Estimates of visual acuity in a pelagic freshwater zooplanktivorous fish, the white crappie (Pomoxis annularis, Centrarchidae), were made using a behavioral measure, the maximum observed prey pursuit distance (MxPD), and a histological measure, the density of cone cells in the retina. The greatest number of pursuits occurs in the 0–30° wedge of the visual field; 87% of all pursuits occur in the first 40°. The longest pursuits (200 mm) also occur in this area and generally get shorter from 0 to 180° (from forward-directed) in the visual field. Consistent with the behavioral results, the largest number of cone photoreceptors (13,000/mm²) is found in the far temporal retina along the eye's horizontal meridian. Cone cell densities in the corresponding region of the nasal retina are approximately half this value. The number of cones decreases dorsally and ventrally from the horizontal meridian. Although the absolute values of visual acuity calculated from cone cell topography (i.e. MxPDs of 500 mm) are 2–3 times greater than those observed behaviorally (i.e. MxPDs of 200 mm), the trends in visual acuity across the visual field obtained from both measures are consistent. We suggest that overestimates of visual acuity obtained from cone cell counts alone result from this measure's not accounting for, among other properties of the nervous system, cone cell convergence onto ganglion cells and higher brain centers. Behavioral measures of visual acuity are, therefore, likely to yield a more accurate estimate of an animal's visual abilities.

The uptake, metabolism, and cellular distribution of 3H-docosahexaenoic acid (3H-22:6) in the frog retina during in vitro incubation were studied. An initial diffuse labeling throughout the retina was detected by autoradiography and was followed by an active steady increase in labeled photoreceptor cells. After 6 hr of incubation, 92% of the label was concentrated in photoreceptor cells. Among these cells, 435 rods (green rods) labeled heavily and showed two to three times higher uptake than the 502~rods (red rods). Cone uptake labeling was the lowest, showing negligible labeling throughout the cytoplasm. However, oil droplets of the 575 cones actively concentrated 22:6. The high uptake of 3H-22:6 by photoreceptor cells was followed by its rapid esterification into phospholipids. After 6 hr of labeling, only 5% of the radioactivity in the retina was free 22:6, whereas 66% was esterified into phospholipids. The remaining 22:6 was distributed equally in triacylglycerols (TAGS) and diacylglycerols. When 3H-22:6 (0.11 PM) of high specific activity was used, early incubation times showed phosphatidylinositol (PI) labeling to be of the same order of magnitude or greater than that of phosphatidylcholine (PC) or phosphatidylethanolamine (PE). Although the amount of endogenous 22:6 esterified into PI accounted for less than 2% of the 22:6 in retinal phospholipids, 27% of 3H-22:6 labeling was recovered in this phospholipid. When 'C-22:6 at a final concentration of 70 PM was used, a different profile of lipid labeling was observed. Forty percent of the labeling remained in the free fatty acid pool, followed by TAG (24%), PC (14%), and PE (12%). PI showed the smallest increase in picomoles of 14C-22:6 incorporated, when compared with 3H-22:6. In conclusion, a selective and differential uptake of 3H-22:6 by photoreceptor cells is coupled to its active utilization for phospholipid biosynthesis, mainly that of PC, PE, and PI. The differential uptake of 3H-22:6 among photoreceptor cells may reflect involvement of this fatty acid in cell-specific functions.

Retinal detachment induces longlasting changes in lipid metabolism which are reflected in lower labeling of [3H]DHA-phospholipids. Metabolic changes, sustained through 48 hours, may lead to inadequate synthesis/turnover of phospholipids, among them, those containing DHA, possibly resulting in defective disc membrane assembly with subsequent deterioration of visual cells.