The southern bluefin tuna (Thunnus maccoyii) is a tuna of the family Scombridae found in open southern Hemisphere waters of all the world's oceans mainly between 30°S and 50°S, to nearly 60°S. At up to and weighing up to , it is among the larger bony fishes.
Southern bluefin tuna, like other pelagic tuna species, are part of a group of bony fishes that can maintain their body core temperature up to above the ambient temperature. This advantage enables them to maintain high metabolic output for predation and migrating large distances. The southern bluefin tuna is an opportunistic feeder, preying on a wide variety of fish, crustaceans, cephalopods, salps, and other marine fishes and crustaceans.
Environmental/physical challenges The southern bluefin tuna is a predatory organism with a high metabolic need. These are pelagic animals, but migrate vertically through the water column, up to in depth. They also migrate between tropical and cool temperate waters in the search for food. The seasonal migrations are between waters off the coast of Australia and the Indian Ocean. Although the preferred temperature range for southern bluefin tuna is from , they can endure temperatures as low as at low depths, and as high as , when spawning.
This wide range of temperature and depth changes poses a challenge to the respiratory and circulatory systems of the southern bluefin tunas. Tuna swim continuously and at high speeds and, therefore, have a high demand for oxygen. The oxygen concentration in the water changes with the change in temperature, being lower at high temperatures. Tuna are, however, driven by the availability of food, not by thermal properties of water. Bluefin tuna, unlike other species of tuna, maintain a fairly constant red muscle (swimming muscle) temperature over a wide range of ambient temperatures. So, in addition to being endotherms, bluefin tuna are also thermoregulators. The species is listed as Endangered by the IUCN.
Physiology
Respiratory physiology Respiratory systems of southern bluefin tunas are adapted to their high oxygen demand. Bluefin tunas are obligate ram ventilators: they drive water into the buccal cavity through their mouth, then over the gills, while swimming. Therefore, unlike most other teleost fish, the southern bluefin tuna does not require a separate pump mechanism to pump water over the gills. Ram ventilation is said to be obligatory in southern bluefin tunas, because the buccal-opercular pump system used by other teleost fish became incapable of producing a stream of ventilation vigorous enough for their needs. All species of tuna in general have lost the opercular pump, requiring a quicker movement of oxygenated water over the gills than induced by the suction of the opercular pump. Therefore, if they stop swimming, tunas suffocate due to a lack of water flow over the gills.
The oxygen need and oxygen uptake of the southern bluefin tuna are directly related. As the tuna increases its metabolic need by swimming faster, water flows into the mouth and over the gills more quickly, increasing the oxygen uptake. Additionally, since there is no energy required to pump the water over the gills, the tunas have adapted an increased energy output to swimming muscles. The oxygen and nutrient uptake in the circulatory system is transported to these swimming muscles rather than to tissues required to pump water over the gills in other teleost fish.
Based on the principles of the Fick equation, the rate of the gas diffusion across the gas exchange membrane is directly proportional to the respiratory surface area, and inversely proportional to the thickness of the membrane. Tunas have highly specialized gills, with a surface area 7–9 times larger than that of other aquatic environment organisms. This increased surface area allows more oxygen to be in contact with the respiratory surface and therefore diffusion to take place more quickly (as represented by the direct proportionality in the Fick equation). This massive increase in surface area of the gills of the southern bluefin tuna is due to a higher density of secondary lamella in the gill filaments.
The southern bluefin tuna, like other tuna species, has a very thin gas-exchange membrane. Tunas have a barrier thickness of 0.5μm, compared with 10μm of dogfish, 5μm of toadfish and less than 5μm of trouts. This means that the oxygen must diffuse a short distance across the respiratory surface to get to the blood. Similarly to the increased surface area, this allows the highly metabolic organism to take oxygenated blood into the circulatory system more quickly. On top of a quicker rate of diffusion in the respiratory system of southern bluefin tuna, there is a significant difference in the efficiency of the oxygen uptake. While other teleost fish typically utilize 27–50% of the oxygen in the water, the tuna's utilization rates have been observed as high as 50-60%. This overall high oxygen uptake works in close coordination with a well-adapted circulatory system to meet the high metabolic needs of the southern bluefin tuna.
The oxygen dissociation curves for southern bluefin tunas show a reverse temperature effect between , and temperature insensitivity between . Reverse temperature shift might prevent premature oxygen dissociation from hemoglobin as it is warmed in rete mirabile. Root effect and a large Bohr factor were also observed at .
Circulatory physiology The cardiovascular system of tunas, as in many fish species, can be described in terms of two RC networks, in which the system is supplied by a single generator (the heart). The ventral and dorsal aorta feed resistance of the gills and systemic vasculature, respectively. The heart in tunas is contained inside a fluid-filled pericardial cavity. Their hearts are exceptionally large, with ventricle masses and cardiac output roughly four to five times larger than those of other active fishes. They consist of four chambers, as in other teleosts: sinus venosus, atrium, ventricle, and bulbus arteriosus.
