Larger Gill Surface Area Research Articles

Gill surface area provides a clue for the respiratory basis of brain size in the blacktip shark (Carcharhinus limbatus).

Brain size varies dramatically, both within and across species, and this variation is often believed to be the result of trade-offs between the cognitive benefits of having a large brain for a given body size and the energetic cost of sustaining neural tissue. One potential consequence of having a large brain is that organisms must also meet the associated high energetic demands. Thus, a key question is whether metabolic rate correlates with brain size. However, using metabolic rate to measure energetic demand yields a relatively instantaneous and dynamic measure of energy turnover, which is incompatible with the longer evolutionary timescale of changes in brain size within and across species. Morphological traits associated with oxygen consumption, specifically gill surface area, have been shown to be correlates of oxygen demand and energy use, and thus may serve as integrated correlates of these processes, allowing us to assess whether evolutionary changes in brain size correlate with changes in longer-term oxygen demand and energy use. We tested how brain size relates to gill surface area in the blacktip shark Carcharhinus limbatus. First, we examined whether the allometric slope of brain mass (i.e., the rate that brain mass changes with body mass) is lower than the allometric slope of gill surface area across ontogeny. Second, we tested whether gill surface area explains variation in brain mass, after accounting for the effects of body mass on brain mass. We found that brain mass and gill surface area both had positive allometric slopes, with larger individuals having both larger brains and larger gill surface areas compared to smaller individuals. However, the allometric slope of brain mass was lower than the allometric slope of gill surface area, consistent with our prediction that the allometric slope of gill surface area could pose an upper limit to the allometric slope of brain mass. Finally, after accounting for body mass, individuals with larger brains tended to have larger gill surface areas. Together, our results provide clues as to how fishes may evolve and maintain large brains despite their high energetic cost, suggesting that C. limbatus individuals with a large gill surface area for their body mass may be able to support a higher energetic turnover, and, in turn, a larger brain for their body mass.

Variable Capacity for Acute and Chronic Thermal Compensation of Physiological Rates Contributes to Inter-Individual Differences in Growth Rate in Mussels (Mytilus galloprovincialis)

The aim of this study was to ascertain if the capacity for acute and chronic compensation of thermal effects on physiological rates represents a trait contributing to inter-individual growth rate differences in the mussel Mytilus galloprovincialis. Juvenile mussels (10-11 mm shell length) were collected, transported to the laboratory, and divided into two groups: one group was maintained at 20oC (warm treatment), and the other at 10oC (cold treatment). The mussels were reared at these two temperatures (continuously fed), until clear size differences allowed us to select fast- (F) and slow (S)-growing individuals from both groups (F20/S20 at 20oC and F10/S10 at 10oC). Selected F and S mussels were then exposed to three experimental temperatures (10, 15, and 20oC), and the time-course of their response, in terms of clearance rate (CR: L/h) and routine oxygen consumption (VO2: mLO2/h), was monitored. The overall growth rate of mussels in the warm treatment group was significantly higher than in the cold treatment group. For both treatments, significant differences were found in key physiological parameters between F and S mussels: F mussels had a higher CR and a larger gill surface area than their S counterparts. Although no significant differences in the thermal sensitivity of the clearance or metabolic rates were observed between F20 and S20 mussels reared at 20oC, when exposed to acute temperature changes, experiments with mussels reared at 10oC revealed a different outcome: in response to acute warming (from 10oC to 15 and 20oC), F10 were capable of compensating for the thermal effect on CR and VO2; however, no such compensatory response was observed in S10. We conclude that two significant factors contribute to endogenous differences in the growth rate of mussels: (i) the capacity to exhibit intense filtering activity, which appears to be functionally correlated with the gill surface area and ii) the capacity to compensate for the effects of temperature on filtration and metabolic rate. The second trait does not appear to make a significant contribution to the inter-individual size-differentiation observed in mussels maintained in warm environments, but explains a significant proportion of inter-individual growth rate differences in cold environments.

