Muscle growth and development in Atlantic cod larvae (Gadus morhua L.), related to different somatic growth rates.

The present study describes the development of the axial musculature in first-feeding larvae of Atlantic cod (Gadus morhua L.) with different somatic growth rates achieved by using different nutritional conditions. Muscle growth was assessed by determining the number of muscle fibres (hyperplasia) and the growth of existing fibres (hypertrophy). Larvae were fed rotifers containing a high (1. 4; treatment 1) or low (0.2; treatment 2) ratio of docosahexaenoic acid to eicosapentaenoic acid from day 5 after hatching. From day 17, the larvae were fed Artemia nauplii with the same enrichment in both treatments. Treatment 1 gave the highest somatic growth rate and hence the highest dry mass at the end of the experiment, but no difference in larval standard length was found between treatments. In slow-growing larvae, higher priority was thus put into reaching a certain length than into increasing muscle mass. The largest fibres, which were present from hatching, increased in cross-sectional area during larval development, but no differences were found between treatments in the cross-sectional area of individual fibres or the total cross-sectional area of these fibres at the end of the experiment. The first white recruitment fibres were observed at the dorsal and ventral apices of the myotome at approximately the onset of first feeding (larval length 4.5 mm). In larvae 8.5 mm long, the total cross-sectional area of white muscle fibres in the treatment 2 group was 75 % of that in the treatment 1 group. The highest somatic growth rate was associated with an increased contribution of hyperplasia to axial white muscle growth. In the faster-growing larval group, the relative contribution of hyperplasia to the total white muscle cross-sectional area was 50 %, whereas it was 41 % in the slower-growing larval group. The subsequent growth potential may thus be negatively affected by inadequate larval feeding.