer values in all three tunas. Values at physiological pH were comparable to those reported in less active species such as carp and eel. The fixed acid Haldane effect was large (maximal uptakes of close to 4 mol H(+) per mol Hb tetramer upon deoxygenation). Thus, the Hb-H(+) binding properties of very active tuna species correspond to other teleosts. Low Hb buffer values may be a pre-requisite for the regulation of red blood cell pH via Na(+)/H(+) exchange. Approximately nine "neutral" groups were titratable in tuna Hbs, suggesting that two alpha-amino groups and seven histidine residues are titrated within each tetramer.