Anatomy and Histology of the Eye of the Nutria Myocastor coypus: Evidence of Adaptation to a Semi-aquatic Life.

The nutria is a large, semi-aquatic rodent that, being invasive, is having a growing impact on the ecosystem in western Japan. Knowledge regarding physical adaptations to the nutria's lifestyle and habitual activities would be useful for effectively controlling and preventing their spread. Nutrias spend time on land and in water, feeding on agricultural crops and wild grasses growing near the waterside, as well as aquatic plants and shellfish. In the current study, the nutria's visual organ was analyzed anatomically and histologically, and aquatic and light environmental adaptations were evaluated. The results revealed that the nutria eyeball was almost spherical, and the cornea was rounded. The lens was convex and slightly thicker than previously reported for other rodents. These features were not characteristic of aquatic adaptations observed in the eyes of fish or marine mammals. The ratio of lens diameter to eyeball diameter was 0.6, similar to that of nocturnal species. The pupil was a vertical slit, suggesting an ability to adjust the amount of light entering the eyeball during twilight. Photoreceptors were sparsely distributed across the whole retina, and no fovea was observed. Retinal thickness was 90-100 μm, thinner than that in other rodent species. Visual acuity was 1.44-1.58 cycles/degree, higher than that in other rodents, likely because of the nutria's large eyeball and body. These results suggest that the nutria visual system is adapted to recognize large shadows of distant predators rather than viewing objects in detail.