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The main brachial nerve in Lingula anatina. (A) Cross semi-thin section of the main brachial nerve. (B) A portion of the main brachial nerve: several types of perikarya; TEM. Striated rootlets are indicated by arrowheads. (C) Cross section of a portion of the neuropil with neurites aligned in different directions: longitudinal (lgn) and transversal (tn) neurites; TEM. (D) The giant nerve fiber (gnf) in the neuropile of the main brachial nerve. Abbreviations: ct, connective tissue; ep, epidermis; gc, glial cell; he, hemidesmosome; mc, mitochondrion; mt, microtubules; n, neuropil; p, perikarya; p1, p2, perikarya of different types; pgc, projections of glial cells; to, tonofilaments.
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Evolutionary relationships among members of the Lophophorata remain unclear. Traditionally, the Lophophorata included three phyla: Brachiopoda, Bryozoa or Ectoprocta, and Phoronida. All species in these phyla have a lophophore, which is regarded as a homologous structure of the lophophorates. Because the organization of the nervous system has been...
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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 nam...
Citations
... The homology of the lophophore in phoronids, brachiopods, and bryozoans was doubted before (1). However, some recent studies on the lophophore innervation in phoronids suggest the possibility of homologization of this structure in phoronids, brachiopods, and bryozoans (64,65). Is Polyzoa an artificial clade? ...
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.
... The lophophore is an outgrowth of the body wall bearing ciliated tentacles, which surround the mouth, but never surround the anus. Although many molecular studies rejected the monophyly of lophophorates (Hejnol et al., 2009;Kocot et al., 2017), it has been supported by both morphological (Temereva & Tsitrin, 2015; Temereva Temereva & Kosevich 2016, 2017a, 2017b | 1 between lophophorates are still not well-resolved: although phoronids and bryozoans have the same bauplan (Temereva & Malakhov, 2011), a number of studies (Cohen et al., 1998;Cohen, 2000Cohen, , 2013Cohen & Weydmann, 2005) support the separate Brachiozoa group uniting phoronids and brachiopods. ...
... Ctenostome bryozoans lack calcium skeleton, and considered as being ancestral for both stenolaemate and cheilostome bryozoans . Molecular phylogenies showed the paraphyly of ctenostomes and strictly speaking they cannot be regarded as a taxon (Fuchs et al., 2009;Waeschenbach et al., 2012). However, all ctenostomes have some morphological traits in common, and are traditionally considered as a clade. ...
... Because the monophyly of lophophorates has been supported by recent molecular and morphological data (Laumer et al., 2019;Marletaz et al., 2019;Temereva & Kosevich, 2016Temereva & Tsitrin, 2015;Temereva, 2017aTemereva, , 2017bTemereva, , 2020aTemereva, , 2020bZverkov et al., 2019), we can suggest that the lophophore is a homologous structure, which had been inherited by recent lophophorates from their last common ancestor. New morphological data on F. hispida lophophore innervation support this idea and allow defining homologous nerves of the lophophore in bryozoans, phoronids and brachiopods. ...
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 lophophore is an outgrowth of the body wall bearing ciliated tentacles, which surround the mouth, but never surround the anus. Although many molecular studies rejected the monophyly of lophophorates (Hejnol et al., 2009;Kocot et al., 2017), it has been supported by both morphological (Temereva & Tsitrin, 2015; Temereva Temereva & Kosevich 2016, 2017a, 2017b | 1 between lophophorates are still not well-resolved: although phoronids and bryozoans have the same bauplan (Temereva & Malakhov, 2011), a number of studies (Cohen et al., 1998;Cohen, 2000Cohen, , 2013Cohen & Weydmann, 2005) support the separate Brachiozoa group uniting phoronids and brachiopods. ...
... Ctenostome bryozoans lack calcium skeleton, and considered as being ancestral for both stenolaemate and cheilostome bryozoans . Molecular phylogenies showed the paraphyly of ctenostomes and strictly speaking they cannot be regarded as a taxon (Fuchs et al., 2009;Waeschenbach et al., 2012). However, all ctenostomes have some morphological traits in common, and are traditionally considered as a clade. ...