Tunas have type IV hearts, which have more than 30% compact myocardium with coronary arteries in compact and spongy myocardium. Their ventricles are large, thick-walled, and pyramidal in shape, allowing for generation of high ventricular pressures. The muscle fibers are arranged around the ventricle in a way that allows rapid ejection of stroke volume, because ventricles can contract both vertically and transversely at the same time. Myocardium itself is well vascularized, with highly branched arterioles and venules, as well as a high degree of capillarization.
Major arteries and veins run longitudinally to and from the red swimming muscles, which are found close to the spinal column, just underneath the skin. Small arteries branch off and penetrate the red muscle, delivering oxygenated blood, whereas veins take deoxygenated blood back to the heart. The red muscles also have a high myoglobin content and capillary density, where many of the capillaries branch off. This helps increase surface area and red-cell residence time. The veins and arteries are organized in a way that allows countercurrent heat exchange. They are juxtaposed and branched extensively to form rete mirabile. This arrangement allows the heat produced by the red muscles to be retained within them, as it can be transferred from the venous blood to the ingoing arterial blood.Tunas have the highest arterial blood pressure among all fishes, due to a high resistance of blood flow in the gills. They also have a high heart rate, cardiac output, and ventilation rate. To achieve high cardiac outputs, tunas increase their heart rate exclusively (other teleosts may increase their stroke volume as well). High cardiac outputs in southern bluefin tuna are necessary to achieve their maximum metabolic rates. The bulbus arteriosus can take up an entire stroke volume, maintaining a smooth blood flow over the gills through diastole. This might, in turn, increase the rate of gas exchange. Their heart rate is also affected by temperature; at normal temperatures can it reach up to 200 beats/min.
The blood of southern bluefin tuna is composed of erythrocytes, reticulocytes, ghost cells, lymphocytes, thrombocytes, eosinophilic granulocytes, neutrophilic granulocytes, and monocytes. Southern bluefin tuna has a high blood hemoglobin content (13.25—17.92 g/dl) and, therefore, a high oxygen carrying capacity. This results from an increased hematocrit and mean cellular hemoglobin content (MCHC). The erythrocyte content in the blood ranges from 2.13 to 2.90 million/l which is at least twice that of adult Atlantic salmon, reflecting the active nature of southern bluefin tuna. Because the MCHC is high, more blood can be delivered to tissues without an increase in energy used to pump more viscous blood. For southern bluefin tuna, this is important in blood vessels that are not protected by heat exchangers when they migrate to colder environments.
Integration of respiratory and circulatory organs Tunas are more mobile than any terrestrial animals and are some of the most active fish; therefore, they require highly efficient respiratory and circulatory systems. Southern bluefin tuna, as well as other species of tunas, have developed many adaptations in order to achieve this. Their respiratory system has adapted to rapidly take up oxygen from water. For example, tunas switched from a buccal-opercular pump system to ram ventilation, which allows them to drive large quantities of water over their gills. Gills have, in turn, become highly specialized to increase the rate of oxygen diffusion. The circulatory system works together with the respiratory system to rapidly transport oxygen to tissues. Due to high hemoglobin levels, the blood of southern bluefin tuna has a high oxygen carrying capacity. Furthermore, their large hearts, with a characteristic organization of muscle fibres, allow for comparatively high cardiac outputs, as well as rapid ejection of stroke volume. This, together with the organization of blood vessels and a countercurrent heat exchange system, allows the southern bluefin tuna to rapidly deliver oxygen to tissue, while preserving energy necessary for their active lifestyle.
Osmoregulation
Environmental osmotic conditions Southern bluefin tuna migrate between a variety of different ocean regions, however the osmotic conditions faced by the tuna stay relatively similar. This species of tuna inhabits ocean areas that are relatively high in salinity compared to the rest of the world's oceans. Like other marine teleost fish, the southern bluefin tuna maintain a constant ion concentration in both their intracellular and extracellular fluids. This regulation of an internal ion concentration classifies southern bluefin tuna as osmoregulators.
The blood plasma, interstitial fluid, and cytoplasm of cells in southern bluefin tuna are hyposmotic to the surrounding ocean water. This means that the ion concentration within these fluids is low relative to the seawater. The standard osmotic pressure of seawater is 1.0 osmole/L, while the osmotic pressure in the blood plasma of the southern bluefin tuna is approximately half of that. Without the mechanism of osmoregulation present, the tuna would lose water to the surrounding environment and ions would diffuse from the seawater into the fluids of the tuna to establish equilibrium.