Eyes of a lower vertebrate are susceptible to the visual environment.

PURPOSE: Recently, it has been found that form deprivation myopia can be induced in fish (tilapia). This study examined the sensitivity of the tilapia eye to positive and negative lenses. It further investigated the sensitivity of the fish eye to form deprivation by examining the effect of fish weight.
METHODS: Twenty-five Nile tilapia (Oreochromis niloticus; group 1) were weighed (range, 26-101 g) and killed, and their eyes were measured to provide normative data regarding fish eye size, body weight, and refractive state. Goggles with lenses of refractive powers in water of either +15 D (group 2, n = 7) or -12 D (group 3, n = 7) were sutured over the right eye of for 2 weeks to induce hyperopia or myopia. The untreated contralateral eye served as a control. An additional six fish (group 4), each wearing a goggle with an open central area, were used to evaluate the effect of the goggle itself. Refractive measurements for these 20 fish were made before and after treatment, after which the fish were killed, the eyes were removed, and axial lengths were measured from frozen sections. Another 21 fish were treated with goggles with lenses for 2 weeks, after which the goggle was removed and the refractive states of both eyes were measured every day for 6 days (day 19) and then after 28 days. These fish were placed in one group (group 5) wearing negative (-12 D) lenses (n = 8; average weight, 25.5 g) and two groups (groups 6, 7) of different size (average weights, 13.9 g [n = 5] and 26.9 g [n = 8], respectively) wearing positive (+15 D) lenses during the treatment period. In addition, translucent goggles were applied for 2 weeks to induce form deprivation myopia in three groups of fish (groups 8, 9, 10) of different weights, averaging 16.0 g (n = 7), 57.4 g, (n = 8), and 98.4 g, (n = 7), to provide an evaluation of the effect of weight on the development of form deprivation myopia.
RESULTS: In untreated fish (group 1), the axial length of the eye, ranging from 5.86 mm to 7.16 mm, was proportional to weight (26.5-101 g), whereas refractive state shifted from hyperopia (+15D for 10-g fish) toward emmetropia. The +15D lens-treated fish (group 2) became hyperopic relative to the contralateral eye (+7.7 +/- 1.6 D; mean +/- SD), whereas the -12 D lenses (group 3) induced myopia relative to the control eye (-8.4 +/- 0.8 D) within 2 weeks. Hyperopic eyes were shorter (4.16 +/- 0.11 mm vs. 4.28 +/- 0.06 mm) and myopic were eyes longer (3.96 +/- 0.36 mm vs. 3.84 +/- 0.27 mm) than their contralateral control eyes. There were no significant differences in eye size or refractive state between treated and untreated eyes of fish wearing open goggles. In the groups that were allowed to recover (groups 5, 6, 7), the fish treated with minus lenses developed an average of -9.8 +/- 1.9 D myopia, whereas +15 D lenses induced average hyperopia amounts of +8.1 +/- 1.4 D (group 6) and +6.2 5 +/- 2.87 D (group 7). All these fish recovered completely within 2 weeks once the goggles with lenses were removed. Pretreatment and posttreatment refractive results indicated that the contralateral control eyes were affected by the positive and negative lens treatments, though to a lesser extent. Form deprivation myopia was induced in all three different weight groups, averaging -11.9 +/- 2.9 D for group 8, 6.3 +/- 2.5 D for group 9, and -2.3 +/- 1.0 D for group 10. All form-deprived eyes and those treated with positive and negative lenses recovered-i.e., little or no difference resulted in refractive state or dimensions between the treated and untreated eyes-to pretreatment levels within 1 week of goggle removal.
CONCLUSIONS: Tilapia, a lower vertebrate species, exhibits positive and negative lens-induced refractive change, as is the case for higher vertebrates. In addition, the level of sensitivity to form deprivation is weight dependent.