COMPARISON OF BEHAVIORAL AND MORPHOLOGICAL MEASURES OF VISUAL-ACUITY DURING ONTOGENY IN A TELEOST FISH, FORSTERYGION-VARIUM, TRIPTERYGIIDAE(FORSTER, 1801)

Ocular morphology was examined in larval, juvenile and adult F. varium . There was a 26-fold increase in eve size from 0.28 mm in the smalles t larva (5.0 mm in length) to a maximum diameter of 7.2 mm in a 110 mm long adult. Larval fish had pure cone retinae at hatching, however, p utative rod precursor cells were also present. Juvenile and adult fish had a duplex retina with cones arranged in a square mosaic in which 4 equal double cones surrounded a central single cone. Hypertrophy of c one ellipsoids with increasing eve size resulted in maintenance of a c losely packed array in fishes of all sizes. Theoretical sensitivity, a ssessed in terms of convergence of rods:bipolars, rod density, and pho toreceptor outer segment length. increased during the juvenile phase b ut was constant across the adult size range. Angular density of cones increased with increasing eve size such that theoretical spacial acuit y was poor in smallest fish (1-degrees 8') and improved to an asymptot ic value of about 9' in adults. Behavioural acuity of a 1-day-old larv a determined using the optokinetic response (29-degrees), was very muc h poorer than histological estimates (1-degrees 8'). Behavioural acuit y improved to 4-degrees 18' at 14 days of age, compared to a theoretic al value of 54'. An estimate of Matthiessen's ratio based on histologi cal measurements suggests that the larval eye is initially strongly my opic, and grows into focus. Development of the retractor lentis muscle was first apparent 7 days after hatching with the result that larval eyes are incapable of accommodative lens movements to correct for a re fractive error. This apparent myopia is thought to account for at leas t part of the mismatch between theoretical and behavioural spatial acu ity.