Reconstructing the muscular ground pattern of phylactolaemate bryozoans: first data from gelatinous representatives

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.

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.

Renewed perspectives on the sedentary-pelagic last common bilaterian ancestor

Various evaluations of the last common bilaterian ancestor (lcba) currently suggest that it resembled either a microscopic, non-segmented motile adult; or, on the contrary, a complex segmented adult motile urbilaterian. These fundamental inconsistencies remain largely unexplained. A majority of multidisciplinary data regarding sedentary adult ancestral bilaterian organization is overlooked. The sedentary-pelagic model is supported now by a number of novel developmental, paleontological and molecular phylogenetic data: (1) data in support of sedentary sponges, in the adult stage, as sister to all other Metazoa; (2) a similarity of molecular developmental pathways in both adults and larvae across sedentary sponges, cnidarians, and bilaterians; (3) a cnidarian-bilaterian relationship, including a unique sharing of a bona fide Hox-gene cluster, of which the evolutionary appearance does not connect directly to a bilaterian motile organization; (4) the presence of sedentary and tube-dwelling representatives of the main bilaterian clades in the early Cambrian; (5) an absence of definite taxonomic attribution of Ediacaran taxa reconstructed as motile to any true bilaterian phyla; (6) a similarity of tube morphology (and the clear presence of a protoconch-like apical structure of the Ediacaran sedentary Cloudinidae) among shells of the early Cambrian, and later true bilaterians, such as semi-sedentary hyoliths and motile molluscs; (7) recent data that provide growing evidence for a complex urbilaterian, despite a continuous molecular phylogenetic controversy. The present review compares the main existing models and reconciles the sedentary model of an urbilaterian and the model of a larva-like lcba with a unified sedentary(adult)-pelagic(larva) model of the lcba.

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

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.

Key novelties in the evolution of the aquatic colonial phylum Bryozoa: evidence from soft body morphology

Molecular techniques are currently the leading tools for reconstructing phylogenetic relationships, but our understanding of ancestral, plesiomorphic and apomorphic characters requires the study of the morphology of extant forms for testing these phylogenies and for reconstructing character evolution. This review highlights the potential of soft body morphology for inferring the evolution and phylogeny of the lophotrochozoan phylum Bryozoa. This colonial taxon comprises aquatic coelomate filter-feeders that dominate many benthic communities, both marine and freshwater. Despite having a similar bauplan, bryozoans are morphologically highly diverse and are represented by three major taxa: Phylactolaemata, Stenolaemata and Gymnolaemata. Recent molecular studies resulted in a comprehensive phylogenetic tree with the Phylactolaemata sister to the remaining two taxa, and Stenolaemata (Cyclostomata) sister to Gymnolaemata. We plotted data of soft tissue morphology onto this phylogeny in order to gain further insights into the origin of morphological novelties and character evolution in the phylum. All three larger clades have morphological apomorphies assignable to the latest molecular phylogeny. Stenolaemata (Cyclostomata) and Gymnolaemata were united as monophyletic Myolaemata because of the apomorphic myoepithelial and triradiate pharynx. One of the main evolutionary changes in bryozoans is a change from a body wall with two well-developed muscular layers and numerous retractor muscles in Phylactolaemata to a body wall with few specialized muscles and few retractors in the remaining bryozoans. Such a shift probably pre-dated a body wall calcification that evolved independently at least twice in Bryozoa and resulted in the evolution of various hydrostatic mechanisms for polypide protrusion. In Cyclostomata, body wall calcification was accompanied by a unique detachment of the peritoneum from the epidermis to form the hydrostatic membraneous sac. The digestive tract of the Myolaemata differs from the phylactolaemate condition by a distinct ciliated pylorus not present in phylactolaemates. All bryozoans have a mesodermal funiculus, which is duplicated in Gymnolaemata. A colonial system of integration (CSI) of additional, sometimes branching, funicular cords connecting neighbouring zooids via pores with pore-cell complexes evolved at least twice in Gymnolaemata. The nervous system in all bryozoans is subepithelial and concentrated at the lophophoral base and the tentacles. Tentacular nerves emerge intertentacularly in Phylactolaemata whereas they partially emanate directly from the cerebral ganglion or the circum-oral nerve ring in myolaemates. Overall, morphological evidence shows that ancestral forms were small, colonial coelomates with a muscular body wall and a U-shaped gut with ciliary tentacle crown, and were capable of asexual budding. Coloniality resulted in many novelties including the origin of zooidal polymorphism, an apomorphic landmark trait of the Myolaemata.

