An electron microscopic study of early gonadogenesis in the hermaphroditic appendicularian Oikopleura gracilis (Tunicata,Oikopleuridae)

The postembryonic development of the gonad in the hermaphroditic appendicularian O. gracilis was studied using transmission electron microscopy. The primordial germ cells were detected first in 10-h-old larvae and represent migrating primordial germ syncytium (mPGS) localized in the hemocoel of the tail/trunk junction and several haemocoel areas of the digestive compartment. The mPGS consisted of primordial germ nuclei (PGN) 2 μm in diameter, and elongate somatic-line nuclei 1.8 μm in diameter. In 12.5-h-old juveniles the gonad primordium 40 × 90 μm in size, was separated by a narrow space of haemocoel between the gut and the epidermis of the reproductive compartment. The gonad primordium consisted of the central syncytial part of primordial germ nuclei (PGN), enclosing a single layer of somatic epithelium. In 3-day-old juveniles, the gonad was differentiated into testis and ovary. The testis, 400 × 550 μm in size, is a syncytium of spermatogonial nuclei, covered by a single layer of somatic epithelium. The ovaries, 350 × 850 μm in size, consist of a syncytium with nurse nuclei and meiotic nuclei. The hermaphroditic gonad originates from extragonadal mPGS. Early gonadogenesis in appendicularians has ultrastructural features in common with early gonadogenesis in ascidians.     

Ultrastructure of the Circulatory System of the Phoronid Phoronopsis harmeri Pixell, 1912. 2. Main vessels

The ultrastructure of the wall of the main blood vessels of the phoronid Phoronopsis harmeri is described. The walls of the lophophoral and left lateral vessels consist of myoepithelial cells of the coelomic lining (peritoneal cells), a thin basal lamina, and an incomplete endothelial lining. In the head region of the body, the wall of the medial vessel consists of myoepithelial cells of the coelomic lining (peritoneal cells), a basal lamina, and true muscular endothelial cells. The anterior part of the medial vessel functions as the heart. In the anterior part of the body, the medial vessel wall consists of five layers: the external nonmuscular coelothelium, a layer of the extracellular matrix, the internal muscular coelothelium, an internal layer of the extracellular matrix, and an incomplete endothelial lining. The complicated structure of the medial vessel wall may be explained by the superimposition of the lateral mesentery on the ordinary vessel wall.     

Comparative anatomy of the heart–glomerulus complex of Cephalodiscus gracilis (Pterobranchia): structure,function, and phylogenetic implications

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.     

Fine Structure of Heart,Pericardium and Glomerular Vessel in Cephalodiscus gracilis M'Intosh, 1882 (Pterobranchia,Hemichordata)

Heart, pericardium and glomerular vessel of Cephalodiscus gracilis have been studied with the electron microscope. The lumen of the heart is lined by a basal lamina and an associated epithelium, composed of myoepithelial cells with well developed thin and thick myofilaments. The heart is located in the pericardial cavity, which is deliminated by the pericardium. The latter is composed of two flat layers of myoepithelia with fused basal laminae. The outer layer of the pericardium is the protocoelomic lining, and the inner layer is the ‘parietal’ pericardial epithelium. The myoepithelium forming the heart wall can be considered to represent the ‘visceral’ pericardial epithelium. The spacious glomerular vessel is lined by a basal lamina, on which typical podocytes rest. These cells indicate that ultrafiltration takes place through the wall of the glomerular vessel. The lumen of the vessel contains fine granular material (presumably precipitated blood proteins), fibrils with a faint cross striation, suggesting that they represent collagen, and stellate cells, which in part line the vessel. Since ultrafiltration requires hydrostatic pressure, it is inferred that the blood flow is from the dorsal region then through the heart and into the glomerular vessel.     

The pericardium in the deuterostome <Emphasis Type="Italic">Saccoglossus kowalevskii</Emphasis> (Enteropneusta) develops from the ectoderm via schizocoely

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 ().     

Ultrastructural study of the heart and its relationship with the connective tissue sheath of the stomach in Rhynchonella psittacea (Tentaculata,Brachiopoda)

