Hyperplastic and hypertrophic growth of lateral muscle in blackspot seabream Pagellus bogaraveo from hatching to juvenile.

To understand better the growth mechanisms in the economically important fish Pagellus bogaraveo, in terms of muscle fibre hyperplasia v. hypertrophy, the lateral muscle of this fish was studied morphometrically from hatching to juvenile comparing rostral and caudal locations. Fish were sampled at 0, 5, 23, 40, 70, 100, 140 and 180 days. Fibre types were first identified by succinate dehydrogenase (SDH) and immunostaining with a polyclonal antibody against fish slow myosin (4-96). Morphometric variables were then measured in transverse body sections, at both post-opercular and post-anal locations, to estimate the following variables: total muscle area [A (muscle)], total fibre number [N (fibres)], fibre number per unit area of muscle [N(A)(fibres, muscle)] and cross-sectional fibre area [a (fibres)] of the two main muscle fibre types (white and red). Overall, growth throughout the various stages resulted from increases both in the number and in the size of muscle fibres, paralleled by an expansion of the [A (muscle)]. Nonetheless, that increase was not significant between 0-5 days on one hand and 100-140 days, on the other hand. On the contrary, the [N(A)(fibres, muscle)] declined as the body length increased. Analysis of the muscle growth kinetics suggested that, within the important time frame studied, hyperplasia gave the main relative contribution to the increase of white muscle [A (white muscle)], whereas red muscle [A (red muscle)] mainly grew by hypertrophy, with both phenomena occurring at a faster pace posteriorly in the body. Finally, when comparing rostral and caudal locations, a greater [N (fibres)] and [A (muscle)] of the posterior white and red fibres were the consistent features. It was also observed that the proportion of the cross-sectional area of the myotomal muscle comprised of white muscle was greater in the anterior part of the fish.