Muscle growth in yolk sac larvae of the Atlantic halibut (Hippoglossus hippoglossus L.), as influenced by temperature in the egg and yolk sac stage

Atlantic halibut eggs and yolk sac larvae were incubated at 1, 5 and 8°C. The embryos and larvae did not tolerate incubation at 1°C. Egg i ncubation at 8°C gave newly hatched larvae with smaller individual wh ite fibres and total white cross-sectional area than eggs incubated a t 5°C. Transport of eggs two days prior to hatching gave higher survi val rates than when eggs were transported shortly after blastopore cl osure, and at hatching the larvae of the former group were longer wit h larger individual red fibres total red cross-sectional area. During the yolk sac stage, the total white cross-sectional area increased 3 -fold. The increase in white cross-sectional area was entirely due to hypertrophy between hatching and 150 d° (10 mm SL; 50% yolk absorpti on). Hypertrophy was not affected by differences in rearing temperatu re, and the large white fibres constituted more than 90% of the myoto me during the yolk sac stage. Recruitment of new fibres started at 15 0d°. Incubation at 8°C during the yolk sac stage gave a lower surviva l at 230d° than at 5°C, but the larvae of the former group were longe r and had more recruitment fibres. Little is known about the natura l biology of halibut yolk sac larvae, and the timing of the onset of exogenous feeding has therefore been subjected to discussion. Our res ults show a shift from hypertrophy to hyperplasia as the main mechani sm of muscle growth at 150d°, the same age at which many other major ontogenetic changes important for feeding occur. Our results also ind icate that rearing temperature during the yolk sac stage affects musc le hyperplasia, but not hypertrophy.