Metamorphosis induces a light-dependent switch in Senegalese sole (Solea senegalensis) from diurnal to nocturnal behavior.

Light plays a key role in the development of biological rhythms in fish. Recent research in Senegal sole has revealed that spawning and hatching rhythms, larval development, and growth performance are strongly influenced by lighting conditions. However, the effect of light on the daily patterns of behavior remains unexplored. Therefore, the aim of this study was to investigate the impact of different photoperiod regimes and white, blue, and red light on the activity rhythms and foraging behavior of Solea senegalensis larvae up to 40 days posthatching (DPH). To this end, eggs were collected immediately after spawning during the night and exposed to continuous white light (LL), continuous darkness (DD), or light-dark (LD) 12L:12D cycles of white (LD(W)), blue (LD(B), λ(peak) = 463 nm), or red light (LD(R), λ(peak) = 685 nm). A filming scenario was designed to video record activity rhythms during day and night times using infrared lights. The results revealed that activity rhythms in LD(B) and LD(W) changed from diurnal to nocturnal on days 9 to 10 DPH, coinciding with the onset of metamorphosis. In LD(R), sole larvae remained nocturnal throughout the experimental period, while under LL and DD, larvae failed to show any rhythm. In addition, larvae exposed to LD(B) and LD(W) had the highest prey capture success rate (LD(B) = 82.6% ± 2.0%; LD(W) = 75.1% ± 1.3%) and attack rate (LD(B) = 54.3% ± 1.9%; LD(W) = 46.9% ± 3.0%) during the light phase (ML) until 9 DPH. During metamorphosis, the attack and capture success rates in these light conditions were higher during the dark phase (MD), when they showed the same nocturnal behavioral pattern as under LD(R) conditions. These results revealed that the development of sole larvae is tightly controlled by light characteristics, underlining the importance of the natural underwater photoenvironment (LD cycles of blue wavelengths) for the normal onset of the rhythmic behavior of fish larvae during early ontogenesis.