Formation and structure of paleae and chaetal arrangement in chrysopetalidae (Annelida)

摘要

Chrysopetalids, more specifically the subfamily Chrysopetalinae, are known for their characteristic, flattened, leaf-like notopodial chaetae. In many chrysopetalid species the animal's body is dorsally covered by these paleae that are arranged in rows similar to the blades of a folding fan. In this study we investigate and compare the chaetal arrangement in two chrysopetalids: Bhawania goodei Webster, 1884 and Chrysopetalum cf. occidentale Johnson, 1897, using serial sectioning and 3D models. Our results reveal details on the topology of the chaetal formative site, where chaetogenesis occurs continuously. The course of chaetogenesis is significantly affected by the very large size of the chaetae, but nevertheless follows the general pattern of chaetogenesis in Annelida. Ultrastructure of the paleae confirms previous observations. The results of the study are the first on chaetogenesis and chaetal topology in a representative of the Phyllodocida. They indicate differences in the number of formative sites in neuro- and notopodia that now need to be studied in a larger number phyllodocid taxa. A special focus has been laid on details of paleal formation to study potential phylogenetic affinities between Wiwaxia and Chrysopetalidae that were proposed due to the significant similarity of the paleae to the sclerites of Wiwaxia.