Fig. 9. The influence of photoperiodic regime on myogenesis in fast muscle of adult seawater stages of Atlantic salmon (Salmo salar L.). (A) The number of fast muscle fibres per myotomal cross-section in fish reared under 24 h continuous light (open circles) or at ambient photoperiod (closed circles). The broken blue line represents sunrise and sunset (Greenwich Mean Time) at Fort William, Scotland where the fish were reared and the green line illustrates daily recordings of sea temperature. The duration of 24 h lighting in the photoperiod manipulated cages is illustrated by the box. The values represent mean ± s.e.m. of 6–12 fish per sample, see original publication for details (Johnston et al., 2003c). (B) The myonuclei content of isolated fast muscle fibres in relation to fibre diameter (D; µm) for the 24 h light-treated fish (open circles) and the ambient day-length fish (closed circles). First-order linear regressions were fitted to the data. For the continuous light-treated fish myonuclei number=1258.1+12.4D (broken line) and for the natural day-length fish myonuclei number=288.6+14.4D (solid line). From (Johnston et al., 2003).