The Respiratory System of Tuna
Tuna are large, fast swimming fish that are known for their delicious meat and valued as game fish. But with their high activity levels, how exactly do tuna get enough oxygen from the water to breathe? Understanding the respiratory system of tuna and how they uptake oxygen can give insights into their biology and evolution.
Gills Allow Gas Exchange from Water
Like all fish, tuna breathe through gills which are specially adapted structures designed for extracting oxygen from water. Tuna have paired gills which consist of filaments filled with blood capillaries to allow gas exchange with the water flowing over them. Dissolved oxygen moves across the thin membranes of the gill filaments into the bloodstream while carbon dioxide moves out of the bloodstream and back into the water.
This countercurrent exchange system allows tuna to efficiently uptake the relatively low levels of oxygen found in water compared to air. The constant flow of water over the gills, maintained by the swimming movements of the tuna, brings fresh oxygenated water over the respiratory surface.
High Blood Supply Fuels Active Lifestyle
Tuna have a high demand for oxygen with their fast, powerful swimming to catch prey. Their blood has a very high hemoglobin content which binds and transports oxygen efficiently. This allows their blood to carry much more oxygen than most fish.
Their circulatory system also transports blood at high pressure and volume throughout the body and maintains a steady supply to the gills. This helps move oxygen quickly from the gills to the organs and muscle tissues that need it.
Special Adaptations Increase Breathing Efficiency
Tuna have undergone evolutionary adaptations that set them apart from other fish to help improve the efficiency of their respiration system for an active, predatory lifestyle chasing down fast moving prey.
Complex Gills Have Large Surface Area
While most fish have four pairs of gills, tuna maintain five pairs of gills even into adulthood for increased capacity for oxygen uptake. Their gills are also much longer and finer than typical fish gills, with greater surface area and more secondary lamellae structures to allow for rapid gas exchange.
This expanded respiratory system combined with covered gill slits maintains water flow and prevents spillage or loss of oxygenated water. It gives tuna the advantage when swimming at high sustained speeds or making quick bursts to catch their food.
Specialized Blood Vessels for Efficiency
Not only do tuna have a high volume of blood, their circulatory system maximizes the tuna's ability to uptake and transport oxygen. They maintain a complex network of capillaries around the gills with a rete mirabile, which functions to greatly slow blood flow around the gills.
This increased gill blood contact time, combined with countercurrent exchange with lamellar blood flow, allows tuna blood to achieve oxygen saturation levels near 100%. This gives a considerable advantage for delivery of oxygen to bodily tissues.
Importance for Fisheries and Conservation
The efficient respiratory adaptations of tuna allow them to thrive in ocean habitats and undertake long migrations across vast distances in search of prey or for spawning behaviors. However, they also contribute to issues in overexploitation due to high catch rates by fisheries.
Supports Demanding Lifestyles
The suite of enhancements to their respiratory system allow tuna to lead extremely active, fast-paced lifestyles in comparison to most other fish. They can swim continuously at aerobic maximum speeds and make rapid movements to capture slippery prey.
Their constant motion forces steady water flow over the gill surface. At the same time, their blood has a very high oxygen carrying capacity to deliver oxygen to muscle tissues. Together, these allow tuna to swim such great ocean distances in migrations across tropical to temperate oceans worldwide.
Drives Overfishing Problems
A downside to the adaptations that allow such high performance swimming in tuna is increased vulnerability to some fishing practices. Their constant motion forces them to keep swimming even when encountered with nets or lines.
So once hooked or netted, they can be hauled in rapidly by fishers because they do not tire out as quickly as many species. Their unique physiology comes back to bite them in the face of industrialized commercial fisheries, resulting in overexploitation of stocks.