Kathryn A. Dickson

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.

Tuna comparative physiology

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.

Anatomical and physiological specializations for endothermy

Publisher Summary This chapter discusses the anatomical and physiological specializations for endothermy in fishes. Tunas are endothermic, which means that they utilize metabolic heat to elevate and maintain regional body temperatures that are warmer than the ambient seawater temperature. Tuna red muscle (RM) is both more anterior and nearer to the vertebral column and is completely surrounded by white muscle This RM position and its specialized connective-tissue linkage to the caudal fin affects swimming mechanics and results in the unique tuna swimming mode, which is termed as “thunniform locomotion.” The features of the thunniform locomotion pattern include minimal lateral body flexion and changes in the relationships between muscle activation and the strain cycle. It is found that in the four species— Euthynnus lineatus, Katsuwonus pelamis, Thunnus albacares, and Thunnus alalunga —the rete arterial vessel walls contain two or three layers of smooth muscle, while the venule walls have a single layer. Varying the contractile state of the vascular smooth muscle provides a possible mechanism whereby vessel diameter, and thus blood-flow and heat-exchange effectiveness may be adjusted in vivo . The link among tuna vascular anatomy, activity level, and an elevated body temperature is also described in the chapter.

Swimming performance studies on the eastern Pacific bonito Sarda chiliensis, a close relative of the tunas (family Scombridae) I. Energetics.

SUMMARY A large swim tunnel respirometer was used to quantify the swimming energetics of the eastern Pacific bonito Sarda chiliensis (tribe Sardini) (45–50 cm fork length, FL) at speeds between 50 and 120 cm s-1 and at 18±2°C. The bonito rate of oxygen uptake (V̇O2)–speed function is U-shaped with a minimum V̇O2 at 60 cm s-1, an exponential increase in V̇O2 with increased speed, and an elevated increase in V̇O2 at 50 cm s-1 where bonito swimming is unstable. The onset of unstable swimming occurs at speeds predicted by calculation of the minimum speed for bonito hydrostatic equilibrium (1.2 FL s-1). The optimum swimming speed (Uopt) for the bonito at 18±2°C is approximately 70 cm s-1 (1.4 FL s-1) and the gross cost of transport at Uopt is 0.27 J N-1 m-1. The mean standard metabolic rate (SMR), determined by extrapolating swimming V̇O2 to zero speed, is 107±22 mg O2 kg-1 h-1. Plasma lactate determinations at different phases of the experiment showed that capture and handling increased anaerobic metabolism, but plasma lactate concentration returned to pre-experiment levels over the course of the swimming tests. When adjustments are made for differences in temperature, bonito net swimming costs are similar to those of similar-sized yellowfin tuna Thunnus albacares (tribe Thunnini), but the bonito has a significantly lower SMR. Because bonitos are the sister group to tunas, this finding suggests that the elevated SMR of the tunas is an autapomorphic trait of the Thunnini.

Functional Morphology and Biochemical Indices of Performance: Is there a Correlation Between Metabolic Enzyme Activity and Swimming Performance?

Abstract Comparative physiologists and ecologists have searched for a specific morphological, physiological or biochemical parameter that could be easily measured in a captive, frozen, or preserved animal, and that would accurately predict the routine behavior or performance of that species in the wild. Many investigators have measured the activity of specific enzymes in the locomotor musculature of marine fishes, generally assuming that high specific activities of enzymes involved in aerobic metabolism are indicators of high levels of sustained swimming performance and that high activities of anaerobic metabolic enzymes indicate high levels of burst swimming performance. We review the data that support this hypothesis and describe two recent studies we have conducted that specifically test the hypothesis that biochemical indices of anaerobic or aerobic capacity in fish myotomal muscle correlate with direct measures of swimming performance. First, we determined that the maximum speed during escapes (C-starts) for individual larval and juvenile California halibut did not correlate with the activity of the enzyme lactate dehydrogenase, an index of anaerobic capacity, in the myotomal muscle, when the effects of fish size are factored out using residuals analysis. Second, we found that none of three aerobic capacity indices (citrate synthase activity, 3-hydroxy-o-acylCoA dehydrogenase activity, and myoglobin concentration) measured in the slow, oxidative muscle of juvenile scombrid fishes correlated significantly with maximum sustained speed. Thus, there was little correspondence between specific biochemical characteristics of the locomotor muscle of individual fish and whole animal swimming performance. However, it may be possible to identify biochemical indices that are accurate predictors of animal performance in phylogenetically based studies designed to separate out the effects of body size, temperature, and ontogenetic stage.