Interspecific Differences in the Metabolic Rate, Gill Dimension and Hematology of Fish in an Amazonian Floodplain Lake

It has been hypothesized that respiratory physiology in fish is closely associated with ecological traits. Therefore, data on gill morphometrics (lamellae frequency, gill surface area and barrier thickness), metabolic rate (oxygen consumption) and blood oxygen capacity (hematology) were analyzed in several fish, including benthic, bentho-pelagic and pelagic species collected in an Amazon floodplain lake. Similar to other teleostean species, the 2nd and 3rd gill arches have numerous large filaments in both pelagic and benthic species, as these characteristics tend to increase the gill surface area. A large gill area (4 to 7 cm2 g-1, mass-specific) is associated with a high (100 to 300 mg O2 h-1 kg-1) routine oxygen consumption rate and has been observed in active pelagic swimmers, such as Cichla monoculus and Pygocentrus nattereri. Benthic dwelling fish (e.g., Pterygoplichthys pardalis and Sorubim lima) have low metabolic rates (20 to 50 mg O2 h-1 kg-1), small gill dimensions (2 to 3 cm2 g-1, mass-specific), low hemoglobin levels (3 to 5 g dL-1), reduced numbers of circulating red blood cells (1 to 2 Í106 mm-3) and low hematocrit values (25 to 35%) compared to pelagic species. These results demonstrated that pelagic fish have high routine oxygen consumption rates compatible with their large gill surface area and high blood oxygen capacity, whereas benthic species have low metabolic rates, small gill dimensions and reduced blood oxygen capacity.

Gill transcriptomic analysis in fast- and slow-growing individuals of Mytilus galloprovincialis

The molecular basis underlying the mechanisms at the origin of growth variation in bivalves is still poorly understood, although several genes have been described as upregulated in fast-growing individuals. In the present study, we reared mussel spat of the species Mytilus galloprovincialis under diets below the pseudofaeces threshold (BP) and above the pseudofaeces threshold (AP). After 3 months, F and S mussels from each condition were selected to obtain 4 experimental groups: FBP, SBP, FAP and SAP. We hypothesized that the nurturing conditions during the growing period would modify the molecular basis of their growth rate differences.To decipher the molecular mechanisms underlying the growth variation, the gill transcriptomes for the four mussel groups were analysed. Gene expression analysis revealed i) a low number (12) of genes differentially expressed in association with diet and ii) 117 genes differentially expressed by the fast- and slow-growing mussels. According to Biological Process GO term analysis transcriptomic differences between the F and S mussels were mainly based on the upregulation of: response to the stimulus, growth and cell activity. Regarding the KEGG terms, carbohydrate metabolism and the Krebs cycle were upregulated in F mussels, whereas biosynthetic processes were upregulated in S mussels. In accordance with their larger gill surface area and higher rates of feeding and growth, the F individuals overexpressed genes in their gill tissues, and these were involved in i) growth (insulin-like growth factors and myostatin); ii) maintenance of the structure and functioning of extracellular matrix (collagen, laminin, fibulin and decorin); iii) filtration and ciliary activity (mucin, fibrocystin, dynein and tilB homolog protein genes); iv) aerobic metabolism (citrate synthase and carbonic anhydrase); and v) the immune-system, probably in association with haemocytes. In contrast, S individuals overexpressed a different series of genes pertaining to immune system (leucine-rich repeat protein and galectin), along with genes involved in the response to cellular stress (Heat shock protein (HSP24) and metalloendopeptidase) as well as anaerobic metabolism (C4-dicarboxylate transporter). These results might suggest that S individuals would have a greater prevalence of pathogens/diseases or a higher susceptibility to the pathogens.

Gill morphometrics of the thresher sharks (Genus Alopias): Correlation of gill dimensions with aerobic demand and environmental oxygen.

Gill morphometrics of the three thresher shark species (genus Alopias) were determined to examine how metabolism and habitat correlate with respiratory specialization for increased gas exchange. Thresher sharks have large gill surface areas, short water-blood barrier distances, and thin lamellae. Their large gill areas are derived from long total filament lengths and large lamellae, a morphometric configuration documented for other active elasmobranchs (i.e., lamnid sharks, Lamnidae) that augments respiratory surface area while limiting increases in branchial resistance to ventilatory flow. The bigeye thresher, Alopias superciliosus, which can experience prolonged exposure to hypoxia during diel vertical migrations, has the largest gill surface area documented for any elasmobranch species studied to date. The pelagic thresher shark, A. pelagicus, a warm-water epi-pelagic species, has a gill surface area comparable to that of the common thresher shark, A. vulpinus, despite the latter's expected higher aerobic requirements associated with regional endothermy. In addition, A. vulpinus has a significantly longer water-blood barrier distance than A. pelagicus and A. superciliosus, which likely reflects its cold, well-oxygenated habitat relative to the two other Alopias species. In fast-swimming fishes (such as A. vulpinus and A. pelagicus) cranial streamlining may impose morphological constraints on gill size. However, such constraints may be relaxed in hypoxia-dwelling species (such as A. superciliosus) that are likely less dependent on streamlining and can therefore accommodate larger branchial chambers and gills.

