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Mastophorus muris (Gmelin, 1790) is a globally distributed parasitic nematode of broad range mammals. The taxonomy within the genus Mastophorus and the cryptic diversity among the genus are controversial among taxonomists. This study provides a detailed morphological description of M. muris from Mus musculus combined with a molecular phylogenetic approach. Moreover, descriptions and molecular data of M. muris from non-Mus rodents and wildcats complement our findings and together provide new insights into their taxonomy. The analysis of M. muris was based on light microscopy and scanning electron microscopy. The morphological description focused on the dentition pattern of the two trilobed pseudolabia. Additionally, we described the position of the vulva, arrangement of caudal pairs of papillae, spicules and measured specimens from both sexes and the eggs. For the molecular phylogenetic approach, we amplified the small subunit ribosomal RNA gene and the internal transcribed spacer, and the cytochrome c oxidase subunit 1. Mastophorus morphotypes based on dentition patterns and phylogenetic clustering indicate a subdivision of the genus in agreement with their host. We recognize two groups without a change to formal taxonomy: One group including those specimens infecting Mus musculus, and the second group including organisms infecting non-Mus rodents. Our genetic and morphological data shed light into the cryptic diversity within the genus Mastopohorus. We identified two host-associated groups of M. muris. The described morphotypes and genotypes of M. muris allow a consistent distinction between host-associated parasites. [ABSTRACT FROM AUTHOR]

The planktonic Oikopleura dioica belongs to Tunicata, the probable sister taxon to Craniota, and might show plesiomorphic characters, conserved from the common lineage of Tunicata and Craniota. In O. dioica a pericardium in a position similar to other chordates but also to the heart and pericardium of craniates is found. Surprisingly, little is known about the ultrastructure of the pericardium in O. dioica. Here, we show based on electron microscopy that the pericardium is completely lined by a single layer of 16 epithelial cells: 6 epithelial myocardial cells on the left side of the pericardium and 10 peritoneal cells constituting the right side. One of the peritoneal cells, situated at the ventral border between peritoneal cells and myocardial cells has an extension that anchors the pericardium to the basal lamina beneath the latero-ventral epidermis. The primary body cavity of O. dioica appears quite uniformly clear in electron microscopic aspect but several sheets, resembling the basal lamina of the pericardium cross the larger spaces of the body cavity and connect to the pericardial basal lamina. This is the first detailed description of two distinct cell types in the epithelial lining of the pericardium of O. dioica. In comparison with other chordates, we conclude that two cell types can be reconstructed for the last common ancestor of Chordata at least. The position of the pericardium at the intersection of trunk and tail in combination with the basal-lamina like sheets spanning the hemocoel is probably of importance for the function of the circulation of the hemocoelic fluid. Similar to the tail, the axis of the pericardium is shifted through 90 degrees to the left as compared to the main body axis of the trunk and we infer that this shift is an apomorphic character of Appendicularia. [ABSTRACT FROM AUTHOR]

Purpose: A ventral heart positioned posterior to the branchial basket and equipped with a pericardium is homologous in tunicates and their sister group, the craniates, yet the tunicate model organism Ciona intestinalis features a pericardial body, a structure peculiar to few ascidian species. Here, we set out to distinguish between two competing hypotheses regarding the function of the pericardial body found in the literature: (H1) The pericardial body performs a role in the removal of dysfunctional myocardial cells, and (H2) it is a specialized niche of the immune system involved in defense against parasites. Methods: We used histological techniques, transmission electron microscopy, and PCR-based gene sequencing to investigate whether individual ascidians parasitized with apicomplexan protists show signs of infections within the pericardial body. Results: In individuals of C. intestinalis from the German North Sea infested with apicomplexan protists, the pericardial body contains numerous myocardial cells in various stages of degeneration while no remnants of parasitic cells could be identified. Conclusion: Thus, we conclude that H2—the pericardial body is a specialized niche of the immune system involved in defense against parasites—can be refuted. Rather, our observations support H1, the hypothesis that the pericardial body performs a role in the removal of dysfunctional myocardial cells. [ABSTRACT FROM AUTHOR]

