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G Katzir, HC Howland; Accommodation Underwater in Great Cormorants, Phalacrocorax Carbo Sinensis . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1900.
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Purpose: In the submerged eye of pursuit diving birds, the refractive power of the cornea is lost, since the media bathing its inner and outer surfaces (aqueous humor, water) are of similar refractive indices. The lens thus becomes the sole agent for accommodative adjustments and must provide for the refractive power lost by the cornea if image quality is to be retained. To date accommodation in pursuit divers was investigated in restrained birds or excised eyes. This research aimed at measuring the refractive power loss and the capacity to accommodate underwater in free diving cormorants. Methods: We measured corneal power and refractive states in 7 adult, hand-reared great cormorants (Phalacrocorax carbo sinensis). Corneal radii was determined by photokeratometry. Photorefractive reflexes were determined by IR video-photorefraction both in air and when the birds were attempting to capture prey underwater. Results: Corneal radii ranged between 1.72 mm and 2.06 mm, corresponding with refractive powers of 52.1 D and 63.2 D (diopters). In air and underwater the eyes were hyperopic most of the time, and states of emmetropia and myopia were brief. Accommodation upon submergence as well as transitions between refractive states underwater were reached within 40-120 msec (1-3 video frames). In both air and underwater, states of myopia were observed when targets (prey) were approximately one bill length or less from the plane of the eye. Underwater, the pupils were not conspicuously constricted but rather retained a diameter similar to that in air. Conclusion: We demonstrated that free diving cormorants overcome the loss of corneal power and are capable of retaining a sharp image underwater. Our results do not fully agree with earlier experiments in terms of time-course (dynamics) and the coupling of iris constriction with accommodation. Also, they point to the possibility that accommodation underwater is employed for brief periods only during dives.
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