The Role of Shell Granules and Accessory Cells in Eggshell Formation in Convoluta pulchra (Turbellaria, Acoela)

Most turbellarian embryos are surrounded by a sclerotinized eggshell originating from polyphenol-containing eggshell-forming granules (EFGs). Although embryos of the acoel Convoluta pulchra are surrounded by a shell, it is not sclerotinized. Therefore, in the absence of polyphenols as a marker for EFGs, it was not clear which, if any, of the granules of the oocyte function in eggshell synthesis. In this study, electron-opaque, elliptical granules with a characteristic frothy component and a diameter of 480 nm were identified in the oocyte as EFGs by their participation in eggshell formation. In addition, it was shown that accessory cells to the oocyte initiate synthesis of the shell by producing a thin, granular, electron-opaque primary shell, against which the contents of the EFGs are released by exocytosis. Morphological components of the shell and stages of its synthesis are described. A second type of membrane-bound granule and the lipid droplets that occur in the ooplasm were found not to be involved in eggshell formation and are probable sources of nutrients for the developing embryo. Possible implications of the findings for taxonomy and phylogeny are discussed.