Development in the colonial ascidian genus Diplosoma is unusual in producing two zooids already during the free-swimming larval period. The development of D. listerianum had been investigated in two studies. The present study reports embryonic development of a second species D. migrans. In D. migrans the pharynx of the oozooid develops from two separate primordia. The intestinal tract develops as well from two distinct primordia that fuse and subsequently connect to the prospective pharynx. The pharynx of the blastozooid develops from the paired epicardia of the oozooid. In the oozooid the primordia of the peribranchial sacs often derive from the endodermal wall of the prospective pharynx, but occasionally develop from thickenings of the ectodermal epidermis. No atrial siphon is formed; instead, the body wall retracts and the gill slits open directly to the exterior. In the oozooid the rectum develops from the initial prospective intestine, while the remaining intestine develops from a diverticulum of the stomach. In the blastozooid the main part of the intestinal tract develops from the initial prospective intestine, whereas the rectum develops as a diverticulum of the initial intestine. Due to this developmental mode, the intestinal tracts of oozooid and blastozooid are transitorily connected at the respective border of rectum and proximal part of posterior intestinal tract. The paired epicardia of the two zooids derive from the epicardial outgrowths of the pharyngeal primordia of the oozooid that are also the primordia of the heart and pericardia. Budding in the larva and the adult is highly similar, in particular concerning the development of a new thorax and the heart from paired epicardial Anlagen and the origin of the distal part of the posterior intestine. Discrepancies in the descriptions of the embryonic development of D. listerianum and differences to the development in D. migrans are discussed. The development of the pericardium-heart-complex from epithelially organized tissue constitutes the plesiomorphic condition within Didemnidae and Aplousobranchiata. The development from mesenchymatic cell masses is a secondarily derived simplification. [ABSTRACT FROM AUTHOR]

Thaliacea are marine planktonic animals within the taxon Tunicata. The species-poor taxon is characterized by diverse life cycles, with sexually (blastozooid) and asexually (oozooid) reproducing generations, that usually evolved polymorph phenotypes. While recent molecular phylogenetic studies indicate that tunicates might be closest living relatives to free-living craniates, additional insights into the evolution of the nervous systems of planktonic tunicates are overdue. To test the hypothesis that polymorphism in the different stages correlates with different distribution patterns of neurotransmitters, we comparatively conducted immunostaining experiments utilizing antibodies against serotonin and tyrosinated-α-tubulin, and confocal laser scanning microscopy in planktonic Thaliacea. We considerably increase the available data on serotonin-like immunoreactive (serotonin-lir) in Thaliacea and found that the pattern of distribution of serotonin-lir cells clearly differs between oozooid and blastozooid stages. Thereby, we reveal that the distribution of serotonin-lir cells in different tissues of thaliaceans can be considered as independent characters. Further, we test the potential phylogenetic signal present in serotonin-lir in thaliaceans, propose a phylogenetic mapping of conceptualized characters, and analyze the evolution of serotonin-lir cells. Comparing our results with data from previous studies indicated that it was necessary to reevaluate already described distribution patterns of serotonin-lir. Due to the complex life cycles of Salpida and Doliolida, erroneous descriptions of the localization of serotonin-lir occur in thaliacean literature. To facilitate the evaluation of data on serotonin-lir in thaliaceans for future readers, we suggest depicting whole-mount stainings of complete animals. [ABSTRACT FROM AUTHOR]

