| This study describes development of slow-twitch myotomal muscle (SM) in juvenile yellowfin tuna, Thunnus albacares. The SM powers continuous swimming and generates the heat needed to maintain elevated SM temperatures in tunas. T. albacares samples 40-74 mm fork length (FL) were obtained at the Inter-American Tropical Tuna Commission Laboratory in Panama. Larger juveniles, 118-344 mm FL, were collected by hook and line off of Oahu, Hawaii. Frozen fish were sectioned at 50%, 60%, and 70% FL with a cryostat and SM fibers were identified by staining for the mitochondrial enzyme succinic dehydrogenase and with SM-specific antibodies. The Scion Image analysis program was used to calculate the amount of SM as a percentage of total cross-sectional area (% SM) at 60% FL, the position of maximal % SM in larger T. albacares. As fish increased in length, the total amount of SM at 60% FL increased exponentially, and % SM gradually increased with FL and with age. The proportion of the SM cross-sectional area not contained in the lateral wedge (medial SM) also increased with FL. The distribution of SM within cross-sections varied along the body. Most of the SM at 50% FL was proximal to the horizontal septum, whereas more of the SM was medial at 60% FL and 70% FL. As the amount of SM and of medial SM increase with fish size, the potential for heat production increases, allowing larger tuna juveniles to maintain more stable SM temperatures when entering cooler waters. Thus, the development of SM may affect the distribution of T. albacares. |
