Size-related Change in the Visual Resolution of Sunfish (Lepomis spp.)

Abstract

Visual resolution of juvenile sunfish (Lepomis spp.) (8–33 mm standard length (SL)), although extremely poor in comparison with the larger individuals (38–160 mm SL) used in previous studies, improves rapidly as they grow. Histologically and behaviorally determined (mean reaction angle) visual angles of fish between 10 and 33 mm SL decrease by approximately 50 and 100 minutes of arc, respectively, and decline non-linearly with increasing fish size. Behaviorally determined visual resolution of juvenile sunfish based on maximum location distance (MLD) is equivalent to that calculated from intercone spacing. The mean reaction angle used in previous studies may have underestimated behavioral visual resolution of larger (> 38 mm SL) sunfish by approximately 30%. Visual volume and search volume increase by nearly three orders of magnitude in sunfish between 8 and 50 mm SL. After sunfish exceed 50 mm SL (when they can safely return to the pelagic zone), visual resolution increases comparatively slowly as body size increases. Our results suggest that the size-related change in behavioral visual resolution in sunfish is influenced by other factors in addition to the growth-related changes in the resolving power of the retina.