Morphology of ctenostome bryozoans: 4. Pierrella plicata

The genus Pierrella was originally created for a single fossil ctenostome bryozoan species from the Late Cretaceous, which is characterized by runner‐like colonies, with zooids possessing a distinctive radial, folded aperture. Not long ago, a few specimens of a recent deep‐sea congener, Pierrella plicata, were discovered and described from the Russian exploration area of the Clarion‐Clipperton Fracture Zone, eastern Central Pacific Ocean. Owing to the lack of data on the internal morphology of this species, we investigated the soft‐body morphology of P. plicata using serial sectioning and 3D‐reconstruction in order to compare it to other more recently investigated ctenostome bryozoans and to infer the systematic position of the genus. The most striking peculiarity of the examined species is the radial aperture formed by multiple cuticular, pleated folds of the cystid wall. The cuticle is thickened into triangular‐shaped folds in this area. An orifical sphincter underlies the folded aperture. Apertural muscles are present as a single pair of parieto‐diaphragmatic muscles and four duplicature bands. The remaining polypide anatomy is mainly characterized by its miniature design: the lophophore has eight short tentacles and the digestive tract is one of the shortest and most compact ever observed in any bryozoan. A small intertentacular organ was detected at the lophophoral base. Taken together the genus Pierrella shows unique characters, such as the radial apertural folds that are closed by a series of orificial sphincter muscles, and its particularly small polypide. The general colony morphology resembles arachnidioidean ctenostomes whereas its internal morphology resembles alcyonidioidean species.