As one of the three major chordate taxa, the highly diverse taxon, Tunicata has always played a key role in considerations of evolutionary origins of vertebrates, especially since several larger-scaled molecular phylogenetic analyses support the hypothesis that tunicates are the sister group to vertebrates. Molecular phylogenetic studies of the relationships within Tunicata are highly contradictory, and cladistic analyses of morphological characters for Tunicata remained limited. In order to come to a better understanding of chordate evolution, we comparatively investigated nine different tunicate species belonging to five different higher tunicate taxa: Phlebobranchiata, Aplousobranchiata, Stolidobranchiata, Thaliacea, and Appendicularia using immunofluorescence labeling with antibodies against serotonin in conjunction with confocal laser scanning microscopy. We found that adult ascidians are comparable in regard to their respective patterns of serotonin-like immunoreactive (serotonin-lir) cells, whereas the planktonic tunicates differ in several aspects. The distribution patterns of serotonin-lir cells in tunicates suggest that serotonin-like immunoreactivity can behave as an independent character during evolution. While the distribution pattern of serotonin-lir cells agrees well with classical taxonomic groupings, phylogenetic interpretation remains inconclusive, as ad hoc hypotheses are always necessary to explain contradictory character distribution. Based on light-microscopically observed morphology, we could distinguish three different types of serotonin-lir cells, most probably functionally distinctive. These were approximately spherical serotonin-lir cells, possibly involved in the control of ciliary beating and mucus secretion, elongated serotonin-lir cells potentially involved in hormonal regulation of feeding, and serotonin-lir neurons that might be implicated in the initiation of locomotory behavior. [ABSTRACT FROM AUTHOR]

Concerning the evolution of deuterostomes, enteropneusts (acorn worms) occupy a pivotal role as they share some characteristics with chordates (e.g., tunicates and vertebrates) but are also closely related to echinoderms (e.g., sea urchin). The nervous system in particular can be a highly informative organ system for evolutionary inferences, and advances in fluorescent microscopy have revealed overwhelming data sets on neurogenesis in various clades. However, immunocytochemical descriptions of neurogenesis of juvenile enteropneusts are particularly scarce, impeding the reconstruction of nervous system evolution in this group. We followed morphogenesis of the nervous system in two enteropneust species, one with direct ( Saccoglossus kowalevskii) and the other with indirect development ( Balanoglossus misakiensis), using an antibody against serotonin and electron microscopy. We found that all serotonin-like immunoreactive (LIR) neurons in both species are bipolar ciliary neurons that are intercalated between other epidermal cells. Unlike the tornaria larva of B. misakiensis, the embryonic nervous system of S. kowalevskii lacks serotonin-LIR neurons in the apical region as well as an opisthotroch neurite ring. Comparative analysis of both species shows that the projections of the serotonin-LIR somata initially form a basiepidermal plexus throughout the body that disappears within the trunk region soon after settlement before the concentrated dorsal and ventral neurite bundles emerge. Our data reveal a highly conserved mode of neurogenesis in enteropneusts that is independent of the developing mode and is inferred to be a common feature for Enteropneusta. Moreover, all detected serotonin-LIR neurons are presumably receptor cells, and the absence of serotonin-LIR interneurons from the enteropneust nervous system, which are otherwise common in various bilaterian central nervous systems, is interpreted as a loss that might have occurred already in the last common ancestor of Ambulacraria. [ABSTRACT FROM AUTHOR]

Enteropneusts or acorn worms are marine deuterostomes that have retained many plesiomorphic characters. Thus, enteropneusts are of prime interest in evolutionary comparisons between deuterostomes and protostomes. In the present study, the larval eyes of Glossobalanus marginatus were reconstructed and described based on serial sectioning for transmission electron microscopy. The everse eyes of the late Metschnikoff/early Krohn-stage tornaria larvae of G. marginatus are epidermal structures consisting of two rows of in total 13 shading pigment cells and another two rows of 13 photoreceptor cells. The pigment cells form a shallow cup with a relatively wide opening, making the cup-shaped eye optically unsuitable for picture generation. We demonstrate that the photosensitive cells possess numerous enlarged microvilli and an unmodified apical cilium. Our ultrastructural studies thus corroborate the photoreceptor cells in the eye of G. marginatus to be of a clearly rhabdomeric type. Preliminary immunohistochemical experiments support those findings by demonstrating immunopositive reaction of the tornarian eye photoreceptors with an antibody designed against rhabdomeric sea urchin photopigment (Sp-Opsin4). Observations of living animals indicate that Late Metschnikoff/early Krohn-stage tornaria larvae are negatively phototactic, probably concordant with imminent metamorphosis. [ABSTRACT FROM AUTHOR]

