Innervation of the lophophore suggests that the phoronid Phoronis ovalis is a link between phoronids and bryozoans

Phoronida-A small clade with a big role in understanding the evolution of lophophorates

Phoronids, together with brachiopods and bryozoans, form the animal clade Lophophorata. Modern lophophorates are quite diverse-some can biomineralize while others are soft-bodied, they could be either solitary or colonial, and they develop through various eccentric larval stages that undergo different types of metamorphoses. The diversity of this clade is further enriched by numerous extinct fossil lineages with their own distinct body plans and life histories. In this review, I discuss how data on phoronid development, genetics, and morphology can inform our understanding of lophophorate evolution. The actinotrocha larvae of phoronids is a well documented example of intercalation of the new larval body plan, which can be used to study how new life stages emerge in animals with biphasic life cycle. The genomic and embryonic data from phoronids, in concert with studies of the fossil lophophorates, allow the more precise reconstruction of the evolution of lophophorate biomineralization. Finally, the regenerative and asexual abilities of phoronids can shed new light on the evolution of coloniality in lophophorates. As evident from those examples, Phoronida occupies a central role in the discussion of the evolution of lophophorate body plans and life histories.

Cambrian stem-group ambulacrarians and the nature of the ancestral deuterostome

Deuterostomes are characterized by some of the most widely divergent body plans in the animal kingdom. These striking morphological differences have hindered efforts to predict ancestral characters, with the origin and earliest evolution of the group remaining ambiguous. Several iconic Cambrian fossils have been suggested to be early deuterostomes and hence could help elucidate ancestral character states. However, their phylogenetic relationships are controversial. Here, we describe new, exceptionally preserved specimens of the discoidal metazoan Rotadiscus grandis from the early Cambrian Chengjiang biota of China. These reveal a previously unknown double spiral structure, which we interpret as a chordate-like covering to a coelomopore, located adjacent to a horseshoe-shaped tentacle complex. The tentacles differ in key aspects from those seen in lophophorates and are instead more similar to the tentacular systems of extant pterobranchs and echinoderms. Thus, Rotadiscus exhibits a chimeric combination of ambulacrarian and chordate characters. Phylogenetic analyses recover Rotadiscus and closely related fossil taxa as stem ambulacrarians, filling a significant morphological gap in the deuterostome tree of life. These results allow us to reconstruct the ancestral body plans of major clades of deuterostomes, revealing that key traits of extant forms, such as a post-anal region, gill bars, and a U-shaped gut, evolved through convergence.

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.

A Cambrian tommotiid preserving soft tissues reveals the metameric ancestry of lophophorates

Among extant animals, Lophotrochozoa accounts for the majority of phyla.¹ This bilaterian clade radiated rapidly during the Cambrian explosion, obfuscating its phylogenetic relationships and rendering many aspects of its early evolution uncertain. Many early lophotrochozoans are known only from isolated skeletal microfossils, "small shelly fossils," often derived from larger animals with complex multi-element skeletons.² The discovery of articulated fossils has revealed surprising insights into the animals from which these skeletal pieces were derived, such as paired shells in the mollusc Halkieria.³ Tommotiids are a key group of phosphatic early skeletal fossils that first appear in the late early Cambrian.^4,5 Although their affinities were previously obscure, discoveries of partial scleritomes and investigations of growth and microstructure⁶ provide links with Brachiopoda^7,8 and Phoronida,⁹ two of the lophophorate phyla. By contrast, the body plan of camenellan tommotiids remains a palaeontological mystery, with hypothetical reconstructions representing motile, benthic, dorsally armored worms.^4,10 Here, we describe an articulated camenellan (Wufengella bengtsoni gen. et sp. nov.) from the Cambrian Chengjiang Biota, China, revealing the morphology of the scleritome and the first soft tissues from an adult tommotiid. Wufengella carries two dorsal rows of sclerites in a highly asymmetric arrangement, flanked by smaller, cap-shaped sclerites. The scleritome was fringed by iterated fascicles of chaetae and two layers of flattened lobes. Phylogenetic analysis confirms that camenellans occupy a deep branch in lophophorate phylogeny, prior to the acquisition of a sessile lifestyle. Wufengella reveals direct evidence for a metameric body plan reminiscent of annelids early in the evolutionary history of lophophorates.^11,12.

