Department of Biology, Institute of Zoology (Neurobiology, Animal Physiology), University of Cologne, 50674, Cologne, Germany.
Grant Institute, School of Geosciences, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, UK.
Sci Rep. 2023 Mar 8;13(1):3917. doi: 10.1038/s41598-023-31089-7.
Arthropods typically possess two types of eyes-compound eyes, and the ocellar, so called 'median eyes'. Only trilobites, an important group of arthropods during the Palaeozoic, seem not to possess median eyes. While compound eyes are in focus of many investigations, median eyes are not as well considered. Here we give an overview of the occurence of median eyes in the arthropod realm and their phylogenetic relationship to other ocellar eye-systems among invertebrates. We discuss median eyes as represented in the fossil record e.g. in arthropods of the Cambrian fauna, and document median eyes in trilobites the first time. We make clear that ocellar systems, homologue to median eyes and possibly their predecessors are the primordial visual system, and that the compound eyes evolved later. Furthermore, the original number of median eyes is two, as retained in chelicerates. Four, probably the consequence of a gene-dublication, can be found for example in basal crustaceans, three is a derived number by fusion of the central median eyes and characterises Mandibulata. Median eyes are present in larval trilobites, but lying below a probably thin, translucent cuticle, as described here, which explains why they have hitherto escaped detection. So this article gives a review about the complexity of representation and evolution of median eyes among arthropods, and fills the gap of missing median eyes in trilobites. Thus now the number of median eyes represented in an arthropod is an important tool to find its position in the phylogenetic tree.
节肢动物通常具有两种类型的眼睛——复眼和所谓的“中央眼”。只有三叶虫，古生代的一个重要节肢动物群体，似乎没有中央眼。虽然复眼是许多研究的焦点，但中央眼并没有得到很好的考虑。在这里，我们概述了节肢动物中中央眼的出现及其与无脊椎动物中其他眼系统的系统发育关系。我们讨论了例如在寒武纪动物群中的节肢动物中代表的中央眼，并且首次记录了三叶虫中的中央眼。我们明确指出，与中央眼同源的眼系统，可能还有它们的前身，是原始的视觉系统，而复眼是后来进化而来的。此外，中央眼的原始数量是两个，这在螯肢动物中得以保留。例如，在基础甲壳类动物中可以发现四个，可能是基因复制的结果，而在三叶虫中，三个是由于中央中央眼融合而形成的衍生数量，并特征化了有颚类。中央眼存在于三叶虫幼虫中，但位于可能很薄的、半透明的表皮下，如这里所描述的，这解释了为什么它们迄今未被发现。因此，本文综述了节肢动物中中央眼的表现和进化的复杂性，并填补了三叶虫中缺失中央眼的空白。因此，现在在节肢动物中代表的中央眼的数量是确定其在系统发育树中的位置的重要工具。
