Developmental changes of the light intensity threshold for school formation in the striped jack Pseudocaranx dentex

Light intensity thresholds for school formation were examined in 3 stages ( 20, 65, and 120 mm) of striped jack Pseudocaranx dentex juveniles with beha vioural and histological approaches. Behaviour under a light intensity of 3 00 to 10(-5) lx was recorded using a video camera, and schooling behaviour, swimming speed, and nearest neighbour distance were analyzed. The retina o f fish adapted to each light intensity was histologically examined. The lig ht intensity threshold for schooling behaviour in 20 and 65 mm fish was 5 x 10(-2) lx, while that of 120 mm fish was 5 x 10(-4) lx. The adaptation rat io, defined as the percentage of cone cell movement, ranged from 30 to 80%, corresponding with 10(-3) to 1 lx of light intensity in 120 mm fish, while 20 mm fish showed a much narrower range of adaptation ratio, i.e, from 50 to 70%. Visual acuity increased exponentially with fish total length from 2 0 to 120 mm, corresponding with the increase of eye diameter. The higher ra nge of adaptation ratio in 120 mm fish should enable them to recognize othe r fish even under low light intensity conditions and make their light inten sity threshold for schooling lower than that of smaller fish. Establishment of behavioural and histological adaptability to lower light intensity shou ld have relevance to the habitat shift that occurs in this juvenile stage f rom bright shallow reef pelagic waters to the offshore dark deeper area.