Giribet Gonzalo, Okusu Akiko, Lindgren Annie R, Huff Stephanie W, Schrödl Michael, Nishiguchi Michele K
Department of Organismic and Evolutionary Biology and Museum of Comparative Zoology, Harvard University, 16 Divinity Avenue, BioLabs 1119, Cambridge, MA 02138, USA.
Proc Natl Acad Sci U S A. 2006 May 16;103(20):7723-8. doi: 10.1073/pnas.0602578103. Epub 2006 May 4.
Monoplacophorans are among the rarest members of the phylum Mollusca. Previously only known from fossils since the Cambrian, the first living monoplacophoran was discovered during the famous second Galathea deep-sea expedition. The anatomy of these molluscs shocked the zoological community for presenting serially repeated gills, nephridia, and eight sets of dorsoventral pedal retractor muscles. Seriality of organs in supposedly independent molluscan lineages, i.e., in chitons and the deep-sea living fossil monoplacophorans, was assumed to be a relic of ancestral molluscan segmentation and was commonly accepted to support a direct relationship with annelids. We were able to obtain one specimen of a monoplacophoran Antarctic deep-sea species for molecular study. The first molecular data on monoplacophorans, analyzed together with the largest data set of molluscs ever assembled, clearly illustrate that monoplacophorans and chitons form a clade. This "Serialia" concept may revolutionize molluscan systematics and may have important implications for metazoan evolution as it allows for new interpretations for primitive segmentation in molluscs.
单板纲动物是软体动物门中最为稀有的类群之一。此前自寒武纪以来仅通过化石为人所知，第一只活体单板纲动物是在著名的第二次加拉泰亚深海探险期间被发现的。这些软体动物的解剖结构令动物学界大为震惊，因为它们呈现出一系列重复的鳃、肾以及八组背腹侧足缩肌。在被认为是独立的软体动物谱系中，即石鳖和深海活化石单板纲动物中，器官的系列性被假定为软体动物祖先分节的遗迹，并且普遍被认为支持了与环节动物的直接关系。我们得以获取一只南极深海单板纲物种的标本用于分子研究。关于单板纲动物的首批分子数据，与有史以来汇集的最大规模的软体动物数据集一同进行分析后，清楚地表明单板纲动物和石鳖构成一个进化枝。这种“系列性动物”概念可能会彻底改变软体动物的系统分类学，并且可能对后生动物的进化具有重要意义，因为它为软体动物原始分节提供了新的解释。
