A study was conducted on common carp (Cyprinus carpio L,) to determine the effects of environmental temperature experienced by embryos and larvae on the development of myotomal white muscle. Eggs from one female were divided into two groups following fertilisation and incubated at constant pre-hatch temperatures of 18 or 28 degreesC, At hatching, larvae from the 18 degreesC-incubated eggs were divided into two groups and either reared at the same temperature of 18 degreesC ('cold' group) or transferred over a period of 5 days (at 2 degreesC per day) to 28 degreesC ('transferred' group), Larvae hatched from eggs incubated at 28 degreesC were reared at the same temperature of 28 degreesC ('warm' group). Larvae mere sampled at two developmental stages (stage 1, inflation of the back chamber of the swimbladder; stage 2, inflation of the front chamber of the swimbladder) and at 26 days post-hatching. The maturation of myotome shape during larval life was studied in parallel with the changes occurring in the organisation of white fibres, At stage 1, the epaxial part of the myotomes surrounding the vent had the shape of lamellae inclined backwards, and only one central layer of white fibres was present. At stage 2, the epaxial part of the myotomes began to acquire a V-shape, which was well developed at 26 days post-hatch. At stage 2 and at 26 days post-hatch, two layers of white fibres were identified: the initial central layer and a second apical layer, These differ in their orientation, the initial central layer being orientated backwards and the apical layer forwards, and in the mean fibre diameter, which is greater in the initial central laver, Studies on the effects of temperature (constant 18 degreesC, constant 28 degreesC, transfer from 18 to 28 degreesC at hatching) were carried out according to both the developmental stage and the length of the larvae. At stage 1, no significant differences were found between the three groups for larval standard length and muscle variables. The number of fibres in one quadrant of epaxial white muscle sectioned at the level of the vent was 100-111, At stage 2, there were significant differences between groups. Larval standard length and mass were higher in the cold group than in the warm group. The transferred larvae were of intermediate standard length but had a significantly higher cross-sectional area of white muscle than either of the other two groups, This increase in surface area was related to a 50% greater fibre number (233) in the transferred larvae compared with the cold (165) or the warm (152) larvae. The increase in fibre number was more marked for large-diameter (>20 mum) white fibres located in the initial central fibre layer (+58-72% in transferred larvae) than in small-diameter (less than or equal to 10 mum) white fibres mainly located in the apical layer (+18-35%). In 26 days post-hatch samples, transferred larvae still showed a higher total number of white fibres than warm larvae, but the difference was no longer significant when the total number of white fibres was regressed against larval standard length, suggesting that this stimulation may be temporary.
