Temperature effects on muscle fibres number at hatch and first feeding in blackspot seabream (Pagellus bogaraveo)

It has been shown that the thermal experience during the earliest phases of development can determine the larval and postlarval growth characteristics of teleosts (Clupea harengus, VIEIRA and JOHNSTON; 1992; Salmo salar, STICKLAND et al., 1988; Dicentrarchus labrax L., AYALA et al., 2000, 2001). In order to investigate the effects of the early temperature regime on prelarval stages of growth of Pagellus bogaraveo, this species was exposed during egg incubation until larvae mouth opening (vitelline phase) at two temperatures: natural temperature (± 14 oC) and 18.0 ± 0.01 oC. Muscle growth was studied by morphometric and immunostaining techniques. The total number of fibres was counted in body cross sections, at both the postopercular and perianal levels. High incubation temperature accelerated embryonic (hatching) and prelarval (mouth opening) development. In newly hatched larvae, myotomes showed two muscular layers: a superficial monolayer of several fibres that extend hypaxially and epaxially away from the lateral line, and a deep layer composed of larger diameter fibres. At hatching, no significant influence of the incubation temperature was observed on muscle fibre number. At the end of the vitelline phase, three layers of muscle fibres were observed, with different antimyosin reactivities: a superficial monolayer, a presumptive slow red (present only as a small group of fibres adjacent to the lateral line nerve) and a fast-white layer. At mouth opening, the highest incubation temperature promoted an increased number of white fibres at the postopercular (N = 199 (CV = 0.17)) vs lowest temperature (N = 146 (CV = 0.07)), being the white muscle fibre hyperplasia greater in the epaxial part of the larvae (P < 0.05). At hatching and mouth opening, the number of superficial monolayer fibres was similar for both tested temperatures. In conclusion, a slight increase of temperature during the vitelline phase of blackspot seabream promoted an epaxial hyperplastic growth of white fibres at the postopercular level, in the prelarval stage. It will be now very important to asses whether the initial fibre number gain can support a faster and/or greater posterior growth of fish under standard conditions.