Respiratory function in common carp with different stress coping styles: a hidden cost of personality traits?

The purpose of this study was to compare investment in structures for gas exchange in common carp, Cyprinus carpio, with different stress coping styles, which are known to differ in resting metabolic rate. Common carp were classified as proactive or reactive on the basis of rate of emergence from cover into a novel, potentially dangerous environment in three successive tests. The fish were then killed and their gill arches removed. Length-independent estimates of the size of the respiratory structures were derived by multivariate analysis of arch length and the number and length of gill filaments and lamellae for all gill arches. Overall gill area was also estimated. Filaments from the second gill arch were sampled for histological estimation of the extent to which the respiratory surface is covered by epithelial cells (hyperplasia). Proactive carp had longer gill filaments and more, longer lamellae, and consequently a larger gill surface area, than reactive carp. In contrast, the extent of hyperplasia was higher in reactive than proactive carp. Thus, compared to reactive fish, proactive carp had a larger respiratory surface, more of which was exposed to the surrounding water rather than being covered by epithelial cells. We suggest that the higher metabolic rate of proactive fish requires greater investment in, and greater exposure of, the respiratory structures. This is likely to make oxygen uptake more effective, but may also impose hidden costs of a proactive, aggressive lifestyle.

Gill surface area and metabolic enzyme activities of demersal fishes associated with the oxygen minimum zone off California

Metabolic enzyme activities and gill surface areas were measured across 10 species of demersal fishes from Monterey Canyon, California, which features a prominent oxygen minimum zone (OMZ). Comparisons were made between species living both within and outside of the OMZ. Enzyme activities showed no significant trend toward aerobic suppression or heightened reliance on anaerobic metabolism in response to the OMZ. While flatfish species living both within and outside of the OMZ had similarly low enzyme activities, the OMZ‐dwelling Microstomus pacificus had 1.8–3 times larger gill surface area than comparably sized flatfishes from higher‐oxygen waters, suggesting a morphological adaptation to low oxygen. In scorpaeniform fishes, high aerobic metabolism was accompanied by large gill surface areas in two routine‐swimming OMZ‐dwelling species (Anoplopoma fimbria and Careproctus melanurus). Low aerobic activities and small gills were found in two Sebastolobus species, suggesting a low oxygen demand resulting from a more sedentary behavior compared to other Scorpaeniformes. In gadiform fishes, no differences were measured in enzyme activity levels, but larger gill surface areas were measured in OMZ‐dwelling Nezumia liolepis. These results indicate adaptation to low oxygen in a variety of ways that balance oxygen demand with supply, with no indication that these species rely on enhanced anaerobic metabolism. With both flatfishes and rattails, adaptation to OMZs is demonstrated through increased gill surface area.

Gill morphometrics in relation to gas transfer and ram ventilation in high‐energy demand teleosts: Scombrids and billfishes

This comparative study of the gill morphometrics in scombrids (tunas, bonitos, and mackerels) and billfishes (marlins, swordfish) examines features of gill design related to high rates of gas transfer and the high-pressure branchial flow associated with fast, continuous swimming. Tunas have the largest relative gill surface areas of any fish group, and although the gill areas of non-tuna scombrids and billfishes are smaller than those of tunas, they are also disproportionally larger than those of most other teleosts. The morphometric features contributing to the large gill surface areas of these high-energy demand teleosts include: 1) a relative increase in the number and length of gill filaments that have, 2) a high lamellar frequency (i.e., the number of lamellae per length of filament), and 3) lamellae that are long and low in profile (height), which allows a greater number of filaments to be tightly packed into the branchial cavity. Augmentation of gill area through these morphometric changes represents a departure from the general mechanism of area enhancement utilized by most teleosts, which lengthen filaments and increase the size of the lamellae. The gill design of scombrids and billfishes reflects the combined requirements for ram ventilation and elevated energetic demands. The high lamellar frequencies and long lamellae increase branchial resistance to water flow which slows and streamlines the ram ventilatory stream. In general, scombrid and billfish gill surface areas correlate with metabolic requirements and this character may serve to predict the energetic demands of fish species for which direct measurement is not possible. The branching of the gill filaments documented for the swordfish in this study appears to increase its gill surface area above that of other billfishes and may allow it to penetrate oxygen-poor waters at depth.