Brachiopod and mollusc biomineralisation is a conserved process that was lost in the phoronid-bryozoan stem lineage

BACKGROUND

Brachiopods and molluscs are lophotrochozoans with hard external shells which are often believed to have evolved convergently. While palaeontological data indicate that both groups are descended from biomineralising Cambrian ancestors, the closest relatives of brachiopods, phoronids and bryozoans, are mineralised to a much lower extent and are comparatively poorly represented in the Palaeozoic fossil record. Although brachiopod and mollusc shells are structurally analogous, genomic and proteomic evidence indicates that their formation involves a complement of conserved, orthologous genes. Here, we study a set of genes comprised of 3 homeodomain transcription factors, one signalling molecule and 6 structural proteins which are implicated in mollusc and brachiopod shell formation, search for their orthologs in transcriptomes or genomes of brachiopods, phoronids and bryozoans, and present expression patterns of 8 of the genes in postmetamorphic juveniles of the rhynchonelliform brachiopod T. transversa.

RESULTS

Transcriptome and genome searches for the 10 target genes in the brachiopods Terebratalia transversa, Lingula anatina, Novocrania anomala, the bryozoans Bugula neritina and Membranipora membranacea, and the phoronids Phoronis australis and Phoronopsis harmeri resulted in the recovery of orthologs of the majority of the genes in all taxa. While the full complement of genes was present in all brachiopods with a single exception in L. anatina, a bloc of four genes could consistently not be retrieved from bryozoans and phoronids. The genes engrailed, distal-less, ferritin, perlucin, sp1 and sp2 were shown to be expressed in the biomineralising mantle margin of T. transversa juveniles.

CONCLUSIONS

The gene expression patterns we recovered indicate that while mineralised shells in brachiopods and molluscs are structurally analogous, their formation builds on a homologous process that involves a conserved complement of orthologous genes. Losses of some of the genes related to biomineralisation in bryozoans and phoronids indicate that loss of the capacity to form mineralised structures occurred already in the phoronid-bryozoan stem group and supports the idea that mineralised skeletons evolved secondarily in some of the bryozoan subclades.

Polyzoa is back: The effect of complete gene sets on the placement of Ectoprocta and Entoprocta

The phylogenomic approach has largely resolved metazoan phylogeny and improved our knowledge of animal evolution based on morphology, paleontology, and embryology. Nevertheless, the placement of two major lophotrochozoan phyla, Entoprocta (Kamptozoa) and Ectoprocta (Bryozoa), remains highly controversial: Originally considered as a single group named Polyzoa (Bryozoa), they were separated on the basis of morphology. So far, each new study of lophotrochozoan evolution has still consistently proposed different phylogenetic positions for these groups. Here, we reinvestigated the placement of Entoprocta and Ectoprocta using highly complete datasets with rigorous contamination removal. Our results from maximum likelihood, Bayesian, and coalescent analyses strongly support the topology in which Entoprocta and Bryozoa form a distinct clade, placed as a sister group to all other lophotrochozoan clades: Annelida, Mollusca, Brachiopoda, Phoronida, and Nemertea. Our study favors the evolutionary scenario where Entoprocta, Cycliophora, and Bryozoa constitute one of the earliest branches among Lophotrochozoa and thus supports the Polyzoa hypothesis.

First Modern Data on the Lophophore Nervous System in Adult Novocrania anomala and a Current Assessment of Brachiopod Phylogeny

Simple Summary

The nervous system of Novocrania anomala adults is described for the first time. A table containing data on the lophophore innervation in species from three brachiopod subphyla is presented. A comparative analysis suggests a close relationship between the Craniiformea and the Rhynchonelliformea, and thereby supports the “Calciata” hypothesis of brachiopod phylogeny.

Abstract

Although the lophophore is regarded as the main synapomorphy of all lophophorates, the evolution of the lophophore in certain groups of lophophorates remains unclear. To date, the innervation of the lophophore has been studied with modern methods only for three brachiopod species belonging to two subphyla: Linguliformea and Rhynchonelliformea. In the third subphylum, the Craniiformea, there are data for juveniles but not for adults. In the current research, the innervation of the lophophore in Novocrania anomala adults was studied by immunocytochemistry and confocal laser scanning microscopy. In the spiral lophophore of adults of the craniiform N. anomala, each arm is innervated by six brachial nerves: main, additional main, accessory, second accessory, additional lower, and lower brachial nerves. Compared with other brachiopod species, this complex innervation of the lophophore correlates with the presence of many lophophoral muscles. The general anatomy of the lophophore nervous system and the peculiarities of the organization of the subenteric ganglion of the craniiform N. anomala have a lot in common with those of rhynchonelliforms but not with those of linguliforms. These findings are consistent with the “Calciata” hypothesis of the brachiopod phylogeny and are inconsistent with the inference that the Craniiformea and Linguliformea are closely related.

