The Method for Ophthalmoscopic Observation on Ocular Fundus of Fishes

Abstract

We invented a method for observing the ocular fundus of fishes with ophthalmoscopes for human beings or with a portable fundus camera for small laboratory animals. Almost all fishes have two characteristic ocular features which make observation of the fundus difficult: the extremely refractive lense (Ca. 145 D in carp, Ca. 523 D in crucian carp), and the other streamlined surface of the cornea, which causes astigmatism in air. However, the fish eye is not astigmatic when in water, because the refractive index of the fish cornea approximately equals that of water, so that little refraction occurs at the corneal surface. Difficulties caused by these features in air observation were eliminated by covering the cornea with a contact lens, and by filling the space between the contact lens and cornea with distilled water.

Prior to ophthalmoscopy, the fish (carp) was anesthetized in well aerated 220 ppm MS222 (tricaine methanesulfonate) solution. Anesthesia was maintained during ophthalmoscopy with well aerated 100 ppm MS222 solution by which the gill surfaces of the fish were irrigated through the mouth.