First transcriptome analysis of bryozoan Fredericella sultana, the primary host of myxozoan parasite Tetracapsuloides bryosalmonae

Bryozoans are aquatic invertebrate moss animals that are found worldwide. Fredericella sultana is a freshwater bryozoan and is the most common primary host of myxozoan parasite, Tetracapsuloides bryosalmonae. However, limited genomic resources are available for this bryozoan, which hampers investigations into the molecular mechanisms of host-parasite interactions. To better understand these interactions, there is a need to build a transcriptome dataset of F. sultana, for functional genomics analysis by large-scale RNA sequencing. Total RNA was extracted from zooids of F. sultana cultivated under controlled laboratory conditions. cDNA libraries were prepared and were analyzed by the Illumina paired-ends sequencing. The sequencing data were used for de novo transcriptome assembly and functional annotation. Approximately 118 million clean reads were obtained, and assembled into 85,544 contigs with an average length of 852 bp, an N50 of 1,085 bp, and an average GC content 51.4%. A total of 23,978 (28%) contigs were annotated using BLASTX analysis. Of these transcripts, 4,400 contigs had highest similarity to brachiopod species Lingula anatina. Based on Gene ontology (GO) annotation, the most highly scored categories of biological process were categorized into cellular process (27%), metabolic process (24%), and biological regulation (8%) in the transcriptome of F. sultana. This study gives first insights into the transcriptome of F. sultana and provides comprehensive genetic resources for the species. We believe that the transcriptome of F. sultana will serve as a useful genomic dataset to accelerate research of functional genomics and will help facilitate whole genome sequencing and annotation. Candidate genes potentially involved in growth, proteolysis, and stress/immunity-response were identified, and are worthy of further investigation.

Myoanatomy of the Lophophore in Adult Phoronids and the Evolution of the Phoronid Lophophore

Confocal laser scanning microscopy was used to study the myoanatomy of the lophophore of three phoronids with different types of lophophore: Phoronis ijimai, Phoronis australis, and Phoronopsis harmeri. A four-part ground plan of the lophophoral musculature was detected in all three species and was previously reported for Phoronis ovalis. The ground plan includes (i) a circular muscle, (ii) longitudinal muscles of the tentacular lamina, (iii) groups of paired distal muscles of the tentacular lamina, and (iv) frontal and abfrontal muscles of the tentacles. In P. australis, the tentacular lamina contains strong abfrontal and numerous frontal muscles. Phoronis harmeri has an inner circular muscle and arch-like muscles. Among all studied phoronids, the four-part ground plan of the lophophoral musculature is least complex in P. ijimai, which has a horseshoe-shaped lophophore. The results suggest two possible scenarios by which the morphology of the phoronid lophophore has transformed over evolutionary time. According to the first scenario, the morphology of the ancestral horseshoe-shaped lophophore became more complicated in the case of most phoronids but became simplified in the case of P. ovalis and bryozoans. According to the second scenario, the lophophore gradually transformed from a simple oval shape to a horseshoe shape and then to a spiral shape. The four-part ground plan of the lophophoral musculature is also present in bryozoans, which is consistent with the view that the lophophorates are monophyletic.

