Department of Marine Science and Technology, Tokyo University of Fisheries, Tokyo 108-8477, Japan
The development of visual acuity in yellowtail were investigated by histological examination of the retina for 43 individuals (BL 15-392mm). The lens diameter increased from 0.5mm to 7.1mm proportionally with the growth of body length. The cone density was found to fit in an exponentially declining curve, where it steeply decreases in the range of 15-73mm BL and becomes a gentle slope to be around 650-304cells/0.01mm2 for samples over 179mm BL. According to the results, the visual acuity was increased with growth; from 0.04 for 15mm BL to 0.23 for 390mm BL specimen. The improvement of acuity can be attributed to the increase of the lens diameter rather than the effect of the cone density. The visual acuity (V.A.) was shown to fit to the allometric function of the body length (BL) as follows,
V.A.=0.0051BL0.6223, (r2=0.97)