Morphology of Stephanella hina (Bryozoa, Phylactolaemata): common phylactolaemate and unexpected, unique characters
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RESEARCH ARTICLE
Open Access
Morphology of Stephanella hina (Bryozoa, Phylactolaemata): common phylactolaemate and unexpected, unique characters Thomas F. Schwaha1*
and Masato Hirose2
Abstract Stephanella hina is a little studied freshwater bryozoan belonging to Phylactolaemata. It is currently the only representative of the family Stephanellidae, which in most reconstructions is early branching, sometimes even sister group to the remaining phylactolaemate families. The morphological and histological details of this species are entirely unknown. Consequently, the main aim of this study was to conduct a detailed morphological analysis of S. hina using histological serial sections, 3D reconstruction, immunocytochemical staining and confocal laser scanning microscopy techniques. The general morphology is reminiscent of other phylactolaemates; however, there are several, probably apomorphic, details characteristic of S. hina. The most evident difference lies in the lophophoral base, where the ganglionic horns/extensions do not follow the traverse of the lophophoral arms but bend medially inwards towards the mouth opening. Likewise, the paired forked canal does not fuse medially in the lophophoral concavity as found in all other phylactolaemates. Additional smaller differences are also found in the neuromuscular system: the rooting of the tentacle muscle is less complex than in other phylactolaemates, the funiculus lacks longitudinal muscles, the caecum has smooth muscle fibres, latero-abfrontal tentacle nerves are not detected and the medio-frontal nerves mostly emerge directly from the circum-oral nerve ring. In the apertural area, several neurite bundles extend into the vestibular wall and probably innervate neurosecretory cells surrounding the orifice. These morphological characteristics support the distinct placement of this species in a separate family. Whether these characteristics are apomorphic or possibly shared with other phylactolaemates will require the study of the early branching Lophopodidae, which remains one of the least studied taxa to date. Keywords: Stephanellidae, Bryozoan evolution, Epistome, Myoanatomy
Introduction Phylactolaemate bryozoans are, from an evolutionary perspective, very interesting, as they comprise the sister group to the two remaining taxa, Stenolaemata and Gymnolaemata [1, 2]. They are a small group of ~ 80 recent species, and due to lack of calcification and hard * Correspondence: [email protected] 1 Department of Evolutionary Biology, University of Vienna, Althanstraße 14, 1090 Vienna, Austria Full list of author information is available at the end of the article
tissues, they rarely fossilize. Several characteristics are typical of this taxon: a horseshoe-shaped lophophore (suspension-feeding tentacle crown), body-wall musculature and an epistome—a flap-like ciliated bulge protruding over the mouth opening, probably involved in feeding [3, 4]. Morphological analyses usually unite bryozoans with brachiopods and phoronids as “lophophorates” due to similarities in
