Collin S P, Pottert I C
Department of Zoology, University of Western Australia, Nedlands, WA, Australia.
Brain Behav Evol. 2000 Mar;55(3):120-38. doi: 10.1159/000006647.
A description is given of the morphological and ultrastructural characteristics of the eye of Mordacia mordax, one of the two parasitic species of the southern hemisphere lamprey family Mordaciidae. The avascular retina is well-differentiated, containing two ganglion cell sublaminae, two layers of horizontal cells and a large population of displaced bipolar cells. Intermediate filaments, which stain positively for glial fibrillary acidic protein (GFAP), lie in perpendicularly-arranged bundles between the two layers of horizontal cells. M. mordax contains only a single type of rod-like photoreceptor, in contrast to representatives in the other two lamprey families Geotriidae and Petromyzontidae, which possess three (two cone-like and one rod-like) and two (one cone and one rod) types of photoreceptor, respectively. The large photoreceptors of M. mordax possess a large mitochondrial ellipsosome, a cylindrical outer segment with incisures, and a nucleus immediately abutting the outer limiting membrane. The photoreceptors are arranged in an hexagonal array with their outer segments surrounded by a retinal tapetum which elicits a yellow fundal eyeshine. The tapetum contains both diffuse (vitread spheroidal granules) and specular (sclerad needles) reflectors. The characteristics of the eye of M. mordax are related to the diurnal burrowing of this species during its upstream migration, a feature not found in the other two extant families of lampreys. The absence of cone-like photoreceptors and the presence of a tapetum suggest that although photopic vision is probably greatly diminished, characteristics have been evolved by the eye to maximize light capture and increase sensitivity in the low light intensities encountered during this species' nocturnal migration.
本文描述了南半球七鳃鳗科莫氏七鳃鳗属两个寄生物种之一的莫氏七鳃鳗眼睛的形态学和超微结构特征。其无血管视网膜分化良好，包含两个神经节细胞亚层、两层水平细胞和大量移位双极细胞。对胶质纤维酸性蛋白（GFAP）呈阳性染色的中间丝，以垂直排列的束状形式位于两层水平细胞之间。与七鳃鳗科和盲鳗科的其他代表物种不同，莫氏七鳃鳗仅含有单一类型的杆状光感受器，七鳃鳗科和盲鳗科分别拥有三种（两种锥状和一种杆状）和两种（一种锥状和一种杆状）光感受器。莫氏七鳃鳗的大型光感受器具有一个大的线粒体椭圆体、一个有切迹的圆柱形外段以及紧邻外限制膜的细胞核。光感受器呈六边形排列，其外段被视网膜反光层包围，该反光层会引发黄色眼底眼辉。反光层包含漫反射（玻璃体侧球状颗粒）和镜面反射（巩膜侧针状）反射器。莫氏七鳃鳗眼睛的这些特征与该物种在上游洄游期间的日间穴居行为有关，这一特征在现存的其他两个七鳃鳗科中并未发现。缺少锥状光感受器和存在反光层表明，尽管明视觉可能大大减弱，但眼睛已经进化出相应特征，以在该物种夜间洄游期间遇到的低光照强度下最大化光捕获并提高敏感度。
