Behavioural and anatomical measures of visual acuity in first-feeding Yellowtail Kingfish (Seriola lalandi) larvae

Ontogenetic change in the visual acuity of Seriola lalandi larvae was measured using both behavioural and anatomical techniques. Visual acuity improved over early development (day 4 to day 7 post-hatch), although for all three larval ages examined estimates of anatomical acuity were consistently lower (higher acuity) than estimates of behavioural acuity. At hatching the eyes of larval kingfish were characterized by an undifferentiated retina surrounding a spherical lens, by day 4 post-hatch the eyes appeared to be functional, the retina was fully pigmented and the optic nerve had made contact with the optic tectum. Analysis of prey search behaviour indicated that larvae employ a saltatory type search behaviour in which brief stationary periods are interspersed with repositioning movements. The mean reactive angle increased between day 4 and day 7 post-hatch indicating that the horizontal visual field was expanding with development, thereby increasing the search area of larvae. Pre-strike distances of early larvae were substantially less than one body length, being constantly around a 1/3 of a body length for all larval ages examined.