The neuromuscular system of the cyclostome bryozoan Crisia eburnea (Linnaeus, 1758)

Unusual lophophore innervation in ctenostome Flustrellidra hispida (Bryozoa)

Since ctenostomes are traditionally regarded as an ancestral clade to some other bryozoan groups, the study of additional species may help to clarify questions on bryozoan evolution and phylogeny. One of these questions is the bryozoan lophophore evolution: whether it occurred through simplification or complication. The morphology and innervation of the ctenostome Flustrellidra hispida (Fabricius, 1780) lophophore have been studied with electron microscopy and immunocytochemistry with confocal laser scanning microscopy. Lophophore nervous system of F. hispida consists of several main nerve elements: cerebral ganglion, circumoral nerve ring, and the outer nerve ring. Serotonin-like immunoreactive perikarya, which connect with the circumoral nerve ring, bear the cilium that directs to the abfrontal side of the lophophore and extends between tentacle bases. The circumoral nerve ring gives rise to the intertentacular and frontal tentacle nerves. The outer nerve ring gives rise to the abfrontal neurites, which connect to the outer groups of perikarya and contribute to the formation of the abfrontal tentacle nerve. The outer nerve ring has been described before in other bryozoans, but it never contributes to the innervation of tentacles. The presence of the outer nerve ring participating in the innervation of tentacles makes the F. hispida lophophore nervous system particularly similar to the lophophore nervous system of phoronids. This similarity allows to suggest that organization of the F. hispida lophophore nervous system may reflect the ancestral state for all bryozoans. The possible scenario of evolutionary transformation of the lophophore nervous system within bryozoans is suggested.

The epithelial layers of the body wall in hornerid bryozoans (Stenolaemata: Cyclostomatida)

Bryozoans are small colonial coelomates. They can be conceptualised as "origami-like" animals, composed of three complexly folded epithelial layers: epidermis of the zooidal/colonial body wall, gut epithelium and coelothelium. We investigated the general microanatomy and ultrastructure of the hornerid (Cyclostomatatida) body wall and polypide in four taxa, including three species of Hornera and one species belonging to an undescribed genus. We describe epithelia and their associated structures (e.g., ECM, cuticle) across all portions of the hornerid body wall, including the terminal membrane, vestibular wall, atrial sphincter, membranous sac and polypide-skeletal attachments. The classic coelomate body wall composition (epidermis-ECM-coelothelium) is only present in an unmodified form in the tentacle sheath. Deeper within a zooid it is retained exclusively in the attachment zones of the membranous sac: [skeleton]-tendon cell-ECM-coelothelium. A typical invertebrate pattern of epithelial organisation is a single, continuous sheet of polarised cells, connected by belt desmosomes and septate junctions, and resting on a collagenous extracellular matrix. Although previous studies demonstrated that polypide-specific epithelia of Horneridae follow this model, here we show that the body wall may show significant deviations. Cell layers can lose the basement membrane and/or continuity of cell cover and cell contacts. Moreover, in portions of the body wall, the cell layer appears to be missing altogether; the zooidal orifice is covered by a thin naked cuticle largely devoid of underlying cells. Since epithelium is a two-way barrier against entry and loss of materials, it is unclear how hornerids avoid substance loss, while maintaining intracolonial metabolite transport with imperfect, sometimes incomplete, cell layers along large portions of their outer body surface.

Polypide anatomy of hornerid bryozoans (Stenolaemata: Cyclostomatida)

Bryozoans are small colonial coelomates whose colonies are made of individual modules (zooids). Like most coelomate animals, bryozoans have a characteristic body wall composition, including an epidermis, an extracellular matrix (ECM) and a coelothelium, all pressed together. The order Cyclostomatida, however, presents the most striking deviation, in which the ECM and the corresponding coelothelium underlying major parts of the skeletal wall epidermis are detached to form an independent membranous sac. It forms a separate, much smaller compartment, suspended in the zooid body cavity and working as an important element of the cyclostome lophophore protrusion mechanism. The polypide anatomy and ultrastructure of this group is best known from studies of one family, the Crisiidae (Articulata). Here, we examined four species from the phylogenetically and ecologically contrasting family Horneridae (Cancellata) from New Zealand, and provide the first detailed ultrastructural description of the hornerid polypide, including tentacles, mouth region, digestive system and the funiculus. We were able to trace continuity and transitions of cell and ECM layers throughout the whole polypide. In addition, we identified that the funiculus is a lumen-free ECM cord with two associated muscles, disconnected from interzooidal pores. Except for funicular core composition, the polypide anatomy of hornerids agrees well with the general cyclostomate body plan.

A comparative analysis of the nervous system of cheilostome bryozoans

Background

Bryozoans are sessile aquatic suspension feeders in mainly marine, but also freshwater habitats. Most species belong to the marine and calcified Cheilostomata. Since this taxon remains mostly unstudied regarding its neuroanatomy, the focus of this study is on the characterization and ground pattern reconstruction of the autozooidal nervous system based on six representatives.

