Morphology of Stephanella hina (Bryozoa, Phylactolaemata): common phylactolaemate and unexpected, unique characters

The unplumatellid Plumatella fruticosa found its home: Hirosella gen. nov. morphological arguments for the systematic placement of a freshwater bryozoan

Bryozoans are colonial, suspension-feeding lophotrochozoans. The phylum consists of the large group of chiefly marine Myolaemata and the exclusively limnic Phylactolaemata. Each colony consists of individual zooids that comprise the protective cystid and the retractable polypide. Phylactolaemates are a small group of approximately 90 species in 6 families. They feature a body wall, that can either be gelatinous, as in the families Stephanellidae, Lophopodidae, Cristatellidae and Pectinatellidae, or encrusted, as in Plumatellidae and Fredericellidae. Morphological investigations of the most specious plumatellids are rare and focus on few species. Plumatella fruticosa is of particular interest in this regard, as it shows a mosaic of plumatellid and fredericellids characters. The most recent phylogeny clusters P. fruticosa with cristatellids and pectinatellids as sister groups to fredericellids. Hence, there is considerable doubt, whether P. fruticosa is truly a plumatellid. Therefore, this study aims to reinvestigate the morphology of P. fruticosa with confocal microscopy and section-based three-dimensional reconstruction. The new data show that P. fruticosa has numerous conspicuous stumps from fragmented proliferation buds, which are otherwise only known from fredericellids. Like fredericellids, P. fruticosa grows erect, but in contrast, has a horseshoe-shaped lophophore and floatoblasts. Besides the proportions of the lophophore, the tentacle sheath and digestive tract resemble a fredericellid-like situation. Myoanatomical details like the pronounced longitudinal muscles of the vestibular wall and tentacle sheath differ from plumatellids and favour the recently proposed scenario, which places P. fruticosa next to Pectinatellidae and Cristatellidae. In addition, the intertentacular membrane of P. fruticosa shows structural similarity to cristatellids as it is attached to the tentacles via lamellae. Taking all aspects into account, we erect a new family: Hirosellidae fam. nov. including the new genus Hirosella gen. nov.

Phylogenomics reveals deep relationships and diversification within phylactolaemate bryozoans

Bryozoans are mostly sessile colonial invertebrates that inhabit all kinds of aquatic ecosystems. Extant bryozoan species fall into two clades with one of them, Phylactolaemata, being the only exclusively freshwater clade. Phylogenetic relationships within the class Phylactolaemata have long been controversial owing to their limited distinguishable characteristics that reflect evolutionary relationships. Here, we present the first phylogenomic analysis of Phylactolaemata using transcriptomic data combined with dense taxon sampling of six families to better resolve the interrelationships and to estimate divergence time. Using maximum-likelihood and Bayesian inference approaches, we recovered a robust phylogeny for Phylactolaemata in which the interfamilial relationships are fully resolved. We show Stephanellidae is the sister taxon of all other phylactolaemates and confirm that Lophopodidae represents the second offshoot within the phylactolaemate tree. Plumatella fruticosa clearly falls outside Plumatellidae as previous investigations have suggested, and instead clusters with Pectinatellidae and Cristatellidae as the sister taxon of Fredericellidae. Our results demonstrate that cryptic speciation is very likely in F. sultana and in two species of Plumatella (P. repens and P. casmiana). Divergence time estimates show that Phylactolaemata appeared at the end of the Ediacaran and started to diverge in the Silurian, although confidence intervals were large for most nodes. The radiation of most extant phylactolaemate families occurred mainly in the Palaeogene and Neogene highlighting post-extinction diversification.

Reconstructing the neuromuscular ground pattern of phylactolaemate bryozoans: new data from the Lophopodidae

BACKGROUND

The solely freshwater inhabiting Phylactolaemata is a sister taxon to all other bryozoans. Among phylactolaemates, Lophopodidae represents an early branching clade that is therefore crucial for ground pattern reconstruction. While more recent morphological data of most phylactolaemate families are present, data of lophopodids are scarce. The genus Asajirella especially, which was previously assigned to the family Pectinatellidae, lacks any detailed analysis with more recent morphological methods.

RESULTS

This study provides the first morphological analyses of three lophopodid species using serial-sectioning histology and 3D reconstruction, but also immunocytochemical stainings and confocal laserscanning microscopy. There are several lophopodid-specific traits in the nervous system such as the large ganglion with extensive lumen and two prominent protrusions referred to as epistomial horns. The epistome in all lophopodids is rather small and dome-shaped. Contrary to previous reports, we can confirm that duplicature bands insert at the tentacle sheath rather than the diaphragmatic sphincter in all phylactolaemates. The morphology of the digestive tract of lophopodids is identical to other phylactolaemates and possesses exclusively circular muscles.

CONCLUSIONS

Altogether, this study fills significant gaps in our knowledge on phylactolaemate neuromuscular systems and general morphology. It shows that the insertion of the duplicature bands at the tentacle sheath and the circular musculature of the digestive tract to be the ground pattern in phylactolaemates. In addition, we found apomorphic characters for lophopodids such as the dome-shaped epistome with its musculature and the voluminous ganglion with its epistomial horns, which aid in defining and delineating the family.