... Because the monophyly of lophophorates has been supported by recent molecular and morphological data (Laumer et al., 2019;Marletaz et al., 2019;Temereva & Kosevich, 2016Temereva & Tsitrin, 2015;Temereva, 2017aTemereva, , 2017bTemereva, , 2020aTemereva, , 2020bZverkov et al., 2019), we can suggest that the lophophore is a homologous structure, which had been inherited by recent lophophorates from their last common ancestor. New morphological data on F. hispida lophophore innervation support this idea and allow defining homologous nerves of the lophophore in bryozoans, phoronids and brachiopods. ...
Many data on echiurid anatomy and ultrastructure are obtained for Bonellia viridis and extrapolated to other species. The ultrastructure of the axial blood vessels, which has been described as an “osmotic pump”, is regarded as one of the unusual features of echiurids. In this study, the ultrastructure of the proboscis blood vessels in females of B. viridis is described, illustrated by accurate schemes, and a new reconstruction of the axial blood vessel is suggested. The walls of the axial and lateral vessels of the proboscis are formed by myoepithelial cells, which are connected to each other via adherence junctions, underlined by basal lamina, and therefore form a true epithelium. Apical, middle, and basal parts of the myoepithelial cells form long, thin projections, which extend to the connective tissue (in axial vessel) or coelomic canals (in lateral vessels) and to the lumen of the vessels. The presence of such projections may evidence active cellular transport. Similarity in the fine structure of the myoepithelial cells of axial and lateral blood vessels evidences their common origin from myoepithelial cells of the coelomic lining. However, in evolution, the coelomic canals were retained around the lateral vessels and disappeared around the axial vessel. The reduction of a hypothetical ancestral axial coelom may be caused by the extensive development of the connective tissue and muscles in the central part of the proboscis, where the axial vessel extends. This article is protected by copyright. All rights reserved.
... The lophophore is a tentacular organ that is present in all lophophorates: phoronids, brachiopods, and bryozoans. The monophyly of lophophorates has been recently rebuilt by morphological [15][16][17][18][19][20][21] and molecular [22][23][24] data. The morphological data, which mostly concern the organization of the lophophore nervous system, support the homology of the lophophore in all three phyla of lophophorates. ...
... In adult brachiopods, the organization of the lophophore nervous system has been studied by immunocytochemistry and confocal laser scanning microscopy in three species from two subphyla: Linguliformea (Lingula anatina) [15] and Rhynchonelliformea (Hemithiris psittacea and Coptothyris grayi) [27,28]. These studies revealed two trends in the evolution of the lophophore in brachiopods. ...
... The lophophore nervous system of brachiopods has been mostly studied by histological methods [29,30,33,34], and only a few species have been investigated by electron microscopy, immunocytochemistry, and confocal laser scanning microscopy [15,19,27,28,[35][36][37]. Together, these studies indicate the general pattern of lophophore innervation. ...
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.
... Although the directions of tentacle evolution remain uncertain, we know that some organisms have specialized tentacles [32][33][34][35][36][37][38]. This specialization is expressed in the zonation and co-localization of several organ systems: ciliary bands, nerve cords, and muscles [39][40][41][42][43][44][45][46][47]. Such specialized tentacles are present in the lophophorates [48][49][50][51]. ...
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 tentacles of all of the studied species of brachiopods have peritoneal neurites in the canal coelothelium, which is consistent with previous reports (Temereva & Tsitrin, 2015;Temereva & Kuzmina, 2017. Peritoneal neurites have been described only in lophophorates, which is consistent with the view that the lophophore is an organ that is specific to the lophophorates (Temereva, 2017). ...
