Ontogenetic changes in characteristics required for endothermy in juvenileblack skipjack tuna (Euthynnus lineatus)

To characterize better the development of endothermy in tunas, we assessed how the abilities to generate heat and to conserve heat within the aerobic, slow-twitch (red) myotomal muscle using counter-current heat exchangers (r etia) change with size in juvenile black skipjack tuna (Euthynnus lineatus) above and below the hypothesized minimum size for endothermy of 207 mm for k length (FL), Early juvenile scombrids (10-77 mm FL) collected off the Pac ific coast of Panama were raised to larger sizes at the Inter-American Trop ical Tuna Commission Laboratory at Achotines Bay, Panama. Evidence of centr al and lateral rete blood vessels was found in E. lineatus as small as 95.9 mm FL and 125 mm FL, respectively. In larger E, lineatus juveniles (up to 244 mm FL), the capacity for heat exchange increased with fork length as a result of increases in rete length, rete width and the number of vessel row s. The amount (g) of red muscle increased exponentially with fork length in both E. lineatus (105-255 mm FL) and a closely related ectothermic species , the sierra Spanish mackerel Scomberomorus sierra (151-212 mm FL), but was greater in E. lineatus at a given fork length. The specific activity (inte rnational units g(-1)) of the enzyme citrate synthase in red muscle, an ind ex of tissue heat production potential, increased slightly with fork length in juvenile E. lineatus (84.1-180 mm FL) and S. sierra (122-215 mm FL), Th us, total red muscle heat production capacity (red muscle citrate synthase activity per gram times red muscle mass in grams) increased with fork lengt h, primarily because of the increase in red muscle mass. Below 95.9 mm FL, E. lineatus cannot maintain red muscle temperature (T-m) above the ambient water temperature (T-a) because juveniles of this size lack retia, Above 95 .9 mm FL, the relationship between T-x (T-m-T-a) and FL for E. lineatus div erges from that for the ectothermic S. sierra because of increases in the c apacities for both heat production and heat retention that result in the de velopment of endothermy.