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 neuro-muscular 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.
Highlights
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]
The cells at its distal border always show a large vacuole of homogenous content (Fig. 3), which at least partially stain for anti-acetylated tubulin (Fig. 4b, e)
The high variability of the epistome structure observed in this study indicates that this process is probably influenced by external factors such as stress
Summary
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 (2020) 6:11 horseshoe-shaped lophophore (e.g., [5, 6]). Several older studies agreed that the genus Stephanella, the sole genus of the Stephanellidae, represents an early branch within phylactolaemate diversification, a notion supported by more recent molecular analyses [8, 9]. Most studies concerning this genus dealt with its taxonomy and distribution [10, 11]. Since S. hina remains an important representative, the aim of this work was to use histological as well as fluorescence techniques to investigate this neglected species in more detail to compare it with other phylactolaemates
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