Although the morphology of the brachiopod tentacle organ, the lophophore, is diverse, the organization of tentacles has traditionally been thought to be similar among brachiopods. We report here, however, that the structure of the tentacle muscles differs among brachiopod species representing three subphyla: Lingula anatina (Linguliformea: Linguloidea), Pelagodiscus atlanticus (Linguliformea: Discinoidea), Novocrania anomala (Craniiformea), and Coptothyris grayi (Rhynchonelliformea). Although the tentacle muscles in all four species are formed by myoepithelial cells with thick myofilaments of different diameters, three types of tentacle organization were detected. The tentacles of the first type occur in P. atlanticus, C. grayi, and in all rhynchonelliforms studied before. These tentacles have a well-developed frontal muscle and a small abfrontal muscle, which may reflect the ancestral organization of tentacles of all brachiopods. This type of tentacle has presumably been modified in other brachiopods due to changes in life style. Tentacles of the second type occur in the burrowing species L. anatina and are characterized by the presence of equally developed smooth frontal and abfrontal muscles. Tentacles of the third type occur in N. anomala and are characterized by the presence of only well-developed frontal muscles; the abfrontal muscles are reduced due to the specific position of tentacles during filtration and to the presence of numerous peritoneal neurites on the abfrontal side of the tentacles. Tentacles of the first type are also present in phoronids and bryozoans, and may be ancestral for all lophophorates.
Highlights
• 1. The tentacles with a well-developed frontal muscle and a small abfrontal muscle reflect the ancestral organization of tentacles of brachiopods.
• 2. This type of tentacle has presumably been modified in other brachiopods due to changes in life style.
... Organization of the lophophore nervous system has been studied by different methods in specimens from all three groups, including the following species: N. anomala 23,25 , Discinisca lamellosa 26 , L. anatina 4 , Gryphus vitreus 27 , and H. psittacea 22 . Innervation of the lophophore in brachiopods has mostly been studied via light microscopy [25][26][27] , and only four species have been studied with TEM, immunocytochemistry, and CLSM 4,22,23,28 . According to all data, the central nervous system of rhynchonelliformean brachiopods includes two ganglia, the subenteric and the supraenteric, which are located under and above the mouth, respectively. ...
... The main brachial nerve is connected to the accessory brachial nerve via numerous cross nerves, which extend into the connective tissue of the lophophore arms. These three brachial nerves, i.e., the main, accessory, and lower, are the major nerves of the lophophore in all brachiopods studied to date 4,22,23,[25][26][27] . Two recent studies of lophophore innervation have revealed the presence of a second accessory nerve in the Abbreviations: bit-base of inner tentacle; bot-base of outer tentacle; cn-cross nerve; ecm-extracellular matrix; fm-frontal tentacle muscle; fn-frontal nerve; gc-gland cell; lafn-lateroabfrontal tentacle nerve; mmitochondria; n-nucleus; ne-neurite; ot-outer tentacle; pec-projection of envelop cell; pkII-perikarya of second type; pkIII-perikarya of third type; rer-rough endoplasmic reticulum; sa-second accessory brachial nerve. ...
... ◂ rhynchonelliform H. psittacea 22 and in the juveniles of craniiform N. anomala 23 . Parts of this brachial nerve are represented by groups of FMRF-amide-like immunoreactive perikarya and have been previously described in L. anatina 4 . Brachiopods therefore have four major brachial nerves: the main, accessory, second accessory, and lower. ...
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.
... Phylum Bryozoa was traditionally considered to be a part of the taxon Lophophorata along with Phoronida and Brachiopoda (Hyman, 1959). Although this grouping was disputed by several molecular phylogenetic studies (Dunn et al., 2008;Hejnol et al., 2009;Helmkampf et al., 2008;Kocot et al., 2017;Laumer et al., 2015;Paps et al., 2009;Philippe et al., 2005), the most recent of them corroborated monophyly of Lophophorata (Laumer et al., 2019;Marlétaz et al., 2019;reviewed in Bleidorn, 2019), in congruence with modern morphological data (Temereva and Tsitrin, 2015;Temereva, 2017aTemereva, , 2017breviewed in Schwaha et al., 2020). Nevertheless, studies on specific synapomorphies, including molecular ones, remain an important challenge for our understanding of the lophophorate phylogeny. ...
