Ontogenetic development of larval phototaxis in Japanese amberjack, Seriola quinqueradiata and greater amberjack, Seriola dumerili

The walling behavior of larvae, driven by positive phototaxis, induces jaw malformations and significantly impairs survival in Seriola juveniles, posing substantial challenges to their fingerling production. However, comprehensive data on the specific light intensity at which larvae exhibit phototactic behavior is currently insufficient. This information is crucial for developing effective countermeasures, such as reducing the reflected light intensity from the rearing tank wall to a level below the phototactic threshold. The current study examined the ontogenetic development of horizontal larval phototaxis in Seriola quinqueradiata and S. dumerili. Larval phototaxis was investigated by comparing the horizontal distribution of 100 individuals before and after exposure to light projected from one end of a transparent acrylic rectangular aquarium (21 × 25 × 100 cm) for a duration of 10 min, with light intensity ranging from 0.01 to 10 μmol/m²/s. The larvae of both species exhibited positive phototaxis from the commencement of feeding until approximately two weeks post-hatching at light intensities exceeding 0.10 μmol/m²/s for S. quinqueradiata and 0.01 μmol/m²/s for S. dumerili. The minimum light intensity required to elicit positive phototaxis varied ontogenetically in both species, with both shared and species-specific chracteristics. In S. quinqueradiata larvae, the lower threshold of positive phototaxis elevated with age, increasing from 0.10 μmol/m²/s on two days post-hatch (dph) to 10.00 μmol/m²/s by 14 dph. Positive phototaxis was not detected at 10.00 μmol/m²/s on 16 dph. S. dumerili larvae exhibited positive phototaxis at 3 dph under a light intensity of 0.01 μmol/m²/s, the lowest intensity assessed. This behavior was observed at all light intensities examined from 5 to 10 dph, with the exception of 7 dph. On 13 dph, they exhibited phototaxis exclusively at the minimal light intensity of 0.01 μmol/m²/s. Subsequently, the larvae did not exhibit any further positive phototaxic response. Additionally, larvae of both species exhibited negative phototaxis at the lowest light intensity examined (0.01 μmol/m²/s) at 4 dph. Furthermore, the light intensities inducing a negative phototactic response broadned with age. In S. dumerili, this phenomenon was exclusively observed at the minimal light intensity of 0.01 μmol/m²/s at the older age of 15 dph. The findings of this study advance the criteria for reflected light intensity from the larval rearing tank wall to mitigate larval walling behavior in both species.