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Gadi Katzir, Ruth Almon, Ido Izhaki; Chromatic components in underwater targets do not affect Great cormorants’ (Phalacrocorax carbo) visual resolution. Invest. Ophthalmol. Vis. Sci. 2013;54(15):3029.
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The negative effects of scatter and absorption of light on image formation impair vision underwater. Because scatter and absorption are wavelength dependent, it is expected that visual resolution for targets with different chromatic components, will differ. Great cormorants pursue prey (fish) underwater, facing visual constraints in their frequent transitions between air and water. Visual resolution of cormorants for achromatic targets has been previously established yet the effects of chromatic components have remained open. We aimed here to determine the underwater visual resolution of cormorants for square wave gratings comprising chromatic and achromatic components.
Hand-reared Great cormorants (N=6) were trained and tested for their resolution, in an underwater Y-maze. The visual targets comprised square-wave gratings that were achromatic (black & white) or chromatic (black & color). Colors ranged from “reds” through “yellow greens” to “blues”. Targets with vertically oriented bars were “positive” and targets with horizontally oriented bars were “negative”. Gratings ranged from 1.4 to 12 cpd. Illumination was diffuse daylight and water turbidity ranged 0.3-5.6 NTU. The cormorants’ choice was made at 1.4m from the targets. Each individual provided results from ca. 30 tests on chromatic and ca. 7 tests on achromatic gratings. The proportion of correct choices was used to determine resolution (at p=0.75 level).
Over all cormorants (grouped) the mean maximal underwater visual resolution for achromatic and chromatic gratings was ca. 8cpd. Resolution for achromatic gratings was consistently higher than for chromatic gratings yet the difference was not significant. Chromatic components did not have a significant effect while the effects of gratings frequency and of individual differences on resolution were significant. Individuals showing high resolution reached 6.1-12.2 cpd while individuals showing low resolution reached 3.0 - 4.0 cpd.
Under the experimental conditions here, the chromatic components in the targets did not affect visual resolution. It may well indicate that color patterns of fishes, such as alternating black and chromatic bars, may not significantly affect their detection by foraging cormorants. Marked and consistent individual differences in visual resolution must play a role in prey detection and capture success.
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