Detailed morphology of tentacular apparatus and central nervous system in Owenia borealis (Annelida, Oweniidae)

The Oweniidae are marine annelids with many unusual features of organ system, development, morphology, and ultrastructure. Together with magelonids, oweniids have been placed within the Palaeoannelida, a sister group to all remaining annelids. The study of this group may increase our understanding of the early evolution of annelids (including their radiation and diversification). In the current research, the morphology and ulta-anatomy of the head region of Owenia borealis is studied by scanning electron microscopy (SEM), 3D reconstructions, transmission electron microscopy (TEM), and whole-mount immunostaining with confocal laser scanning microscopy. According to SEM, the tentacle apparatus consists of 8-14 branched arms, which are covered by monociliary cells that form a ciliary groove extending along the oral side of the arm base. Each tentacle contains a coelomic cavity with a network of blood capillaries. Monociliary myoepithelial cells of the tentacle coelomic cavity form both the longitudinal and the transverse muscles. The structure of this myoepithelium is intermediate between a simple and pseudo-stratified myoepithelium. Overall, tentacles lack prominent zonality, i.e., co-localization of ciliary zones, neurite bundles, and muscles. This organization, which indicates a non-specialized tentacle crown in O. borealis and other oweniids with tentacles, may be ancestral for annelids. TEM, light, and confocal laser scanning microscopy revealed that the head region contains the anterior nerve center comprising of outer and inner (=circumoral) nerve rings. Both nerve rings are organized as concentrated nerve plexus, which contains perikarya and neurites extending between basal projections of epithelial cells (radial glia). The outer nerve ring gives rise to several thick neurite bundles, which branch and extend along aboral side of each tentacle. Accordingly to their immunoreactivity, both rings of the anterior nerve center could be homologized with the dorsal roots of circumesophageal connectives of the typical annelids. Accordingly to its ultrastructure, the outer nerve ring of O. borealis and so-called brain of other oweniids can not be regarded as a typical brain, i.e. the most anterior ganglion, because it lacks ganglionic structure.

The nervous system of the most complex lophophore provides new insights into the evolution of Brachiopoda

The lophophore is a tentacle organ unique to the lophophorates. Recent research has revealed that the organization of the nervous and muscular systems of the lophophore is similar in phoronids, brachiopods, and bryozoans. At the same time, the evolution of the lophophore in certain lophophorates is still being debated. Innervation of the adult lophophore has been studied by immunocytochemistry and confocal laser scanning microscopy for only two brachiopod species belonging to two subphyla: Linguliformea and Rhynchonelliformea. Species from both groups have the spirolophe, which is the most common type of the lophophore among brachiopods. In this study, we used transmission electron microscopy, immunocytochemistry, and confocal laser scanning microscopy to describe the innervation of the most complex lophophore (the plectolophe) of the rhynchonelliform species Coptothyris grayi. The C. grayi lophophore (the plectolophe) is innervated by three brachial nerves: the main, second accessory, and lower. Thus, the plectolophe lacks the accessory brachial nerve, which is typically present in other studied brachiopods. All C. grayi brachial nerves contain two types of perikarya. Because the accessory nerve is absent, the cross nerves, which pass into the connective tissue, have a complex morphology: each nerve consists of two ascending and one descending branches. The outer and inner tentacles are innervated by several groups of neurite bundles: one frontal, two lateral, two abfrontal, and two latero-abfrontal (the latter is present in only the outer tentacles). Tentacle nerves originate from the second accessory and lower brachial nerves. The inner and outer tentacles are also innervated by numerous peritoneal neurites, which exhibit acetylated alpha-tubulin-like immunoreactivity. The nervous system of the lophophore of C. grayi manifests several evolutionary trends. On the one hand, it has undergone simplification, i.e., the absence of the accessory brachial nerve, which is apparently correlated with a reduction in the complexity of the lophophore's musculature. On the other hand, C. grayi has a prominent second accessory nerve, which contains large groups of frontal perikarya, and also has additional nerves extending from the both ganglia to the medial arm; these features are consistent with the complex morphology of the C. grayi plectolophe. In brachiopods, the evolution of the lophophore nervous system apparently involved two main modifications. The first modification was the appearance and further strengthening of the second accessory brachial nerve, which apparently arose because of the formation of a double row of tentacles instead of the single row of the brachiopod ancestor. The second modification was the partial or complete reduction of some brachial nerves, which was correlated with the reduced complexity of the lophophore musculature and the appearance of skeletal structures that support the lophophore.