Cephalodiscus gracilis Harmer, 1905 is a semi-sessile deuterostome that shares with fish-like chordates pharyngeal gill slits and a dorsally situated brain. In order to reveal structures potentially homologous among deuterostomes and to infer their functional roles, we investigated the axial complex, associated blood vessels and structures of C. gracilis using transmission electron microscopy, light microscopy, and digital 3D reconstructions. We describe the smooth, bipartite cephalic shield retractor muscles that originate as solid compact muscles and fan out to traverse the protocoel as individual muscle cells. The axial complex consists of a cap-shaped coelomic sac, the pericardium that surrounds the central heart. The pericardium is constituted of myoepithelial cells, with the cells facing the heart being thicker and richer in myofilaments. A prominent dorsal median blood vessel opens into the heart, which gives rise to a short median ventral vessel that opens into the paired glomeruli connected to the ventral side of the stomochord. The tip of the curved stomochord rests precisely above the connection of the dorsal median vessel with the heart, a position that would allow the stomochord to function as a valve facilitating unidirectional blood flow. Glomeruli are lined by podocytes of the spacious protocoel and are considered to be the site of ultrafiltration. Two pairs of blood vessels enter the median dorsal blood vessel from the tentacles. The median dorsal blood vessel is separated from the brain by a thin basement membrane. This arrangement is consistent with the hypothesis that blood vessels in the tentacles increase oxygen supply for the brain. Based on detailed similarities, the heart-glomerulus complex of C. gracilis is considered homologous with the heart-glomerulus complex in Rhabdopleura spp., and Enteropneusta, and the axial complex in Echinodermata. In addition, we hypothesize homology to the excretory complex including Hatschek's nephridium in Cephalochordata. Thus, the heart-glomerulus complex does not support a sister-group relationship between Echinodermata and Hemichordata, whereas the organization of the cephalic shield retractor muscles is consistent with the evolution of pterobranchs within enteropneusts. [ABSTRACT FROM AUTHOR]

Pterobranchs have been interpreted as 'missing links' combining primitive invertebrate features with advanced vertebrate-like characteristics. The first detailed morphological description of an ontogenetic stage of a pterobranch, based on digital 3D-reconstruction at electron microscopic resolution, reveals a triploblastic animal with monociliated epithelia, an extensive coelomic cavity, a through gut with an asymmetrically developed gill slit but no signs of planktonic specializations, such as ciliated bands. Therefore, this crawling larva supports the hypothesis proposed in previous molecular phylogenetic studies that pterobranchs could be derived within enteropneusts rather than being 'missing links'. [ABSTRACT FROM AUTHOR]

Nervous systems are important in assessing interphyletic phylogenies because they are conservative and complex. Regarding nervous system evolution within deuterostomes, two contrasting hypotheses are currently discussed. One that argues in favor of a concentrated, structured, central nervous system in the last common ancestor of deuterostomes (LCAD); the other reconstructing a decentralized nerve net as the nervous system of the LCAD. Here, we present a morphological analysis of the nervous system of the pterobranch deuterostome Cephalodiscus gracilis Harmer, 1905 based on transmission electron microscopy, confocal laser scanning microscopy, immunohistochemistry, and computer-assisted 3D reconstructions based on complete serial histological sections. The entire nervous system constitutes a basiepidermal plexus. The prominent dorsal brain at the base of the mesosomal tentacles contains an anterior concentration of serotonergic neurons and a posterior net of neurites. Predominant neurite directions differ between brain regions and synapses are present, indicating that the brain constitutes a centralized portion of the nervous system. Main structures of the peripheral nervous system are the paired branchial nerves, tentacle nerves, and the ventral stalk nerve. Serotonergic neurites are scattered throughout the epidermis and are present as concentrations along the anterior border of the branchial nerves. Serotonergic neurons line each tentacle and project into the brain. We argue that the presence of a centralized brain in C. gracilis supports the hypothesis that a nerve center was present in the LCAD. Moreover, based on positional and structural similarity, we suggest that the branchial nerves in C. gracilis could be homologous to branchial nerves in craniates, a hypothesis that should be further investigated. [ABSTRACT FROM AUTHOR]

