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A review of the literature indicated contrasts in retinal structure and function between perches (Perca spp.) and pikeperches (Stizostedion spp.). Foremost among these were differences in size and distribution of rods, size and shape of cones, extent of retinomotor responses, possession or lack of reflecting material, and relative concentrations of melanin. The perches are primarily active by daylight, whereas the pikeperches are most active during twilight or at night. The coincidental occupancy of shoal areas by yellow perch (Perca flavescens) and walleye (Stizostedion vitreum vitreum) at twilight is related to decreasing visual acuity of the former and the approach of optimum visual performance in the latter species. On this basis the two species have a classic predator–prey relationship. Key words: histology, light, Perca sp., photopic vision, predation, retina, Stizostedion vitreum vitreum, symbiosis, tapetum lucidum

Daily variations in the melatonin concentration in the blood, pineal organ, and retina of the brook trout, Salvelinus fontinalis, were investigated in July under constant photoperiod (16 h L: 8 h D) and temperature (12 °C). Diel variations in melatonin levels in the blood and pineal organ showed similar patterns, with higher values during the night, whereas melatonin concentrations in the retina increased slightly in the first half of the light period. Light exposure of 1 h duration at mid-dark decreased melatonin levels in the blood and pineal organ in an intensity-dependent manner, whereas retinal melatonin levels increased with increasing light intensities. The minimum light intensity causing significant effects on the melatonin concentrations was 2 lx for the serum and 20 lx for the pineal organ and the retina. The parallelism between pineal and circulating melatonin patterns suggests that the lateral eyes of the trout have no significant endocrine function as far as the melatonin rhythm in the blood is concerned. Supporting evidence comes from the reversed response of retinal melatonin to light exposure at mid-dark, and from the low retinal melatonin concentrations found in this study. Thus, the serum melatonin rhythm of the brook trout seems to be mainly the result of rhythmic melatonin secretion from the pineal organ.

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