Dietary protein content influences both growth and size distribution of anterior and posterior muscle fibres in juveniles of Pagellus bogaraveo (Brunnich).

Muscle cellularity was studied in Pagellus bogaraveo juveniles fed on diets with different protein contents. Measured in transversal body sections, at both post-opercular and post-anal locations, the morphometric variables estimated were: total muscle area (A), total number of fibres (N), number of fibres per unit area of muscle (N (A)) and cross-sectional fibre area (ā) of the two main muscle fibre types. At the end of the experiment fish fed on diets having more than 40% of protein displayed significantly higher body weight. Fish fed on protein-rich diets exhibited greater ā and N. For fish fed on 30 and 50% protein diets the morphometric parameters measured grew linearly with the fish weight. High-protein diets favoured muscle hyperplasia. When comparing rostral and caudal locations, a greater N and a smaller ā of posterior red fibres were the consistent differences found-a fact, to our knowledge, so far unreported for fish.