Bailes Helena J, Robinson Stephen R, Trezise Ann E O, Collin Shaun P
School of Biomedical Sciences, The University of Queensland, St. Lucia, Brisbane, Queensland 4072, Australia.
J Comp Neurol. 2006 Jan 20;494(3):381-97. doi: 10.1002/cne.20809.
The Australian lungfish Neoceratodus forsteri (Dipnoi) is an ancient fish that has a unique phylogenetic relationship among the basal Sarcopterygii. Here we examine the ultrastructure, histochemistry, and distribution of the retinal photoreceptors using a combination of light and electron microscopy in order to determine the characteristics of the photoreceptor layer in this living fossil. Similar proportions of rods (53%) and cones (47%) reveal that N. forsteri optimizes both scotopic and photopic sensitivity according to its visual demands. Scotopic sensitivity is optimized by a tapetum lucidum and extremely large rods (18.62 +/- 2.68 microm ellipsoid diameter). Photopic sensitivity is optimized with a theoretical spatial resolving power of 3.28 +/- 0.66 cycles degree(-1), which is based on the spacing of at least three different cone types: a red cone containing a red oil droplet, a yellow cone containing a yellow ellipsoidal pigment, and a colorless cone containing multiple clear oil droplets. Topographic analysis reveals a heterogeneous distribution of all photoreceptor types, with peak cone densities predominantly found in temporal retina (6,020 rods mm(-2), 4,670 red cones mm(-2), 900 yellow cones mm(-2), and 320 colorless cones mm(-2)), but ontogenetic changes in distribution are revealed. Spatial resolving power and the diameter of all photoreceptor types (except yellow cones) increases linearly with growth. The presence of at least three morphological types of cones provides the potential for color vision, which could play a role in the clearer waters of its freshwater environment.
澳大利亚肺鱼新角齿鱼（角齿鱼目）是一种古老的鱼类，在肉鳍鱼基部类群中有着独特的系统发育关系。在此，我们结合光学显微镜和电子显微镜技术，研究视网膜光感受器的超微结构、组织化学及分布情况，以确定这种活化石中光感受器层的特征。视杆细胞（53%）和视锥细胞（47%）比例相近，这表明新角齿鱼根据其视觉需求优化了暗视觉和明视觉敏感度。暗视觉敏感度通过反光层和极大的视杆细胞（椭球体直径18.62±2.68微米）得以优化。明视觉敏感度则通过理论空间分辨力3.28±0.66周/度得以优化，这一分辨力基于至少三种不同视锥细胞类型的间距：一种含有红色油滴的红色视锥细胞、一种含有黄色椭圆形色素的黄色视锥细胞以及一种含有多个透明油滴的无色视锥细胞。地形图分析显示，所有光感受器类型分布不均，视锥细胞密度峰值主要出现在颞侧视网膜（6,020个视杆细胞/平方毫米、4,670个红色视锥细胞/平方毫米、900个黄色视锥细胞/平方毫米以及320个无色视锥细胞/平方毫米），但分布存在个体发育变化。除黄色视锥细胞外，所有光感受器类型的空间分辨力和直径均随生长呈线性增加。至少三种形态类型视锥细胞的存在为色觉提供了可能，这在其淡水清澈水域环境中可能发挥作用。
