Schoenemann Brigitte, Clarkson Euan N K, Horváth Gábor
Department of Animal Physiology and Institute of Biology Education (Zoology),University of Cologne , Cologne , Germany.
Grant Institute, University of Edinburgh , Edinburgh , United Kingdom.
PeerJ. 2015 Dec 15;3:e1492. doi: 10.7717/peerj.1492. eCollection 2015.
The calcitic lenses in the eyes of Palaeozoic trilobites are unique in the animal kingdom, although the use of calcite would have conveyed great advantages for vision in aquatic systems. Calcite lenses are transparent, and due to their high refractive index they would facilitate the focusing of light. In some respects, however, calcite lenses bear evident disadvantages. Birefringence would cause double images at different depths, but this is not a problem for trilobites since the difference in the paths of the ordinary and extraordinary rays is less than the diameter of the receptor cells. Another point, not discussed hitherto, is that calcite fluoresces when illuminated with UV-A. Here we show experimentally that calcite lenses fluoresce, and we discuss why fluorescence does not diminish the optical quality of these lenses and the image formed by them. In the environments in which the trilobites lived, UV-A would not have been a relevant factor, and thus fluorescence would not have disturbed or confused their visual system. We also argue that whatever the reason that calcite was never again used successfully in the visual systems of aquatic arthropods, it was not fluorescence.
古生代三叶虫眼睛中的方解石晶状体在动物界是独一无二的，尽管在水生系统中使用方解石对视觉会带来极大的优势。方解石晶状体是透明的，并且由于其高折射率，它们有助于光线聚焦。然而，在某些方面，方解石晶状体存在明显的缺点。双折射会在不同深度产生重影，但这对三叶虫来说不是问题，因为寻常光线和非常光线的路径差异小于受体细胞的直径。另一个迄今未被讨论的点是，方解石在紫外光A照射下会发出荧光。在这里，我们通过实验表明方解石晶状体发出荧光，并且我们讨论了为什么荧光不会降低这些晶状体及其所形成图像的光学质量。在三叶虫生活的环境中，紫外光A不会是一个相关因素，因此荧光不会干扰或混淆它们的视觉系统。我们还认为，无论方解石从未再次成功用于水生节肢动物视觉系统的原因是什么，都不是荧光。
