Scombrids (tunas, mackerels, and bonitos) have streamlined bodies that taper on either end, moderately large mouths, and well-developed teeth. Gill membranes are not attached to the isthmus. Scales are cycloid and usually tiny, and body coloration is metallic, often blue and silver. Spanish mackerels have yellow to bronze spots and bonitos and tunas may have dusky bands and fins. The dorsal fin is composed of nine to 27 densely packed rays, and the pelvic fins have six rays. One member of the family, bluefin tuna, can reach 4.2 m and are probably the world’s largest bony fishes. Scombrids are highly adapted to continuous swimming in the open ocean. Their bodies are an ideal streamlined shape, with the thickest part of the body occurring two-fifths of the way back from the head. Their dorsal fins can slot into grooves to reduce drag, and the caudal fin is stiff and sickle-shaped for powerful propulsion. The five to 12 separate finlets behind the anal and second dorsal fins may allow the tail to push against less turbulence by preventing vortices from forming in water flowing toward the tail. The slender caudal peduncle bears at least two keels that reduce drag and may accelerate water flowing over the tail. (Click here to see a fish diagram).
Tunas are negatively buoyant and must swim continuously to avoid sinking. In addition, they require constant movement to ventilate the gills. Through a process called ram gill ventilation, swimming (at speeds no less than 65 cm per second) forces water over the gills. Tunas have numerous lamellae (gill membranes) and very thin lamellar walls, and are able to extract more oxygen from the water than any other fish. Tunas have large hearts and blood volumes. They also have a high proportion of the red muscle that permits sustained swimming, buried centrally along the spinal column to conserve heat. Other members of the family, such as the mackerels, also have red muscle, but located nearer the outside of the fish.
One of the most striking features of the scombrids is that some groups are endothermic, able to maintain a body temperature higher than that of the surrounding water. Tunas (tribe Thunnini) conserve heat produced by swimming muscles through an arrangement of blood vessels called a rete mirabile (“wonderful net”). These blood vessels act as a countercurrent heat-exchanger. In any fish, when blood cycles through the gills to receive oxygen, it also cools to the temperature of the surrounding water. In tunas, this blood is diverted to vessels near the outside of the body instead of traveling directly through the fish’s core. Before flowing inward, the cool, oxygenated blood passes through a network of small vessels, countercurrent to warm blood leaving the swimming muscles, and heat is transferred to the entering blood. In this way much of the heat generated by swimming muscles is conserved. In waters ranging from 7 to 30˚ C, bluefin tuna maintain muscle temperatures between 28 and 33 C. Others keep body temperatures 3 to 7 degrees C warmer than the surrounding water. Some species, such as bigeye tuna, utilize the heat exchanger only when they enter colder water. Endothermy also helps warm parts of the central nervous system, which stabilizes nervous system function in cold water. Butterfly mackerels keep brain and eye temperatures elevated using thermogenic (heat-producing) tissue. (Click here to see a diagram of tuna thermoregulation).
Other Physical Features: endothermic ; heterothermic ; bilateral symmetry