The origin of mesoderm and coelomic compartments has traditionally been given high value for phylogenetic considerations of animal relationships. Two main modes have been distinguished, associated with the two main groups of animals: schizocoely with protostomes and enterocoely with deuterostomes. During enterocoely, coelomic compartments are formed from the endoderm. Here, we show that the pericardium of the deuterostome Saccoglossus kowalevskii, an enteropneust, is ontogenetically derived from the ectoderm and develops by schizocoely. The pericardium develops from a solid cluster of epidermis cells situated underneath the ectodermal nerve net above the basement membrane of the epidermis. The undifferentiated cells are interconnected by spot desmosomes, become separated from the epidermis and develop a central cavity. Pericardial cells become epithelial, by developing apical adherens junctions, a single apical cilium and basal striated myofibres. The differentiated pericardium possesses a cavity and surrounds a central blood vessel, the heart, situated in the basal extracellular matrix. The pericardium is an integral part of the anterior excretory complex, and comparisons to other deuterostomes indicate that pericardia are homologous despite differing ontogenies. Original data generated for the present study are deposited on MorphDBase (). [ABSTRACT FROM AUTHOR]

Ophiurid basket stars belonging to the family Gorgonocephalidae are distributed from the Arctic to the Antarctic and from the shallow subtidal to the deep sea, but their biology remains poorly known. In situ observations at the mouth of the Oslofjord by a remotely operated vehicle showed that Gorgonocephalus caputmedusae had a patchy distribution at 85 to 120 m water depth and frequently occurred in association with the gorgonian Paramuricea placomus and the coral Lophelia pertusa. Morphological and histological studies show that G. caputmedusae is well adapted to capture macroplanktonic prey. Histological examination of the arms revealed the presence of a thick layer of dermal mutable connective tissue which is probably an energy-efficient way to maintain its feeding posture against the current. This layer is connected to the nerve cord suggesting that the passive mechanical properties (stiffness) is controlled by the nervous system. In the distal parts of the arms, each segment has a pair of sticky tube feet and a sophisticated system of spines and hooks, which are connected to muscles and collagenous tendons. In combination, these features were shown, in an experimental flume study, to be used for capturing the locally abundant krill species Meganyctiphanes norvegica. This is the first documentation of G. caputmedusae of this kind. [ABSTRACT FROM AUTHOR]

Abstract: Using immunohistochemistry in combination with confocal laser scanning microscopy, the serotonergic nervous systems of major tunicate taxa were studied in three-dimensional detail. Organisms analyzed included aplousobranchiate, phlebobranchiate, and stolidobranchiate ascidian larvae, appendicularian juveniles and adults, and doliolid oozooids. Outgroup comparisons to notochordates showed that the serotonergic nervous system of the last common ancestor of Chordata consisted of two elements: (i) an anterior concentration of serotonergic cell bodies, and (ii) a fiber network that followed posteriorly and gave rise to fiber tracts that descended towards the effective somatic lateral musculature. Within Tunicata, the nervous systems of Appendicularia and Aplousobranchiata appear serotonin-reduced or negative. This could be interpreted as a heterochronic reduction and a synapomorphy between Appendicularia and Aplousobranchiata. In this hypothesis, free-living Appendicularia are derived within Tunicata, and a biphasic life cycle with a free-swimming larva and a sessile, ascidian-like adult is most parsimoniously reconstructed for the last common ancestor of Tunicata. The close spatial relation of the serotonergic cell cluster with the statocyte complex suggests a function as an integrative control center for the coordination of locomotion. A similar anterior concentration of serotonergic nerve cells is known from tornaria larvae. See also Electronic Supplement at: http://www.senckenberg.de/odes/05-02.htm [Copyright &y& Elsevier]