Metabolic limits and energetics

Tunas are impressive athletes that swim continuously and can undertake extensive vertical and horizontal migrations throughout the oceans of the world (Block et al. 2001; Graham and Dickson 2004; Block et al. 2005). Such athleticism demands a range of morphological, physiological and biochemical adaptations to ensure that energy and oxygen can be obtained and utilized most effectively. Adaptations for athleticism are obvious in the gross external morphology of tunas, including a fusiform body to reduce drag, fin grooves to increase streamlining, a high-aspect-ratio tail with a narrow caudal peduncle, and finlets across the trailing edges of the body (Dewar and Graham 1994; Graham and Dickson 2004). A closer examination reveals thin gill epithelia and a large gill surface area relative to body size, obligate ram ventilation, anterior-medial body position of slow-twitch oxidative (red) muscle, high aerobic capacity of fast-twitch (white) muscle, and countercurrent vascular heat exchangers (retia mirabilia) that function to retain metabolic heat in particular regions of the body and create a thermal excess relative to ambient water (commonly termed regional heterothermy or regional endothermy) (Carey and Teal 1969; Muir and Hughes 1969; Giovane et al. 1980; Carey et al. 1984; Bushnell and Brill 1992; Dewar et al. 1994; Mathieu-Costello et al. 1996; Stevens et al. 2000; Gunn et al. 2001; Marcinek et al. 2001; Graham and Dickson 2004). While these attributes have captured the interest of scientists for many decades, they have also contributed to the inherent difficulties in studying live and unstressed tunas.