Abstract
Bryozoa is a phylum of aquatic, colonial suspension-feeders within the Lophotrochozoa. In the Phylactolaemata embryonic development occurs in an internal brood sac on the body wall accompanied by extraembryonic nutrition. Owing to previous contradictive descriptions, many aspects of their sexual reproduction require restudy. Consequently, this study analyses embryogenesis of the freshwater bryozoan Plumatella casmiana by serial sections, 3D reconstruction and transmission electron microscopy. Early embryos cleave and soon develop into blastulae with a small central cavity. The mesoderm forms by delamination starting from the distal side towards the proximal end. In later embryos two polypides form on the posterior side that ultimately will be covered by a ciliated mantle in the larva. Embryos increase in size during development and form temporary cell contacts to the embryo sac. Mesodermal cells of the embryo sac show signs of transcellular transport indicating that embryos are nourished by transferring nutrients from the maternal coelom towards the brood cavity. This study clarifies several details such as mesoderm formation and the onset of bud development. Embryos are connected to their respective embryo sacs by a variety of temporary cytoplasmic processes formed by both tissues during embryogenesis, including a ‘placental’ ring zone. Although ultrastructural data of these cell contacts are not entirely conclusive about their function, we suggest that embryos absorb nutrients via the entire surface. The close opposition of embryos to the embryo sac implies placentation as matrotrophic mode in phylactolaemate bryozoans, with embryo sacs acting as placental analogues.
Highlights
Bryozoans are a group of mostly colonial, suspension-feeding lophotrochozoans occurring in both marine and freshwater habitats (Ryland 1970, 2005; McKinney and Jackson 1989)
The ovary lies proximal of the embryo sac, which is closest to the orifice (Figs. 2, 3)
The outer surface remains as a thin mesodermal layer of the embryo sac, the ectodermal, and internal layer remains as a thin layer in the proximal, suspended part of the sac, whereas it forms a dense plug of maternal ectodermal cells in the distal attachment area (Fig. 3d)
Summary
Bryozoans are a group of mostly colonial, suspension-feeding lophotrochozoans occurring in both marine and freshwater habitats (Ryland 1970, 2005; McKinney and Jackson 1989). Individual animals of a colony are termed zooids and are traditionally, but artificially, divided into a polypide and a cystid (Ryland 2005; Schwaha 2020a). The former comprises a ciliated tentacle crown (lophophore), the digestive tract, and associated neural and muscular tissue. Plumatellidae is the most speciose taxon with a branching colony morphology. Colonies of most species are repent, but some are erect and branching (Wood 2014). Plumatellids are the most studied family and most investigations focus on Plumatella repens and P. fungosa
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
