Temperature and development in larvae of the turbot Scophthalmus maximus

Turbot (Scophthalmus maximus L.) were reared at 12 and 16°C until 26 d after hatching. At both temperatures, starting at the neural plate stage, somites were initially formed every 75 min. Expressed as a percentage of development time (DT, fertilisation to 90% larvae hatching) somite formation occurred relatively earlier during embryogenesis at 12°C (45% DT) than at 16°C (55% DT). At 12°C, after the 32-somite stage the rate of somite formation decreased to one every 300 min. The larvae hatched after 6 d at 12°C and 3 d at 16°C at a relatively primitive stage of development, prior to the opening of the mouth and anus, with unpigmented eyes, and a straight gut. Temperature altered the relative timing of organogenesis in the larval stages. At 12°C, the following characters appeared (in this order): swimbladder>loop in the gut (at the time of yolk exhaustion)>caudal fin. In contrast, at 16°C, the caudal fin appeared at the same time as the loop in the gut. At 16°C, spines formed on the head in the region of the otic capsule at the time the swimbladder formed and the yolk was exhausted, but were absent in 12°C larvae. At both temperatures, in 1 d-old larvae the myotomes just behind the yolk-sac contained 200 inner muscle fibres (presumptive white muscle). The initial growth of inner muscle was largely due to hypertrophy, but by 26 d at 12°C and 11 d at 16°C hyperplastic growth became important, as evidenced by a significant increase in the number of small fibres (<10 m²). By 26 d the average number of inner muscle fibres had increased to 341 at 12°C and 988 at 16°C. New muscle fibres were added in distinct germinal zones at the dorsal and ventral apices of the myotomes. Metamorphosis was associated with a thickening of the superficial (presumptive red) muscle layer and the appearance of tonic muscle fibres.