Life in a tube: morphology of the ctenostome bryozoan Hypophorella expansa

Bryozoa is a large phylum of colonial aquatic suspension feeders. The boring ctenostome Hypophorella expansa is unique and inhabits parchment-like polychaete tubes. Morphological studies date back to the nineteenth century, but distinct adaptations to this specific habitat have not been properly analysed, which prompted us to reexamine the morphology of this recently encountered species. The colony of H. expansa is composed of elongated stolonal kenozooids with a distal capsule-like expansion. A median transversal muscle is present in the latter, and one autozooid is laterally attached to the capsule. Unique stolonal wrinkles are embedded in the thin parts of the stolons. Single autozooids are attached in an alternating right-left succession on subsequent stolons. Polypide morphology including digestive tract, muscular system and most parts of the nervous system are similar to other ctenostomes. The most obvious apomorphic features of Hypophorella are space balloons and the gnawing apparatus. The former are two fronto-lateral spherical structures on autozooids, which provide space inside the tube. The latter perforates layers of the polychaete tube wall and consists of two rows of cuticular teeth that, together with the entire vestibular wall, are introvertable during the protrusion-retraction process. The apertural muscles are in association with this gnawing apparatus heavily modified and show bilateral symmetry. Adaptations to the unique lifestyle of this species are thus evident in stolonal wrinkles, autozooidal space balloons and the gnawing apparatus. The growth pattern of the colony of H. expansa may aid in rapid colonization of the polychaete tube layers.

Myoanatomy of the phoronid Phoronis ovalis: functional and phylogenetic implications

The myoanatomy of adult phoronids has never been comprehensively studied by fluorescent staining and confocal laser scanning microscopy. Because the organization of the musculature may provide insight into phoronid biology and phylogeny, phoronid myoanatomy warrants detailed investigation. The current study provides the first description based on the use of modern methods of the musculature of the very small phoronid Phoronis ovalis. The musculature of the lophophore base includes radial, longitudinal, and circular muscles; pharynx dilators; and paired lateroabfrontal muscles. The musculature of the anterior part of the body is formed by outer-circular, middle-diagonal, and inner-longitudinal muscles; because all of the cells in these muscles contact the basal lamina, the musculature in the anterior part of the body forms a single layer. In the posterior part of the body, diagonal muscles are absent, and the longitudinal musculature is represented by small, thin bundles. In the terminal end of the body, there is an inversion of circular and longitudinal muscles. The organization of the musculature in the lophophore base and anterior part of the body suggests that the lophophore can move in different directions in order to capture food from local water currents. The organization of the musculature of the terminal end would enable this part of the body to be used for digging into the substratum. The four-partitioned ground plan of the lophophoral musculature in P. ovalis and in bryozoans from all three main groups indicates the homology of the lophophore and the monophyly of the lophophorates as a united clade that includes three phyla: Phoronida, Bryozoa, and Brachiopoda. Some similarities in the organization of the lophophoral musculature, however, may reflect the similarities in the sessile life styles and feeding behaviors of P. ovalis and bryozoans.

Morphology of the bryozoan Cinctipora elegans (Cyclostomata, Cinctiporidae) with first data on its sexual reproduction and the cyclostome neuro-muscular system

BACKGROUND

Cyclostome bryozoans are an ancient group of marine colonial suspension-feeders comprising approximately 700 extant species. Previous morphological studies are mainly restricted to skeletal characters whereas data on soft tissues obtained by state-of-the-art methods are still lacking. In order to contribute to issues related to cyclostome ground pattern reconstruction, we analyzed the morphology of the neuromuscular system Cinctipora elegans by means of immunocytochemical staining, confocal laser scanning microscopy, histological sections and microCT imaging.

RESULTS

Polypides of C. elegans are located in elongated tubular skeletal cystids. Distally, the orifice leads into a prominent vestibulum which is lined by an epithelium that joins an almost complete perimetrical attachment organ, both containing radially arranged neurite bundles and muscles. Centrally, the prominent atrial sphincter separates the vestibulum from the atrium. The latter is enclosed by the tentacle sheath which contains few longitudinal muscle fibers and two principal neurite bundles. These emerge from the cerebral ganglion, which is located at the lophophoral base. Lateral ganglia are located next to the cerebral ganglion from which the visceral neurite bundles emerge that extend proximally towards the foregut. There are four tentacle neurite bundles that emerge from the ganglia and the circum-oral nerve ring, which encompasses the pharynx. The tentacles possess two striated longitudinal muscles. Short buccal dilatators are situated at the lophophoral base and short muscular sets are present at the abfrontal and frontal side of the tentacle base. The pharynx is myoepithelial and triradiate in cross-section. Oocytes are found inside the pharyngeal myoepithelium. The digestive tract contains dense circular musculature and few longitudinal muscles. The membranous sac contains regular, thin, circular and diagonal muscles and neurites in its epithelial lining.

