EMBRYONIC TEMPERATURE MODULATES MUSCLE GROWTH-CHARACTERISTICS IN LARVAL AND JUVENILE HERRING

The influence of embryonic and larval temperature regime on muscle gro wth was investigated in Atlantic herring (Clupea harengus L.). Eggs of spring-spawning Clyde herring were incubated at 5 degrees C, 8 degree s C off 12 degrees C until hatching and then reared until after metamo rphosis at rising temperatures to simulate a seasonal warming, Metamor phosis to the juvenile stage was complete at 37 mm total length (TL), after an estimated 177 days as a larva at 5 degrees C, 117 days at 8 d egrees C and 101 days at 12 degrees C, Growth rate and the development of median fins were retarded in relation to body length at 5 degrees C compared with 8 degrees C and 12 degrees C, Between hatching (at 8-9 mm TL) and 16 mm TL, there was a threefold increase in total muscle c ross-sectional area, largely due to the hypertrophy of the embryonic r ed and white muscle fibres, The recruitment of additional white muscle fibres started at approximately 15 mm TL at all temperatures, and by 37 mm was estimated to be 66 fibres day(-1) at 5 degrees C and 103 fib res day(-1) at 8 degrees C and 12 degrees C, Peptide mapping studies r evealed a change in myosin heavy chain composition in white muscle fib res between 20 and 25 mm TL, Embryonic red muscle fibres expressed fas t myosin light chains until 24-28 mm TL at 5 degrees C and 22 mm TL at 12 degrees C, and new red fibres were added at the horizontal septum starting at the same body lengths, Following metamorphosis, the total cross-sectional area of muscle was similar at different temperatures, although the number of red and white fibres per myotome was significan tly greater at the warmest than at the coldest regime, For example, th e mean number of white muscle fibres per myotome in 50 mm TL juveniles was calculated to be 23.4 % higher at 12 degrees C (12065) than at 5 degrees C (9775), In other experiments, spring-spawning (Clyde) and au tumn-spawning (Manx) herring were reared at different temperatures unt il first feeding and then transferred to ambient seawater temperature and fed an libitum for constant periods, These experiments showed that , for both stocks, the temperature of embryonic development influenced the subsequent rate of muscle fibre recruitment and hypertrophy as we ll as the density of muscle nuclei, Labelling experiments with 5'-brom o-2-deoxyuridine showed that both the hypertrophy and recruitment of m uscle fibres involved a rapidly proliferating population of myogenic p recursor cells. The cellular mechanisms underlying the environmental m odulation of muscle growth phenotype are discussed.