Deconstructing bryozoans: origin and consequences of a unique body plan

The Bryozoa (or Ectoprocta) is an enigmatic phylum of colonial organisms whose phylogenetic position has long been problematic. Although traditionally they have been considered to form a separate ”lo- phophorate” clade with phoronids and brachiopods, genome analyses indicate that they are lophotrochozoans. One of the greatest mysteries regarding bryozoans is the delayed appearance of their body plan in the fossil record. From their earliest record in the Ordovician they comprise five discrete stenolae mate orders. An explanation for their late emergence may be linked to their unusual development. Bryozoans appear to lack tissues derived from endoderm and to possess a gut derived from, ectoderm. During metamorphosis or budding the polypide arises from undifferentiated cells of the body wall of the preancestruia or cystid. This unusual mode of development and the absence of endoderm suggest that the bryozoan stem lineage underwent a unique transformation involving loss of the original gut and gain of a new ectodermally derived gut.