Bryozoans are aquatic colonial suspension-feeders abundant in many marine and freshwater benthic communities. This phylum is under-investigated, however, 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 (except for not found trnY) 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 clades showing 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.
... Although the directions of tentacle evolution remain uncertain, we know that some organisms have specialized tentacles [32][33][34][35][36][37][38]. This specialization is expressed in the zonation and co-localization of several organ systems: ciliary bands, nerve cords, and muscles [39][40][41][42][43][44][45][46][47]. Such specialized tentacles are present in the lophophorates [48][49][50][51]. ...
The Oweniidae are marine annelids with many unusual features of organ system, development, morphology, and ultrastructure. Together with magelionds, 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) and of the morphology of the last common bilaterian ancestor. In the current research, scanning electron microscopy revealed that the tentacle apparatus consists of 10 branched arms. The tentacles are covered by monociliary cells that form a ciliar groove that extends along the oral side of the arm base. Light, confocal, and transmission electron microscopy revealed that head region contains two circular intraepidermal nerves (outer and inner) that give rise to the neurites of each tentacle, i.e., intertentacular neurites are absent. Each tentacle contains a coelomic cavity with a network of blood capillaries. Monociliar myoepithelial cells of the tentacle coelomic cavity form both the longitudinal and the circular muscles. The structure of this myoepithelium is intermediate between simple and pseudo-stratified myepithelium. 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, is probably ancestral for annelids and for all Bilateria. The outer circular nerve of O. borealis is a dorsal medullary commissure that apparently functions as an anterior nerve center and is organized at the ultrastructural level as a stratified neuroepithelium. Given the hypothesis that the anterior nerve center of the last bilateral ancestor might be a diffuse neural plexus network, these results suggest that the ultra anatomy of that plexus brain might be a stratified neuroepithelium. Alternatively, the results could reflect the simplification of structure of the anterior nerve center in some bilaterian lineages.
... The phylum Brachiopoda includes three subphyla: Linguliformea, Craniiformea, and Rhynchonelliformea [21]. Organization of the lophophore nervous system has been studied in specimens from all three groups, including the following species: N. anomala [20,22], Discinisca lamellosa [23], L. anatina [4], Gryphus vitreus [24], and H. psittacea [19]. Innervation of the lophophore in brachiopods has mostly been studied via light microscopy [22][23][24], and only four species have been studied with TEM, immunocytochemistry, and CLSM [4,19,20,25]. ...
... Organization of the lophophore nervous system has been studied in specimens from all three groups, including the following species: N. anomala [20,22], Discinisca lamellosa [23], L. anatina [4], Gryphus vitreus [24], and H. psittacea [19]. Innervation of the lophophore in brachiopods has mostly been studied via light microscopy [22][23][24], and only four species have been studied with TEM, immunocytochemistry, and CLSM [4,19,20,25]. According to all data, the central nervous system of brachiopods includes two ganglia, the subenteric and the supraenteric, which are located under and above the mouth, respectively. ...
... The main brachial nerve is connected to the accessory brachial nerve via numerous cross nerves, which extend into the connective tissue of the lophophore arms. These three brachial nerves, i.e., the main, accessory, and lower, are the major nerves of the lophophore in all brachiopods studied to date [4,[19][20][22][23][24]. Two recent studies of lophophore innervation have revealed the presence of a second accessory nerve in the rhynchonelliform H. psittacea [19] and in the craniiform N. anomala [20]. ...
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 lophophore has been studied 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 and 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 immunoreactivity.
This result supports the following previously proposed hypothesis about the evolution of the lophophore in brachiopods: the morphology of the lophophore has evolved from simple to complex, whereas the innervation of the lophophore has evolved from complex to simple; the latter is indicated by a smaller number of lophophoral nerve tracts in species with complex lophophores. The reduction of the accessory brachial nerve and diminution of the main brachial nerve are associated with general reduction of the prosoma in brachiopods.