Gill area, permeability and Na+,K+-ATPase activity as a function of size and salinity in the blue crab,Callinectes sapidus

Juvenile blue crabs, Callinectes sapidus, extensively utilize oligohaline and freshwater regions of the estuary. With a presumptively larger surface-area-to-body weight ratio, juvenile crabs could experience osmo- and ionoregulatory costs well in excess of that of adults. To test this hypothesis, crabs ranging over three orders of magnitude in body weight were acclimated to either sea water (1,000 mOsm) or dilute sea water (150 mOsm), and gill surface area, water and sodium permeabilities (calculated from the passive efflux of 3H2O and 22Na+), gill Na+, K+ -ATPase activity and expression were measured. Juveniles had a relatively larger gill surface area; weight-specific gill surface area decreased with body weight. Weight-specific water and sodium fluxes also decreased with weight, but not to the same extent as gill surface area; thus juveniles were able to decrease gill permeability slightly more than adults upon acclimation to dilute media. Crabs < 5 g in body weight had markedly higher activities of gill Na+ ,K+ -ATPase than crabs > 5 g in both posterior and anterior gills. Acclimation to dilute medium induced increased expression of Na+, K+ -ATPase and enzyme activity, but the increase was not as great in juveniles as in larger crabs. The increased weight-specific surface area for water gain and salt loss for small crabs in dilute media presents a challenge that is incompletely compensated by reduced permeability and increased affinity of gill Na+, K+ -ATPase for Na+. Juveniles maintain osmotic and ionic homeostasis by the expression and utilization of extremely high levels of gill Na+, K+ -ATPase, in posterior, as well as in anterior, gills.

Allometry of gill weights, gill surface areas, and foot biomass δ13C values of the chemoautotroph–bivalve symbiosis Solemya velum

The protobranch bivalve Solemya velum Say, 1822 has large gills, which harbor chemolithoautotrophic bacteria that supply the majority of the clam’s organic carbon. A substantial portion of the CO2, O2, H2S, and other nutrients necessary for symbiont autotrophy and host heterotrophy are acquired from the environment through the gills, whose large size may be necessary to facilitate the acquisition of sufficient O2 from S. velum’s habitat to meet the combined demands of the host and symbionts. Large gills may also result in an oversupply of CO2, which may in turn be responsible for the isotopically depleted δ13C values observed in S. velum biomass (−31 to −34‰). Alternatively, gill hypertrophy may simply be an adaptation to house a large population of symbionts adjacent to their environmental source of dissolved gases and other nutrients. To better understand gill function in this symbiosis, gill weights, gill surface areas, and foot δ13C values were measured as a function of total weights. S. velum gill weights were found to be a substantial portion of total clam weight, averaging 38% of wet weight, compared to nonsymbiotic protobranch bivalves Yoldia limatula Say, 1831 (5%) and Nucula proxima Say, 1822 (11%). Gill weights are a smaller percentage of total weight in larger individuals; the allometric equation for gill weight (G) as a function of total weight (M) is G=0.26M0.85. Dry weights scale similarly. Gill surface areas are immense; the average gill surface area measured was 107 cm2 g−1 total soft tissue wet weight, the highest value for any marine invertebrate. Gill surface area (SA) also scales with size (SA=69.8M0.85). When gill surface areas were calculated with respect to gill wet weights, they did not scale with size. The δ13C values do not scale with size either, consistent with high rates of CO2 supply at all sizes. Extraordinarily high rates of CO2 supply relative to demand are supported by a model for CO2 delivery based on Fick’s law and the allometric relationship between surface areas and total weight, consistent with a role for large gill surface areas in the generation of isotopically depleted tissue δ13C values.

