Development of the Rete Mirabile with Growth and Muscle Temperature in the Young Bluefin Tuna

The weight of red and white muscles, the number of vessels in 1 mm long segment of rete and muscle temperature were measured to elucidate the development of the ability to conserve metabolic heat during the growth of young bluefin tuna Thunnus thynnus (20-39 cm in fork length). Maximum weight of boiled red and white muscles was located in the region of 40 to 55% FL (percent of fork length from the snout), where number of rete vessels and body thickness were maximum. Also, maximum muscle temperature was found at the same region in the body. Both boiled red and white muscle masses markedly increased, but the former was superior in the rate of increase to the latter. The number of rete vessels conspicuously increased, too. It was considered that the increase in the number of rete vessles contributed not only to transporting more blood volume in proportion to increase of red muscle mass, but also to improving the efficiency of heat conservation. Indeed, maximum muscle temperature tended to be higher and distribute more broadly in larger specimens. Maximum muscle temperature of the specimens of 32.1-39 cm in fork length were maintained 3-4°C over environmental seawater temperature of 24.2°C. Accordingly, it was appeared that the ability to conserve metabolic heat had already been developing.