The fine structure of the heart and connective tissue sheath surrounding the stomach of the brachiopod Rhynchonella psittacea has been studied. The stomach wall is lined externally with peritoneal epithelium. Between the bases of the peritoneal epithelial cells and those of the stomach epithelial cells is an extracellular amorphous matrix. The exterior part of the matrix is occupied by smooth muscle cells and the interior part by fibroblasts. The heart wall shows continuity with the peritoneal epithelium covering the stomach wall and consists of three layers: an outer layer of smooth myoepithelial and epithelial cells, an intermediate thick layer of extracellular matrix, and an inner discontinuous layer of fibroblasts. In myoepithelial cells, nucleated heads protruding freely into the coelom and contractile parts embedded in the extracellular matrix can easily be distinguished. These cells contain no sarcoplasmic reticulum or any elements of a T system. The epithelial cells are non-muscular mononucleated cells scattered among the myoepithelial cells and closely associated with these basally. They possess a well-developed rough endoplasmic reticulum. In rare cases, a small amount of myofibrils occurs basally in the epithelial cells. Morphologically the epithelial cells in the myocardium are very similar to the peritoneal epithelial cells covering the stomach wall. Both epithelial and myoepithelial cells are ciliated. No nerve elements have been found in the brachiopod heart. The structure of the brachiopod heart is compared with that of other invertebrates; similarity of cellular composition of the brachiopod heart and stomach cover is considered evidence of origin of the heart cells from the cells of the connective tissue sheath of the stomach. The myogenic role of the peritoneal cells and epithelial cells of the myocardium is suggested. J. Morphol. 234:69–77, 1997. © 1997 Wiley-Liss, Inc.     

Ultrastructure of somatic tissues in the ovaries of a chaetognath (Ferosagitta hispida)

Transmission electron microscopy reveals that the ovaries of Ferosagitta hispida contain four somatic tissues. A myoepithelial ovary wall, continuous with a thin layer of peritoneocytes lining the coelomic cavity, encloses a fluid-filled ovarian space in which oocytes develop. Lamellar extensions of a “follicular reticulum” branch throughout the ovarian space and ensheath developing oocytes. This tissue has been overlooked in most previous studies of chaetognath ovaries. A bipartite oviductal complex extends the length of each ovary just within the lateral ovary wall. It consists of a flattened, blindly ending cellular tube, herein referred to as the cellular sheath, and an enclosed syncytium. Sheath cells secrete an electron-dense product into the ovarian space. Those sheath cells directly bordering the syncytium are contractile and are joined to the to the syncytium by gap junctions and microvillar interdigitations. The syncytium contains a complex of membrane-bounded lumina. The latter sometimes enclose sperm received during mating or ovulated eggs. Thus the syncytium serves both as a seminal receptacle and as a duct for passage of eggs to the outside. Contrary to several classical reports, the cellular sheath and syncytium of the oviductal complex do not separate at ovulation to form a temporary oviductal lumen.     

Adaptive morphology of the heart of Southern‐Fur‐Seal (Arctocephalus australis – Zimmermamm, 1783)

The Southern‐fur‐seal belongs to the order Carnivora, suborder Pinnipedia, and Otariidae family. This species inhabits aquatic and terrestrial environments, thus presenting important morphophysiological adaptive changes, especially in the cardiac system. For this purpose, Southern‐fur‐seal (Arctocephalus australis) hearts were used from animals that died from natural causes. Gross morphology observations were supported by light, scanning and transmission electron microscopy. The heart was long and flat; it was lined by pericardium and partly covered by lungs. Structurally, atrium and ventricle muscle fibers exhibit typical features of cardiac fibers revealing myofibrils bundles, mitochondria, plate‐shaped junctions, anastomosis between myofibrils bundles, and electron‐dense granule natriuretic around the nucleus and mitochondria of atrium muscle cells. The Southern‐fur‐seal heart was structurally similar to other mammals; however, it presented morphological changes that assist in their adaptation to their environment.     

Ultrastructure of the coelom in the brachiopod Lingula anatina

The organization of the coelomic system and the ultrastructure of the coelomic lining are used in phylogenetic analysis to establish the relationships between major taxa. Investigation of the anatomy and ultrastructure of the coelomic system in brachiopods, which are poorly studied, can provide answers to fundamental questions about the evolution of the coelom in coelomic bilaterians. In the current study, the organization of the coelom of the lophophore in the brachiopod Lingula anatina was investigated using semithin sectioning, 3D reconstruction, and transmission electron microscopy. The lophophore of L. anatina contains two main compartments: the preoral coelom and the lophophoral coelom. The lining of the preoral coelom consists of ciliated cells. The lophophoral coelom is subdivided into paired coelomic sacs: the large and small sinuses (= canals). The lining of the lophophoral coelom varies in structure and includes monociliate myoepithelium, alternating epithelial and myoepithelial cells, specialized peritoneum and muscle cells, and podocyte‐like cells. Connections between cells of the coelomic lining are provided by adherens junctions, tight‐like junctions, septate junctions, adhesive junctions, and direct cytoplasmic bridges. The structure of the coelomic lining varies greatly in both of the main stems of the Bilateria, that is, in the Protostomia and Deuterostomia. Because of this great variety, the structure of the coelomic lining cannot by itself be used in phylogenetic analysis. At the same time, the ciliated myoepithelium can be considered as the ancestral type of coelomic lining. The many different kinds of junctions between cells of the coelomic lining may help coordinate the functioning of epithelial cells and muscle cells.     

