Ontogenetic changes in phototactic behavior of Pacific bluefin tuna (Thunnus orientalis) larvae and juveniles

In this study, to elucidate the development of visual function in Pacific bluefin tuna (PBT; Thunnus orientalis ), we analyzed ontogenetic changes in the expression of visual opsin genes from hatching through the juvenile stage and evaluated wavelength- and intensity-dependent photobehavior using projected light. RT-PCR analysis detected transcripts for all opsin genes except RH2A2, a green-sensitive RH2 opsin subtype, in larvae from the yolk-sac stage (1 day post-hatching; dph) onward. In addition, RH2A2 expression was first confirmed in post-flexion larvae (15 dph) and showed no marked change through the juvenile stage. Next, we examined attraction to and avoidance of projected red, green, and blue light, as well as responses across a range of brightness levels. Larvae and juveniles avoided red light and low-brightness conditions, whereas they were attracted to blue and green light and to high-brightness conditions. These results suggest that PBT larvae can visually discriminate among wavelengths from an early developmental stage and orient toward preferred wavelengths. Moreover, the findings suggest substantial potential for improving seed production by controlling fish behavior using projected light, for example by combining avoidance of red light with attraction to blue light.