| Summary: | In the present study, Senegalese sole eggs incubated at 20 °C were reared at three different temperatures (15 °C, 18 °C or 21 °C) during the pelagic phase, and then transferred to a common temperature (20 °C) from benthic stage until 100 days post-hatch (dph). Somatic growth, fast muscle cellularity and expression of 15 growth-related genes were compared at the same developmental stage for each temperature (pre-metamorphic larvae, metamorphic larvae, post-metamorphic larvae) and at 83 dph and 100 dph early juveniles. During pre-metamorphosis and metamorphosis, larvae from 21 °C weighed significantly more than those reared at 18 °C or 15 °C (P < 0.001). Relative growth rate (RGR) of pelagic larvae and survival of newly-settled larvae were also higher at 21 °C (P < 0.05). Furthermore, an increase in muscle growth towards the highest temperatures was observed concomitantly with an increase in gene expression, namely myogenic regulatory factors (MRFs), myosins, igf-I and fgf6. After transfer to a common temperature (20 °C), the 15 °C group initiated a process of compensatory growth, inverting relative growth rate values that became the highest, particularly between 83 dph and 100 dph (P < 0.05). This increased growth effort in the 15 °C group was accompanied by an up-regulation of gene expression in fast muscle, particularly in the 83 dph juveniles. Mrf4 and myHC were up-regulated at 15 °C and positive correlations with growth were also found for myog or pax7 (P < 0.05). In contrast, mstn1 was down-regulated at 15 °C (P < 0.05), suggesting a boost in muscle growth. By 100 dph, the 15 °C group had already reached the same weight as the 18 °C and the same length as the 21 °C ones. At this age, the 18 °C juveniles had the largest mean muscle fibre diameter (P < 0.001), and thus fibre hypertrophy seems to be a major growth mechanism relatively to the other groups. Our work shows that temperature during pelagic phase greatly influences the growth trajectory of Senegalese sole juveniles.
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