Physiological refugia: swamps, hypoxia tolerance and maintenance of fish diversity in the Lake Victoria region

In Lake Nabugabo, Uganda, a satellite of Lake Victoria, ≈50% of the indigenous fishes disappeared from the open waters subsequent to the establishment of the introduced predatory Nile perch, Lates niloticus. This pattern is similar to the faunal loss experienced in the much larger Lake Victoria. Several of these species persisted in wetland refugia (e.g. ecotonal wetlands, swamp lagoons); however, deep swamp refugia (habitats lying well within the dense interior of fringing wetlands), are available only to a subset of the basin fauna with extreme tolerance to hypoxia. Although air-breathers are common in deep swamp refugia; we also documented a surprisingly high richness and abundance of non-air-breathing fishes. We describe several mechanisms that may facilitate survival in deep swamp refugia including high hemoglobin concentration, high hematocrit, large gill surface area and a low critical oxygen tension ( P c). In addition, swamp-dwelling fishes showed lower P o 2 thresholds for onset of aquatic surface respiration than the lake-dwelling fishes. This suggests higher tolerance to hypoxia in the swamp fishes because they are able to withstand a lower oxygen tension before approaching the surface. We suggest that physiological refugia may be important in modulating the impact of Nile perch and indigenous fishes in the Lake Nabugabo region; this highlights the need to evaluate relative tolerance of introduced predators and indigenous prey to environmental stressors.

Respiratory adaptations in a deep-sea orbiniid polychaete from Gulf of Mexico brine pool NR-1: metabolic rates and hemoglobin structure/function relationships.

Methanoaricia dendrobranchiata Blake (Polychaeta; Orbiniidae) occurs in large numbers in association with communities of the mussel Bathymodiolus childressi at hydrocarbon seeps on the Louisiana Slope of the Gulf of Mexico. Its microhabitat can be strongly hypoxic (oxygen is often undetectable) and sulfidic (sulfide concentrations can reach millimolar levels), which may seriously challenge aerobic metabolism. We describe a suite of adaptations to its low-oxygen environment. The worms are capable of regulating their rate of oxygen consumption down to partial pressures of approximately 870 Pa oxygen. This capability correlates with a large gill surface area, a small diffusion distance from sea water to blood, a very high hemoglobin oxygen-affinity (P50=27.8 Pa at 10 degrees C and pH 7.6) and a Bohr effect that is pronounced at high oxygen saturations. When fully saturated, the hemoglobin binds sufficient oxygen for only 31 min of aerobic metabolism. However, these polychaetes can withstand extended periods of anoxia both in the absence and presence of 1 mmol l(-1) sulfide (TL50=approx. 5.5 and 4 days, respectively).

Implications of hypoxia for the brain size and gill morphometry of mormyrid fishes

AbstractThe mormyrids are well known for their remarkable electrogenic and electrolocation capabilities and exceptionally large cerebellum that may account for much of their total oxygen consumption. Mormyrids living in oxygen‐deficient waters may use oxygen efficiently, protecting the brain from hypoxia damage; and/or brain size may be reduced. This study compares the TFL, gill lamellar density, gill lamellar area, total gill surface area, and brain size of two species of mormyrids from extremely hypoxic waters in Uganda (Gnathonemus victoriae and Petrocephalus catostoma) to two open‐water species (Mormyrus kannume and Gnathonemus longibarbis) from the same region. In addition, interdemic variation was considered by comparing swamp populations of G. victoriae and P. catostoma to open‐water populations of the same species. Total gill surface area of all species fell within the upper range for freshwater fishes. However, there were both intraspecific and interspecific differences in gill characters. Interdemic comparisons showed larger gill size in swamp‐dwelling populations. Brain size varied among species; larger brains were characteristic of species from well‐oxygenated waters. Large gill surface area may permit survival of mormyrids in oxygen‐stressed environments; however, mechanisms compensating for hypoxia seem to be inadequate to support a brain size as large as that seen in fish from well‐oxygenated waters.

