Embryonic development and matrotrophy in the phylactolaemate bryozoan Plumatella casmiana

Julian Bibermair, Andrew N. Ostrovsky, Andreas Wanninger, Thomas Schwaha

Research outputpeer-review


Bryozoans are an aquatic group of colonial sessile filter-feeders within the Lophotrochozoa. In the exclusively freshwater phylactolaemates, reproduction involves embryonic incubation inside the internal brood sac developing on the body wall, and accompanied by extraembryonic nutrition. Although detailed studies on the phylactolaemate sexual reproduction date back to the late 19th and early 20th century, many aspects of their sexual reproduction need to be restudied owing to previous contradictive descriptions.

This study focuses on the embryogenesis of the fresh-water bryozoan Plumatella casmiana by using histological serial-sections added by 3D-reconstruction techniques and transmission electron microscopy. Early embryo develops into a single-layered blastula with a small central cavity, and forms a mesodermal layer by delamination. Subsequently, two polypides are developed via ingression and cell proliferation, resulting in a ciliated “mantle larva”. The embryo increases in size forming temporary cell contacts to the embryo sac wall. Ultrastructural study found no evidence that ‘contact cells’ participate in the extraembryonic nutrition. In contrast, mesoderm cells of the embryo sac showed the signs of transcytosis indicating that the embryo is nourished as the embryo sac takes up nutrients from the maternal coelom fluid and releases them into the brood cavity.

Based on the modern methods our study confirmed the exisitng data on sexual reproduction in plumatellids and showed some new details e.g. the delamination process during mesoderm formation and the timing of bud development. We also found that embryo and embryo sac are connected via variety of temporary cytoplasmic processes formed by both of them during embryogenesis, including the ‘placental’ ring zone. At least, some of these cell contacts are obviously not involved in the extraembryonic nutrition, and the embryo supposedly absorbs nutrients via the entire surface. Still, since the embryo is closely opposed to the wall of the embryo sac this matrotrophic mode should be defined as placentation, and the embryo sac as a placental analogue.
Original languageEnglish
Number of pages42
JournalOrganisms Diversity and Evolution
Publication statusSubmitted - 2021

Scopus subject areas

  • Agricultural and Biological Sciences(all)

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