COMPARISON OF BEHAVIORAL AND MORPHOLOGICAL MEASURES OF VISUAL RESOLUTION DURING ONTOGENY OF ROACH (RUTILUS-RUTILUS) AND YELLOW PERCH (PERCA-FLAVESCENS)

Changes in visual resolution of two fish species, the yellow perch Per ca flavescens, and the roach, Rutilus rutilus, were measured during on togenetic development from first feeding to adult fish using (i) morph ological measurements (eye lens diameter and cone density) and (ii) a behavioural correlate of visual resolution (reactive distance). Both m ethods were used to calculate visual resolution expressed as minimum s eparable angle (MSA). Morphological estimates of MSA were similar in b oth species whereas behaviourally determined MSA values were higher in roach. Initially, predicted performance based on morphological measur ements was 3-10 times better than behavioural measurements of MSA for both species. With growth, behavioural measures of visual resolution i mproved, resulting in similar MSA values at 35 mm standard length in p erch and 100 mm standard length in roach. Above these sizes, both spec ies outperformed the morphologically predicted MSA values. The sensiti vity of calculations of MSA from eye lens diameter and cone density to both variables was analysed and ways to correct for differences in me thods (e.g., the use of fixed or unfixed material, whether cell counts were made within specific retinal areas, and changes in Mathiessen's ratio) were exemplified to facilitate comparison of absolute MSA value s.