Functional respiratory anatomy of a deep‐sea orbiniid polychaete from the Brine Pool NR‐1 in the Gulf of Mexico

Abstract. An undescribed species of Orbiniidae (Annelida; Polychaeta) is found in large numbers associated with communities of the mussel, Bathymodiolus childressi at hydrocarbon seeps on the Louisiana slope (Gulf of Mexico). Their microhabitat is often hypoxic and sulfidic, which poses serious respiratory challenges for an aerobic metazoan. They display several anatomical features that are quite unusual for this family, which likely allow them to live in their food‐rich, but oxygen‐limited, habitat. The anterior gills are hypertrophied whereas the posterior gills are not. These anterior gills provide the worms with a large gill surface area (≤9.9 cm2/g wet weight), which represents 90–95% of the total gill surface area. The gills contain two blood vessels: a central blood vessel, delimited by a coelomic epithelium, and an intra‐epidermal vessel. The diffusion distance between this latter and the environment is only 3 μ‐m in the anterior gills, which facilitates gas diffusion. Only the anterior gills are ciliated, which may also facilitate gas exchange across this respiratory surface. The gill cells also contain numerous mitochondria and other electron‐dense organelles that might be involved in sulfide detoxification.

Respiratory characters of three species of haplochromine cichlids: Implications for use of wetland refugia

Respiratory characters of three east African haplochromine cichlid species that differ in their use of hypoxic wetlands were examined to consider the potential of dissolved oxygen as one factor affecting habitat use. All three species had a large gill surface area, ranging from the 67th (Pseudocrenilabrus multicolor victoriae) to 98th (Astatotilapia velifer) percentile of the known gill size range for freshwater fishes. Pseudocrenilabrus multicolor victoriae was the most tolerant to hypoxia exhibiting the lowest aquatic surface respiration (R S ) thresholds and lowest critical oxygen tension of the three species. Astatotilapia velifer had the highest ASR thresholds, gill ventilation rates, and level of surface activity of the three species, indicating a relatively low tolerance to hypoxia. Prognathochromis venator was intermediate in its response to hypoxia. These findings are discussed in light of survivorship and distribution patterns of these species following Nile perch introduction into Lake Nabugabo.

Gill Morphometry in Growth Hormone Transgenic Pacific Coho Salmon, Onchorhynchus kisutch, Differs Markedly from that in GH Transgenic Atlantic Salmon

In a previous study we showed that many of the morphological features of the respiratory system of GH (growth hormone) transgenic Atlantic salmon are greater than similarly sized control salmon. Here we show that the manifestation of GH transgene is similar in two different lines of GH transgenic Pacific coho salmon, but that it is very different from that in the GH transgenic Atlantic salmon. The GH transgenic Pacific coho salmon do not have a larger gill surface area than similarly sized control fish.

Life at stable low oxygen levels: adaptations of animals to oceanic oxygen minimum layers.

Zones of minimum oxygen level are found at intermediate depths in most of the world's oceans and, although the oxygen partial pressure in some of these 'oxygen minimum layers' is only a fraction of a kilopascal, populations of pelagic metazoans exist there. These oxygen minimum layers are areas of the water column and the associated benthos with stable conditions of continuously low oxygen level and low temperature at intermediate depths (400-1000 m depth) over vast areas. Off California, where PO2 at the oxygen minimum is 0.8 kPa, there are abundant populations of animals both in the water column and on the bottom. Farther to the south in the eastern tropical Pacific, oxygen partial pressures of less than approximately 0.4 kPa result in very low biomasses and diversity of animals at minimum layer depths. At the minimum oxygen levels found off California, most animals which inhabit the minimum zones appear to support their routine metabolic demands via aerobic metabolism. They do this by being very effective at removing oxygen from water. Among the adaptations of pelagic crustaceans to these conditions are: (1) enhanced ventilatory abilities, (2) enhanced percentage removal of O2 from the ventilatory stream, (3) large gill surface areas, (4) short diffusion distances from the water to the blood, and (5) hemocyanin respiratory proteins with a very high affinity for O2, high cooperativity and large Bohr effects. The lower O2 consumption rates of many deeper-living species are also functionally adaptive in that they facilitate aerobic survival at low PO2. However, they are not adaptations to the minimum layer, since similarly low rates are found in the same and comparable species living at the same depths in regions without well-developed minima, and these animals are unable to survive at the low PO2 values of the minima. While anaerobic metabolism may be important for metabolic rates above the routine level for most animals in the minimum layer, there is little evidence for the use of sustained anaerobiosis in the species studied. In summary, given the stable presence of very low O2 levels in the minima, the primary adaptations of animals living within them are those that support aerobic metabolism by giving the animals remarkable abilities to extract O2 from water. These abilities are notably better than those of animals adapted to unstable hypoxic environments, such as intertidal mudflats, while the latter animals rely to a much greater extent on anaerobiosis and perhaps on metabolic suppression to survive periods of anoxia.

