Due to the otter's amphibious nature, the refractive contribution of the cornea is routinely negated when the eve is submerged in water. To maintain acute vision in both air and water would require the possession of an accommodative range that is equal to the refractive contribution of the cornea in air. Using the techniques of static photorefraction and dynamic photorefraction, we found the sea otter to be within a few diopters of emmetropia in both air and water. Photokeratoscopy of live animals and geometric analysis of freshly enucleated globes demonstrated the refractive contribution of the cornea in air (and hence the otter's accomodative range) to be approximately 60 diopters (4X the range of a 10 year old child. Morphologic examination revealed the otter eye to contain an extensive iridalsphincter muscle and very well developed iridal dilator and meridional ciliary muscles. A similar muscular arrangement has been described for amphibious birds in which iridal contraction plays an active role in lenticular deformation. We suggest a similar mechanism is employed by the sea otter eye.

Funded in part by the Veterinary Optometric Society.