CONCLUSIONS

The general structure of the neuro-muscular system is more reminiscent of the condition found in Gymnolaemata rather than Phylactolaemata, which supports a close relationship between Cyclostomata and Gymnolaemata. Several characters of C. elegans such as the lateral ganglia or loss of the cardia are probably apomorphic for this species. For the first time, oocytes that surprisingly develop in the pharyngeal wall are reported for this species.

Unity in diversity: a survey of muscular systems of ctenostome Gymnolaemata (Lophotrochozoa, Bryozoa)

BACKGROUND

Myoanatomical studies of adult bryozoans employing fluorescent staining and confocal laser scanning microscopy (CLSM) have been chiefly conducted on freshwater bryozoans. The diversity of muscular systems in the marine bryozoans is currently not well known with only two species being studied in more detail. The aim of this study is to unravel the diversity of muscle systems of 15 ctenostome bryozoans by phalloidin-coupled fluorescence stainings combined with CLSM.

RESULTS

In general, the myoanatomy of the selected ctenostomes shows significant similarities and consists of 1) muscles associated with the body wall, 2) apertural muscles, 3) lophophoral muscles, 4) tentacle sheath muscles, 5) digestive tract muscles and 6) the prominent retractor muscles. Differences are present in the arrangement of the apertural muscles from generally three muscles sets of four bundles, which in some species can be partially reduced or modified into a bilateral arrangement. The cardiac region of the digestive tract shows a distinct sphincter in four of the six studied clades. In some cases the cardiac region forms a prominent proventriculus or gizzard. Tentacle sheath muscles in victorelloideans and walkerioideans are arranged diagonally and differ from the simple longitudinal muscle arrangements common to all other taxa. Lophophoral base muscles consist of four sets that vary in the size of the sets and in the shape of the inner lophophoral ring, which either forms a complete ring or separate, intertentacular muscle bundles. The stolon-forming walkeridiodean ctenostomes show prominent transverse muscles in their stolons. These are always present in the shorter side stolons, but their occurrence in the main stolon seems to depend on the colony form, being present in creeping but absent in erect colony forms.

CONCLUSIONS

This study represents the first broad survey of muscular systems in adult ctenostome bryozoans and shows a certain degree of conservation in a series of diverse colony forms belonging to five major clades. However, several myoanatomical features such as the cardiac sphincter, basal (possibly transitory) cystid muscles, tentacle sheath muscles or apertural muscle arrangement vary across taxa and thus show a high potential for the assessment of character evolution within ctenostomes. As such, this study represents an essential contribution towards determining and reconstructing the character states of the bryozoan ground pattern once a reliable phylogenetic tree of the whole phylum becomes available.

Morphology and ontogeny of Lophopus crystallinus lophophore support the epistome as ancestral character of phylactolaemate bryozoans

Phylactolaemate bryozoans are the sister-group to all remaining bryozoan taxa. Consequently, their study is essential to reveal and analyze ancestral traits of Phylactolaemata and Bryozoa in general. They are the only bryozoans to possess an epistome which traditionally has been regarded as shared with phoronids and brachiopods. Contrary to older observations, an epistome was recently reported to be missing in the early branching phylactolaemate Lophopus crystallinus. In this study, the ontogeny of the lophophoral base and also its three-dimensional structure in adult specimens was reinvestigated to assess whether an epistome is never formed during ontogeny and absent in adult specimens. The results show that organogenesis during the budding process in this species is similar to other, previously investigated, species. The epistome anlage in L. crystallinus forms in early buds from the outer budding layer which penetrates the two shanks of the u-shaped gut. This ingression of the epithelium further proceeds distally and starts to wrap over the forming ganglion. The adult epistome is a rather short, but present bulge above the cerebral ganglion with prominent muscle bundles traversing its cavity. Distally it is arched by the forked canal that in L. crystallinus has a particularly thick and prominent epithelium in the three median tentacles. This study shows that neither during ontogeny nor in the adult stage an epistome is absent. The epistome is less pronounced than in other phylactolaemates, but otherwise similar in its general structure. Consequently, an epistome can be assumed to be present in the ground pattern of Phylactolaemata.