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.

Mitochondrial gene order of the freshwater bryozoan Cristatella mucedo retains ancestral lophotrochozoan features

Bryozoans are aquatic colonial suspension-feeders abundant in many marine and freshwater benthic communities. At the same time, the phylum is under studied on both morphological and molecular levels, and its position on the metazoan tree of life is still disputed. Bryozoa include the exclusively marine Stenolaemata, predominantly marine Gymnolaemata and exclusively freshwater Phylactolaemata. Here we report the mitochondrial genome of the phylactolaemate bryozoan Cristatella mucedo. This species has the largest (21,008 bp) of all currently known bryozoan mitogenomes, containing a typical metazoan gene compendium as well as a number of non-coding regions, three of which are longer than 1500 bp. The trnS1/trnG/nad3 region is presumably duplicated in this species. Comparative analysis of the gene order in C. mucedo and another phylactolaemate bryozoan, Pectinatella magnifica, confirmed their close relationships, and revealed a stronger similarity to mitogenomes of phoronids and other lophotrochozoan species than to marine bryozoans, indicating the ancestral nature of their gene arrangement. We suggest that the ancestral gene order underwent substantial changes in different bryozoan cladesshowing mosaic distribution of conservative gene blocks regardless of their phylogenetic position. Altogether, our results support the early divergence of Phylactolaemata from the rest of Bryozoa.

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.

Genes with evidence of positive selection as potentially related to coloniality and the evolution of morphological features among the lophophorates and entoprocts

Evolutionary mechanisms that underlie the origins of coloniality among organisms are diverse. Some animal colonies may be comprised strictly of clonal individuals formed from asexual budding or comprised of a chimera of clonal and sexually produced individuals that fuse secondarily. This investigation focuses on select members of the lophophorates and entoprocts whose evolutionary relationships remain enigmatic even in the age of genomics. Using transcriptomic data sets, two coloniality-based hypotheses are tested in a phylogenetic context to find candidate genes showing evidence of positive selection and potentially convergent molecular signatures among solitary species and taxa-forming colonies from aggregate groups or clonal budding. Approximately 22% of the 387 orthogroups tested showed evidence of positive selection in at least one of the three branch-site tests (CODEML, BUSTED, and aBSREL). Only 12 genes could be reliably associated with a developmental function related to traits linked with coloniality, neuroanatomy, or ciliary fields. Genes testing for both positive selection and convergent molecular characters include orthologues of Radial spoke head, Elongation translation initiation factors, SEC13, and Immediate early response gene5. Maximum likelihood analyses included here resulted in tree topologies typical of other phylogenetic investigations based on wider genomic information. Further genomic and experimental evidence will be needed to resolve whether a solitary ancestor with multiciliated cells that formed aggregate groups gave rise to colonial forms in bryozoans (and perhaps the entoprocts) or that the morphological differences exhibited by phoronids and brachiopods represent trait modifications from a colonial ancestor.

First data on the organization of the nervous system in juveniles of Novocrania anomala (Brachiopoda, Craniiformea)

The organization and development of the nervous system are traditionally used for phylogenetic analysis and may be useful for clarification of evolution and phylogeny of some poor studied groups. One of these groups is brachiopods: most data on their nervous system organization were obtained in 19^th century. In this research, antibody staining and confocal laser scanning microscopy were used to study the nervous system of early ontogenetic stages of the brachiopod Novocrania anomala. Although N. anomala adults are thought to lack a supraenteric ganglion, a large supraenteric ganglion exists in N. anomala juveniles with either a trocholophe or a schizolophe. During ontogenesis, the supraenteric ganglion in the juvenile changes its shape: the commissure between the two lobes of the ganglion extends. This commissure possibly gives rise to the main brachial nerve in adults. The supraenteric ganglion gives rise to the cross (transversal) nerves that extend to the accessory brachial nerve, which gives rise to the tentacular nerves. In juveniles with a trocholophe, the accessory brachial nerve gives rise to the frontal and intertentacular nerves of tentacles that form a single row. When the trocholophe transforms into the schizolophe, the second row of tentacles appears and the innervation of the tentacles changes. The intertentacular nerves disappear and the second accessory nerve forms and gives rise to the laterofrontal tentacular nerves of the inner and outer tentacles and to the abfrontal nerves of the inner tentacles. The so-called subenteric ganglion, which was described as a ganglion in N. anomala adults, is represented by a large circumvisceral nerve in N. anomala juveniles.The results suggest that ‘phoronid-like’ non-specialized tentacles may be regarded as the ancestral type of tentacles for brachiopods and probably for all lophophorates. The presence of intertentacular nerves is the ancestral feature of all lophophorates. The transformation of the juvenile supraenteric ganglion into the main brachial nerve of N. anomala adults suggests that research is needed on the development and organization of the supraenteric ganglion and the main brachial nerve in other brachiopods, whose adults have a prominent supraenteric ganglion.

