AGE-RELATED-CHANGES IN THE VISUAL-ACUITY OF LARVAE OF NEW-ZEALAND SNAPPER, PAGRUS-AURATUS

Snapper Pagrus auratus (Bloch & Schneider) (Pisces: Sparidae) larvae a re visual feeders which initiate exogenous feeding four days after hat ching. Visual acuity of cultured snapper larvae was determined morphol ogically from cone spacing within the retina, and improved from 2-degr ees-10' minimum separable angle (MSA) in a 4-d-old larva (3-2 mm SL) t o 52' in a fish 22 d old (5.8 mm SL). Visual acuity determined behavio urally using the optokinetic response, was poorer at the time of first feeding than acuity determined morphologically. Behaviourally determi ned acuity improved with growth from 38-degrees in 4-d-old fish (mean SL 3.1 mm) to 8-degrees-8' at 16 d of age (mean SL 4.9 mm). This was s till lower than the theoretical estimate of acuity (55' at 18 d old, 5 .9 mm SL). An estimate of the Matthiessen ratio based upon histologica l measurements suggests that the larval eye is initially strongly myop ic and grows into focus. Development of the lens accommodative system was first apparent in fish at 4 d of age as a pigmented outgrowth of t he ventral iris. A retractor lentis muscle was present in 7-d-old larv ae (3.5 mm SL) and suspensory ligaments were present in 10-d-old fish (3.6 mm SL). As a consequence, larval snapper were initially incapable of accommodative lens movements to correct for a refractive error. Th e maximum internal jaw dimensions, an estimate of maximum ingestible p rey size, ranged from 152 mum at first feeding to 373 mum in a 22 d ol d larva. Reactive distance to prey based on MSA determined from the op tokinetic response and maximum prey width, were small in first-feeding fish (0-2 mm for prey width of 150 mum), but increased linearly with both body size and prey width. Minimum separable angles of first-feedi ng snapper determined optokinetically, were larger (lower acuity) than the visual angles determined from feeding events of first-feeding fis h larvae reported elsewhere. This may reflect the fact that optokineti c experiments involve a single sensory modality and chemical sense and developing lateral line may contribute to visually oriented feeding.