The standard metabolic rate of dolphin fish

The standard metabolic rates (SMRs) of 11 (1.395–4.125 kg) dolphin fish (mahimahi or dorado, Coryphaena hippurus) were measured at 25°± 0.5°C. Fish were prevented from swimming with neuromuscular blocking agents and force ventilated. Heart rates were determined simultaneously. SMRs (358–726 mg O2 h –1) were several times those of other similarly sized active teleosts such as salmonids, but close to those of tunas. Heart rates (84–161 beats min –1) were also high, but alike those of tunas under similar circumstances. As in tunas, the high SMR of dolphin fish may result from high osmoregulatory costs engendered by their large gill surface areas and/or other adaptations necessary for achieving exceptionally high maximum metabolic rates.

Respiratory gill surface area of a facultative air-breathing loricariid fish, Rhinelepis strigosa

The respiratory surface area of the gill in relation to body mass of the facultative air-breathing loricariid fish Rhinelepis strigosa was analyzed using logarithmic transformation (log Y = log a + b log W) of the equation Y = aWb. The data revealed differences in growth pattern for each gill element. The increase in gill surface area was not isometric with body mass (b = 0.76). The total number of secondary lamellae (b = 0.38) and the average bilateral surface area of the secondary lamellae (b = 0.46) contributed most to the rate of development of the gill surface area (total area of the secondary lamellae) with increase in body mass. Gill filament length (b = 0.339) was more important than the frequency (number/mm) of secondary lamellae in determining the increase in the total number of secondary lamellae. The number of gill filaments showed the lowest b value, 0.072. Rhinelepis strigosa has a larger gill surface area than most other air-breathing fish, indicating that it is better adapted for breathing in water than in air.

Oxygen transport and cardiovascular responses in skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) exposed to acute hypoxia.

Responses to acute hypoxia were measured in skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) (approximately 1-3 kg body weight). Fish were prevented from making swimming movements by a spinal injection of lidocaine and were placed in front of a seawater delivery pipe to provide ram ventilation of the gills. Fish could set their own ventilation volumes by adjusting mouth gape. Heart rate, dorsal and ventral aortic blood pressures, and cardiac output were continuously monitored during normoxia (inhalant water (PO2 greater than 150 mmHg) and three levels of hypoxia (inhalant water PO2 approximately 130, 90, and 50 mmHg). Water and blood samples were taken for oxygen measurements in fluids afferent and efferent to the gills. From these data, various measures of the effectiveness of oxygen transfer, and branchial and systemic vascular resistance were calculated. Despite high ventilation volumes (4-7 l.min-1.kg-1), tunas extract approximately 50% of the oxygen from the inhalant water, in part because high cardiac outputs (115-132 ml.min-1.kg-1) result in ventilation/perfusion conductance ratios (0.75-1.1) close to the theoretically ideal value of 1.0. Therefore, tunas have oxygen transfer factors (ml O2.min-1.mmHg-1.kg-1) that are 10-50 times greater than those of other fishes. The efficiency of oxygen transfer from water in tunas (approximately 65%) matches that measured in teleosts with ventilation volumes an order of magnitude lower. The high oxygen transfer factors of tunas are made possible, in part, by a large gill surface area; however, this appears to carry a considerable osmoregulatory cost as the metabolic rate of gills may account for up 70% of the total metabolism in spinally blocked (i.e., non-swimming) fish. During hypoxia, skipjack and yellowfin tunas show a decrease in heart rate and increase in ventilation volume, as do other teleosts. However, in tunas hypoxic bradycardia is not accompanied by equivalent increases in stroke volume, and cardiac output falls as HR decreases. In both tuna species, oxygen consumption eventually must be maintained by drawing on substantial venous oxygen reserves. This occurs at a higher inhalant water PO2 (between 130 and 90 mmHg) in skipjack tuna than in yellowfin tuna (between 90 and 50 mmHg). The need to draw on venous oxygen reserves would make it difficult to meet the oxygen demand of increasing swimming speed, which is a common response to hypoxia in both species.(ABSTRACT TRUNCATED AT 400 WORDS)