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.

Neurons and Glia Cells in Marine Invertebrates: An Update

The nervous system (NS) of invertebrates and vertebrates is composed of two main types of cells: neurons and glia. In both types of organisms, nerve cells have similarities in biochemistry and functionality. The neurons are in charge of the synapse, and the glial cells are in charge of important functions of neuronal and homeostatic modulation. Knowing the mechanisms by which NS cells work is important in the biomedical area for the diagnosis and treatment of neurological disorders. For this reason, cellular and animal models to study the properties and characteristics of the NS are always sought. Marine invertebrates are strategic study models for the biological sciences. The sea slug Aplysia californica and the squid Loligo pealei are two examples of marine key organisms in the neurosciences field. The principal characteristic of marine invertebrates is that they have a simpler NS that consists of few and larger cells, which are well organized and have accessible structures. As well, the close phylogenetic relationship between Chordata and Echinodermata constitutes an additional advantage to use these organisms as a model for the functionality of neuronal cells and their cellular plasticity. Currently, there is great interest in analyzing the signaling processes between neurons and glial cells, both in vertebrates and in invertebrates. However, only few types of glial cells of invertebrates, mostly insects, have been studied, and it is important to consider marine organisms' research. For this reason, the objective of the review is to present an update of the most relevant information that exists around the physiology of marine invertebrate neuronal and glial cells.

Are hyoliths Palaeozoic lophophorates?

The phylogenetic position of hyoliths has long been unsettled, with recent discoveries of a tentaculate feeding apparatus (‘lophophore’) and fleshy apical extensions from the shell (‘pedicle’) suggesting a lophophorate affinity. Here, we describe the first soft parts associated with the feeding apparatus of an orthothecid hyolith, Triplicatella opimus from the Chengjiang biota of South China. The tuft-like arrangement of the tentacles of T. opimus differs from that of hyolithids, suggesting they collected food directly from the substrate. A reassessment of the feeding organ in hyolithids indicates that it does not represent a lophophore and our analysis of the apical structures associated with some orthothecids show that these represent crushed portions of the shell and are not comparable to the brachiopod pedicle. The new information suggests that hyoliths are more likely to be basal members of the lophotrochozoans rather than lophophorates closely linked with the Phylum Brachiopoda.

The neuroanatomy of Barentsia discreta (Entoprocta, Coloniales) reveals significant differences between bryozoan and entoproct nervous systems

BACKGROUND

Entoprocta affinities within Lophotrochozoa remain unclear. In different studies, entoprocts are considered to be related to different groups, including Cycliophora, Bryozoa, Annelida, and Mollusca. The use of modern methods to study the neuroanatomy of Entoprocta should provide new information that may be useful for phylogenetic analysis.

RESULTS

The anatomy of the nervous system in the colonial Barentsia discreta was studied using immunocytochemistry and transmission electron microscopy. The ganglion gives rise to several main nerves: paired lateral, aboral, and arcuate nerves, and three pairs of tentacular cords that branch out into tentacular nerves. The serotonergic nervous system includes paired esophageal perikarya and two large peripheral perikarya, each with a complex net of neurites. Each tentacle is innervated by one abfrontal and two laterofrontal neurite bundles. Sensory cells occur regularly along the abfrontal side of each tentacle. Star-like nerve cells are scattered in the epidermis of the calyx. The stalk is innervated by paired stalk nerves.

CONCLUSIONS

The neuroanatomy of the colonial Barentsia discreta is generally similar to that of solitary entoprocts but differs in the anatomy and ultrastructure of the ganglion, the number of neurite bundles in the calyx, and the distribution of serotonin in the nerve elements. A comparison of the organization of the nervous system in the Entoprocta and Bryozoa reveals many differences in tentacle innervations, which may indicate that these groups may not be closely related. Our results can not support with any certainty the homology of nervous system elements in adult entoprocts and adult "basal mollusks".

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.

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.