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G. Katzir, T. Strod, Z. Arad, I. Izhaki; Underwater Visual Acuity in Great Cormorants, Phalacrocorax carbo sinensis in Clear and Turbid Water . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2127.
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Purpose: Great cormorants, Phalacrocorax carbo, are pursuit diving birds that need to constantly cope with the demands of amphibious vision. It has been demonstrated previously that unrestrained cormorants are able to compensate for the loss of >50 of corneal power upon submergence. The purpose of the present study was to determine the visual acuity of cormorants in air and in water, under different levels of turbidity. Methods: We tested 5 mature, hand-reared unrestrained great cormorants. The birds were tested on square wave, high contrast, gratings in a 2-stimuli, forced choice situation. Choice (at a 0.75 level) was made between horizontal and vertical gratings. Underwater tests were performed in a Y-maize mesh tunnel. Turbidity levels (0.5-10 NTU) were controlled by adding suspended particles. Results: Mean visual acuity in air (n=3) was 3.8' (min. of arc) while mean visual acuity in clear water (<0.2 NTU at 20-40 µEin/m2/sec) was 8.9' (n=5). Visual acuity declined with increased turbidity as M=3.7*T+7.6 (M-minimum angle of resolution in min. of arc; T- turbidity, NTU). Conclusions: Visual acuity of cormorants suffers a two-fold decrease upon submergence yet remains at levels reported for other species in air.Turbidity had a pronounced, quantifiable, effect on grating acuity, .
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