Miyazaki Taeko, Naritsuka Yukako, Yagami Michihiro, Kobayashi Shuji, Kawamura Koichi
Department of Marine Bioresources, Graduate School of Bioresources, Mie University, 1577 Kurimamachiya-cho, Tsu, Mie 514-8507, Japan. E-mail:
Department of Zoology, Faculty of Science, Okayama University of Science, Ridai-cho 1-1, Kita-ku, Okayama 700-0005, Japan. E-mail:
Zool Stud. 2022 May 10;61:e18. doi: 10.6620/ZS.2022.61-18. eCollection 2022.
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.
海狸鼠是一种大型半水生啮齿动物，作为入侵物种，它对日本西部的生态系统影响日益增大。了解海狸鼠对其生活方式和日常活动的身体适应性，将有助于有效控制和防止它们的扩散。海狸鼠在陆地和水中活动，以水边生长的农作物、野草以及水生植物和贝类为食。在本研究中，对海狸鼠的视觉器官进行了解剖学和组织学分析，并评估了其对水生和光照环境的适应性。结果显示，海狸鼠的眼球几乎呈球形，角膜呈圆形。晶状体呈凸形，比之前报道的其他啮齿动物略厚。这些特征并非鱼类或海洋哺乳动物眼睛中所观察到的水生适应性特征。晶状体直径与眼球直径的比例为0.6，与夜行性物种相似。瞳孔为垂直狭缝，表明在黄昏时能够调节进入眼球的光量。光感受器在整个视网膜上分布稀疏，未观察到中央凹。视网膜厚度为90 - 100μm，比其他啮齿动物物种的视网膜薄。视力为1.44 - 1.58周/度，高于其他啮齿动物，这可能是由于海狸鼠的眼球和身体较大。这些结果表明，海狸鼠的视觉系统适应于识别远处捕食者的大阴影，而不是详细观察物体。
