Accommodation in the Oscar, Astronotus ocellatus

Highly visual, aggressive feeders such as the oscar, Astronotus ocellatus, need a well-developed and effective accommodative apparatus. Since the refractive power of the cornea becomes negligible when the eye is immersed in water, the spherical lens of the teleost eye is its primary refractive element and has a high refractive index (Sivak 1980). The fish eye provides a unique opportunity to study accommodation since the lens is easily visible within the eye. A smooth muscle, the retractor lentis, controls the position of the lens with respect to the retina. In its contracted state, the retractor lentis muscle pulls the lens temporally toward the posterior retina for viewing far targets. As the muscle relaxes, it allows the lens to move both laterally and nasally away from the retina and toward the cornea for viewing near targets.