Morphological and enzymatic correlates of aerobic and burst performance in different populations of Trinidadian guppies Poecilia reticulata

SUMMARY We examined the mechanistic basis for two whole-animal performance traits, aerobic capacity and burst speed, in six laboratory-reared Trinidadian guppy populations from different native drainages with contrasting levels of predation. Using within- and between-population variation, we tested whether variation in organs and organ systems (heart, gill and swimming motor mass) and the activities of several enzymes that support locomotion (citrate synthetase, lactate dehydrogenase and myofibrillar ATPase) are correlated with aerobic performance (maximum rates of oxygen consumption, V̇O2max) or burst performance (maximum swim speed during escape responses). We also tested for associations between physiological traits and habitat type (different drainages and predation levels). Organ size and enzyme activities showed substantial size-independent variation, and both performance measures were strongly correlated to body size. After accounting for size effects, neither burst nor aerobic performance was strongly correlated to any organ size or enzymatic variable, or to each other. Two principal components (PCI, PC2) in both males and females accounted for most of the variance in the organ size and enzymatic variables. In both sexes, heart and gill mass tended to covary and were negatively associated with citrate synthetase and lactate dehydrogenase activity. In males (but not females), variation in aerobic performance was weakly but significantly correlated to variation in PC1, suggesting that heart and gill mass scale positively with V̇O2max. Neither of the component variables and no single morphological or enzymatic trait was correlated to burst speed in either sex. Evolutionary changes in important life history traits occur rapidly in guppy populations subjected to different predation intensities (high mortality in downstream sites inhabited by large predatory fish; low mortality in upstream sites lacking large predators). We found significant differences between stream drainages in all morphological variables and most enzymatic variables, but only the mass of the swimming motor and LDH activity were significantly affected by predation regime. Overall, our data show that microevolution has occurred in the physiological foundations of locomotor performance in guppies, but evolutionary changes in physiology do not closely correspond to the predation-induced changes in life history parameters.

Evidence for cranial endothermy in the opah (Lampris guttatus).

SUMMARY Cranial endothermy evolved independently in lamnid sharks, billfishes and tunas, and is thought to minimize the effects of ambient temperature change on both vision and neural function during deep dives. The opah, Lampris guttatus, is a large epipelagic–mesopelagic predator that makes repeated dives into cool waters to forage. To determine if L. guttatus exhibits cranial endothermy, we measured cranial temperatures in live, decked fish and identified potential sources of heat and mechanisms to conserve heat. In 40 opah (95.1±7.6 cm fork length), the temperature of the tissue behind the eye was elevated by a mean (±s.e.m.) of 2.1±0.3°C and a maximum of 6.3°C above myotomal muscle temperature (Tm), used as a proxy for ambient temperature. Cranial temperature varied significantly with Tm and temperature elevation was greater at lower Tm. The proximal region of the paired lateral rectus extraocular muscle appears to be the primary source of heat. This muscle is the largest extraocular muscle, is adjacent to the optic nerve and brain and is separated from the brain only by a thin layer of bone. The proximal lateral rectus muscle is darker red in color and has a higher citrate synthase activity, indicating a higher capacity for aerobic heat production, than all other extraocular muscles. Furthermore, this muscle has a layer of fat insulating it from the gill cavity and is perfused by a network of arteries and veins that forms a putative counter-current heat exchanger. Taken together, these results support the hypothesis that the opah can maintain elevated cranial temperatures.