Results

A common neuronal innervation pattern is present in the investigated species: a cerebral ganglion is located at the base of the lophophore, from where neurite bundles embrace the mouth opening to form a circumoral nerve ring. Four neurite bundles project from the cerebral ganglion to innervate peripheral areas, such as the body wall and parietal muscles via the tentacle sheath. Five neurite bundles comprise the main innervation of the visceral tract. Four neurite bundles innervate each tentacle via the circumoral nerve ring. Mediofrontal tentacle neurite bundles emerge directly from the nerve ring. Two laterofrontal- and one abfrontal tentacle neurite bundles emanate from radial neurite bundles, which originate from the cerebral ganglion and circumoral nerve ring in between two adjacent tentacles. The radial neurite bundles terminate in intertentacular pits and give rise to one abfrontal neurite bundle at the oral side and two abfrontal neurite bundles at the anal side. Similar patterns are described in ctenostome bryozoans.

Conclusions

The present results thus represent the gymnolaemate situation. Innervation of the tentacle sheath and visceral tract by fewer neurite bundles and tentacular innervation by four to six tentacle neurite bundles support cyclostomes as sister taxon to gymnolaemates. Phylactolaemates feature fewer distinct neurite bundles in visceral- and tentacle sheath innervation, which always split in nervous plexus, and their tentacles have six neurite bundles. Thus, this study supports phylactolaemates as sistergroup to myolaemates.

Peculiarities of Tentacle Innervation of Flustrellidra hispida and Evolution of Lophophore in Bryozoa

The study of the lophophore organization is of great importance for the reconstruction of lophophorate phylogeny and for understanding the evolutionary transformation in each phylum of Lophophorata. The innervation of the lophophore in ctenostome bryozoan Flustrellidra hispida was studied using immunocytochemistry and confocal laser scanning microscopy. It has been demonstrated that this species has an outer nerve ring giving rise to the tentacle nerves. The outer nerve ring was earlier described in some ctenostomates and cyclostomates, but not as connected with nerves. The discovered feature of lophophore innervation in F. hispida suggests the evolutionary transformation from a hypothetical phoronida-like ancestor lophophore bearing a prominent outer nerve ring with numerous tentacle nerves emanating from it, to the complex bell-shaped lophophore of F. hispida with a well-pronounced outer nervous ring bearing a few tentacle nerves. The next one in this hypothetical row is the lophophore of the other ctenostomates and some cyclostomates with no ring-nerve connection and cheilostomates lophophore with no outer nerve ring at all.

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.

Novel data on the innervation of the lophophore in adult phoronids (Lophophorata, Phoronida)

The structure of the lophophore nervous system may help clarify the status of the clade Lophophorata, whose monophyly is debated. In the current study, antibody labeling and confocal laser scanning microscopy revealed previously undescribed main nerve elements in the lophophore in adult phoronids: Phoronis australis and Phoronopsis harmeri. In both species, the nervous system includes a dorsal ganglion, a tentacle nerve ring, an inner nerve ring, intertentacular groups of perikarya, and tentacle nerves. The dorsal ganglion and tentacle nerve ring contain many serotonin-like immunoreactive perikarya of different sizes. The inner nerve ring is described for the first time in adult phoronids with complex lophophore. It contains a thin bundle of serotonin-like immunoreactive neurites. The tentacles possess abfrontal, frontal, and laterofrontal nerves. The abfrontal nerves originate from the tentacle nerve ring; the frontal tentacle nerves extend from the inner nerve ring in P. harmeri and from the intertentacular frontal nerves in P. australis. The intertentacular groups of perikarya are found in phoronids for the first time. These small nerve centers connect with neither the tentacle nerve ring nor the inner nerve ring, giving rise to the laterofrontal tentacle nerves. The discovery of the inner nerve ring in adult phoronids makes the architecture of the lophophore nervous system similar in all lophophorates and thereby supports the monophyly of this group. The presence of intertentacular nerves, perikarya, and groups of perikarya is a typical feature of the nervous system in lophophorate presumably coordinating movements of the tentacles and thereby increasing the efficiency of lophophore functioning.

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.

The nervous system in the cyclostome bryozoan Crisia eburnea as revealed by transmission electron and confocal laser scanning microscopy

INTRODUCTION

Among bryozoans, cyclostome anatomy is the least studied by modern methods. New data on the nervous system fill the gap in our knowledge and make morphological analysis much more fruitful to resolve some questions of bryozoan evolution and phylogeny.

RESULTS

The nervous system of cyclostome Crisia eburnea was studied by transmission electron microscopy and confocal laser scanning microscopy. The cerebral ganglion has an upper concavity and a small inner cavity filled with cilia and microvilli, thus exhibiting features of neuroepithelium. The cerebral ganglion is associated with the circumoral nerve ring, the circumpharyngeal nerve ring, and the outer nerve ring. Each tentacle has six longitudinal neurite bundles. The body wall is innervated by thick paired longitudinal nerves. Circular nerves are associated with atrial sphincter. A membranous sac, cardia, and caecum all have nervous plexus.

CONCLUSION

The nervous system of the cyclostome C. eburnea combines phylactolaemate and gymnolaemate features. Innervation of tentacles by six neurite bundles is similar of that in Phylactolaemata. The presence of circumpharyngeal nerve ring and outer nerve ring is characteristic of both, Cyclostomata and Gymnolaemata. The structure of the cerebral ganglion may be regarded as a result of transformation of hypothetical ancestral neuroepithelium. Primitive cerebral ganglion and combination of nerve plexus and cords in the nervous system of C. eburnea allows to suggest that the nerve system topography of C. eburnea may represent an ancestral state of nervous system organization in Bryozoa. Several scenarios describing evolution of the cerebral ganglion in different bryozoan groups are proposed.