Hart Nathan S, Bailes Helena J, Vorobyev Misha, Marshall N Justin, Collin Shaun P
School of Biomedical Sciences, The University of Queensland, Brisbane, QLD 4072, Australia.
BMC Ecol. 2008 Dec 18;8:21. doi: 10.1186/1472-6785-8-21.
The transition from water to land was a key event in the evolution of vertebrates that occurred over a period of 15-20 million years towards the end of the Devonian. Tetrapods, including all land-living vertebrates, are thought to have evolved from lobe-finned (sarcopterygian) fish that developed adaptations for an amphibious existence. However, while many of the biomechanical and physiological modifications necessary to achieve this feat have been studied in detail, little is known about the sensory adaptations accompanying this transition. In this study, we investigated the visual system and visual ecology of the Australian lungfish Neoceratodus forsteri, which is the most primitive of all the lungfish and possibly the closest living relative to the ancestors of tetrapods.
Juvenile Neoceratodus have five spectrally distinct retinal visual pigments. A single type of rod photoreceptor contains a visual pigment with a wavelength of maximum absorbance (lambdamax) at 540 nm. Four spectrally distinct single cone photoreceptors contain visual pigments with lambdamax at 366 (UVS), 479 (SWS), 558 (MWS) and 623 nm (LWS). No double cones were found. Adult lungfish do not possess UVS cones and, unlike juveniles, have ocular media that prevent ultraviolet light from reaching the retina. Yellow ellipsoidal/paraboloidal pigments in the MWS cones and red oil droplets in the LWS cones narrow the spectral sensitivity functions of these photoreceptors and shift their peak sensitivity to 584 nm and 656 nm, respectively. Modelling of the effects of these intracellular spectral filters on the photoreceptor colour space of Neoceratodus suggests that they enhance their ability to discriminate objects, such as plants and other lungfishes, on the basis of colour.
The presence of a complex colour vision system based on multiple cone types and intracellular spectral filters in lungfishes suggests that many of the ocular characteristics seen in terrestrial or secondarily aquatic vertebrates, such as birds and turtles, may have evolved in shallow water prior to the transition onto land. Moreover, the benefits of spectral filters for colour discrimination apply equally to purely aquatic species as well as semi-aquatic and terrestrial animals. The visual system of the Australian lungfish resembles that of terrestrial vertebrates far more closely than that of other sarcopterygian fish. This supports the idea that lungfishes, and not the coelacanth, are the closest living relatives of the ancestors of tetrapods.
从水生到陆生的转变是脊椎动物进化过程中的一个关键事件，发生在泥盆纪末期的1500万至2000万年期间。四足动物，包括所有陆生脊椎动物，被认为是从肉鳍鱼（总鳍鱼）进化而来，这些鱼逐渐适应了两栖生活。然而，尽管为实现这一转变所需的许多生物力学和生理变化已得到详细研究，但对于伴随这一转变的感官适应却知之甚少。在本研究中，我们调查了澳大利亚肺鱼新角齿鱼的视觉系统和视觉生态学，新角齿鱼是所有肺鱼中最原始的，可能是四足动物祖先现存最近的亲属。
幼年新角齿鱼有五种光谱上不同的视网膜视觉色素。一种单一类型的视杆光感受器含有一种视觉色素，其最大吸收波长（λmax）为540纳米。四种光谱上不同的单锥光感受器含有视觉色素，其λmax分别为366（紫外视锥，UVS）、479（短波单锥，SWS）、558（中波单锥，MWS）和623纳米（长波单锥，LWS）。未发现双锥。成年肺鱼不具有紫外视锥，且与幼鱼不同，其眼介质可阻止紫外线到达视网膜。中波单锥中的黄色椭圆形/抛物面色素和长波单锥中的红色油滴使这些光感受器的光谱敏感性函数变窄，并分别将其峰值敏感性转移到584纳米和656纳米。对这些细胞内光谱滤光片对新角齿鱼光感受器颜色空间影响的建模表明，它们增强了基于颜色区分物体（如植物和其他肺鱼）的能力。
肺鱼中基于多种视锥类型和细胞内光谱滤光片的复杂颜色视觉系统的存在表明，在陆地或次生水生脊椎动物（如鸟类和海龟）中看到的许多眼部特征可能在登上陆地之前就在浅水中进化了。此外，光谱滤光片对颜色区分的益处同样适用于纯水生物种以及半水生和陆生动物。澳大利亚肺鱼的视觉系统与陆生脊椎动物的视觉系统比与其他肉鳍鱼类的视觉系统更为相似。这支持了肺鱼而非腔棘鱼是四足动物祖先现存最近亲属的观点。