Physiological Thermoregulation in the Albacore Thunnus alalunga

Shipboard experiments demonstrate that the warm-bodied albacore tuna, Thunnus alalunga, uses physiological mechanisms to control heat loss and gain and is able to defend its body temperature (Tb) over a range of ambient temperatures (Ta). Thermocouple-implanted fish that were restrained yet capable of unhindered tail activity maintained stable red and white muscle temperatures (Trm and Twm) for from 15 to 160 min while exposed to Tas ranging from 11.5 to 18.0 C. Below 11.5 C Ta, Trm and Twm could not be maintained and began to cool. Above 18.0 C Ta, both Trm and Twm increased with Ta in the pattern of a constant temperature excess (Tx = Tt − Ta), The mean Ta triggering thermoregulation was 13.6 C and the mean Trm at the initiation of thermoregulation was 19.7 C. Red and white muscle temperatures varied spatially and temporally. In one fish so tested brain temperature was not regulated while Trm was. All test albacore moved their tails at frequencies and amplitudes that would be sufficient to power a free-swimming fish at velocities needed for hydrostatic equilibrium. At low Tas caudal fin activity was shifted to a higher frequency (+20%) and lower specific amplitude (from 13.6% to 8.6%l [fish fork length]). This mode of swimming favors thermoregulation because a higher beat frequency would produce more heat. It may also be less efficient and thus further contribute to heat production by requiring additional muscle fiber activity to power swimming. The albacore probably controls its heat balance by modulating retial exchange efficiency. This can occur in response to rapid increases or decreases in Ta and is also affected by a fishs recent thermal history, activity and Tb. High activity or swimming in warmer surface waters can increase the albacores Tb. Shunting of cool blood around retia and to the body core via the dorsal aorta may be an important mechanism for preventing excessive heating or achieving rapid cooling upon return to a cooler Ta. In its natural environment, the albacore doubtlessly makes short forays into water with Tas that are both above and below its range of physiological thermoregulation. Thermal imbalances resulting from this activity are probably compensated for by changes in heat exchange efficiency and the behavioral selection of a Ta that favors rapid restoration of thermal equilibrium.

Enzyme Activities Support the Use of Liver Lipid–Derived Ketone Bodies as Aerobic Fuels in Muscle Tissues of Active Sharks

Few data exist to test the hypothesis that elasmobranchs utilize ketone bodies rather than fatty acids for aerobic metabolism in muscle, especially in continuously swimming, pelagic sharks, which are expected to be more reliant on lipid fuel stores during periods between feeding bouts and due to their high aerobic metabolic rates. Therefore, to provide support for this hypothesis, biochemical indices of lipid metabolism were measured in the slow‐twitch, oxidative (red) myotomal muscle, heart, and liver of several active shark species, including the endothermic shortfin mako, Isurus oxyrinchus. Tissues were assayed spectrophotometrically for indicator enzymes of fatty acid oxidation (3‐hydroxy‐o‐acyl‐CoA dehydrogenase), ketone‐body catabolism (3‐oxoacid‐CoA transferase), and ketogenesis (hydroxy‐methylglutaryl‐CoA synthase). Red muscle and heart had high capacities for ketone utilization, low capacities for fatty acid oxidation, and undetectable levels of ketogenic enzymes. Liver demonstrated undetectable activities of ketone catabolic enzymes but high capacities for fatty acid oxidation and ketogenesis. Serum concentrations of the ketone &bgr;‐hydroxybutyrate varied interspecifically (means of 0.128–0.978 &mgr;mol mL−1) but were higher than levels previously reported for teleosts. These results are consistent with the hypothesis that aerobic metabolism in muscle tissue of active sharks utilizes ketone bodies, and not fatty acids, derived from liver lipid stores.

Mitochondrial proton leak rates in the slow, oxidative myotomal muscle and liver of the endothermic shortfin mako shark (Isurus oxyrinchus) and the ectothermic blue shark (Prionace glauca) and leopard shark (Triakis semifasciata)

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 tissues 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.