Polypide anatomy of hornerid bryozoans (Stenolaemata: Cyclostomatida)

Abstract

Bryozoans are small colonial coelomates whose colonies are made of individual modules (zooids). Like most coelomate animals, bryozoans have a characteristic body wall composition, including an epidermis, an extracellular matrix (ECM) and a coelothelium, all pressed together. The order Cyclostomatida, however, presents the most striking deviation, in which the ECM and the corresponding coelothelium underlying major parts of the skeletal wall epidermis are detached to form an independent membranous sac. It forms a separate, much smaller compartment, suspended in the zooid body cavity and working as an important element of the cyclostome lophophore protrusion mechanism. The polypide anatomy and ultrastructure of this group is best known from studies of one family, the Crisiidae (Articulata). Here, we examined four species from the phylogenetically and ecologically contrasting family Horneridae (Cancellata) from New Zealand, and provide the first detailed ultrastructural description of the hornerid polypide, including tentacles, mouth region, digestive system and the funiculus. We were able to trace continuity and transitions of cell and ECM layers throughout the whole polypide. In addition, we identified that the funiculus is a lumen‐free ECM cord with two associated muscles, disconnected from interzooidal pores. Except for funicular core composition, the polypide anatomy of hornerids agrees well with the general cyclostomate body plan.

The epithelial layers of the body wall in hornerid bryozoans (Stenolaemata: Cyclostomatida)

“…3. Material and methods in this study mostly overlap those in Tamberg et al ( 2021 ). For more details on sampling and processing please consult this publication.…”

“…Ultrathin sections (80–90 nm) were stained with uranyl acetate and lead citrate and imaged with a JEOL 2200FS electron microscope. In addition to complete sets of combined semi‐thin and ultrathin serial sections (≥5 series per species, ≥5 polypides per series), described in Tamberg et al ( 2021 ), incomplete sets and individual sections were also used in this study.…”

“…Here, we examine this prediction and explore variability of epithelial and non‐epithelial structures of the body wall and exosaccal cavity. This study is a companion piece to Tamberg et al ( 2021 ), it is based on the same material, and examines the same species with light and electron microscopic methods, but with the emphasis on various epithelia of the body wall.…”

Composite branch construction by dual autozooidal budding modes in hornerids (Bryozoa: Cyclostomatida)

“…Resulting series of semi-thin sections (1 m thick) were imaged with a light μ microscope (several Zeiss and Olympus models). In addition, we consulted four serial block-face SEM datasets obtained previously for H. robusta (see Tamberg et al, 2021 for details).…”

“…The polypide bauplan is conserved at the ordinal level (e.g. Boardman & McKinney, 1985; Tamberg et al, 2021). In contrast, the arrangements of zooidal modules within a bryozoan colony vary greatly within families, genera and species, resulting in a diverse array of ‘second-order’ morphologies (Hageman, 2003).…”