Development of the deep‐sea viviparous quill worm Leptoecia vivipara (Hyalinoeciinae,Onuphidae, Annelida)

Development of Leptoecia vivipara, a brooding deep‐sea onuphid polychaete with a circum‐Antarctic distribution, was studied using light microscopy, scanning and transmission electron microscopy, and histology. All specimens examined were brooding viviparous females or juveniles; no male gametes were detected. Anterior segments of juvenile and adult worms bore paired compact ovaries with clusters of vitellogenic oocytes. In adults, the mid‐body region formed a chamber containing up to 12 offspring at different stages of development, from oocyte to 13 chaetigers. Mature oocytes freely floated in the coelomic fluid, while embryos and juveniles were enclosed in peritoneal envelopes. Chaetal replacement in juveniles and the morphology of the provisional maxillae are described. Leptoecia vivipara is argued to be a progenetic species with juvenile‐like external morphology and accelerated sexual maturation. These traits may have arisen as adaptations to epibenthic life in a high‐latitude deep‐sea environment affected by seasonal pulses of organic matter.     

Ultrastructure of coelomic lining in echinoderm podia: significance for concepts in the evolution of muscle and peritoneal cells

Summary Ultrastructural data are presented on the histological organization of coelomic lining in the podia of ten species of the five major groups of extant echinoderms. Further evidence of the incorporation of podial retractor muscle cells (myocytes) into a monociliated myoepithelial coelomic lining is provided. In the podia of the crinoid Nemaster rubinginosa and the ophiuroid Ophiophragmus wurdemani as well as in the feeding tentacles of the holothurian Leptosynapta tenuis, coelomic linings are organized as simple myoepithelia consisting of non-contractile peritoneal cells (peritoneocytes) and myocytes. Coelomic linings in the holothurian Thyonella gemmata, the echinoids Eucidaris cf. tribuloides and Lytechinus variegatus, and the asteroids Asterias forbesi and Astropecten sp. are pseudostratified or bipartite pseudostratified myoepithelia consisting of subapical myocytes and apically situated peritoneocytes. The ophiuroid podia of Ophioderma brevispinum and Ophiothrix angulata exhibit transitions from simple myoepithelia to partially pseudostratified epithelia. Intermediate forms between the extremes in myoepithelial organization also occur in the podial lining of single species (e.g. Eucidaris cf. tribuloides). These data supplement recent ultrastructural studies on the podial lining of echinoderms and, in conjunction with published ultrastructural data on the myoepithelial organization of other coelomic linings in echinoderms and in other coelomates, suggest myoepithelial organization of the coelomic lining is a plesiomorph feature in Bilateria.     

The origin of the endothelial cells: an evo-devo approach for the invertebrate/vertebrate transition of the circulatory system

Circulatory systems of vertebrate and invertebrate metazoans are very different. Large vessels of invertebrates are constituted of spaces and lacunae located between the basement membranes of endodermal and mesodermal epithelia, and they lack an endothelial lining. Myoepithelial differentation of the coelomic cells covering hemal spaces is a frequent event, and myoepithelial cells often form microvessels in some large invertebrates. There is no phylogenetic theory about the origin of the endothelial cells in vertebrates. We herein propose that endothelial cells originated from a type of specialized blood cells, called amoebocytes, that adhere to the vascular basement membrane. The transition between amoebocytes and endothelium involved the acquisition of an epithelial phenotype. We suggest that immunological cooperation was the earliest function of these protoendothelial cells. Furthermore, their ability to transiently recover the migratory, invasive phenotype of amoebocytes (i.e., the angiogenic phenotype) allowed for vascular growth from the original visceral areas to the well-developed somatic areas of vertebrates (especially the tail, head, and neural tube). We also hypothesize that pericytes and smooth muscle cells derived from myoepithelial cells detached from the coelomic lining. As the origin of blood cells in invertebrates is probably coelomic, our hypothesis relates the origin of all the elements of the circulatory system with the coelomic wall. We have collected from the literature a number of comparative and developmental data supporting our hypothesis, for example the localization of the vascular endothelial growth factor receptor-2 ortholog in hemocytes of Drosophila or the fact that circulating progenitors can differentiate into endothelial cells even in adult vertebrates.     

Structure of the body cavity of the sea spider <Emphasis Type="Italic">Nymphon brevirostre</Emphasis> Hodge, 1863 (Arthropoda: Pycnogonida)

The microscopic anatomy and ultrastructure of the body cavity and adjacent organs in the sea spider Nymphon brevirostre Hodge, 1863 (Pycnogonida, Nymphonidae) were examined by transmission electron microscopy. The longitudinal septa subdividing the body cavity are described: (1) Dohrn’s horizontal septum, (2) lateral heart walls, and (3) paired ventral septa consisting of separate cellular bands. The body cavity is a hemocoel, it has no epithelial lining and is only bordered by a basal lamina. The epidermis, heart, and Dohrn’s septum are not separated from each other by basal laminae and may have a common origin. The cellular bands forming the longitudinal ventral septa are not covered with the basal lamina and presumably derive from cells belonging to the hemocoel. The roles of the morphological structures studied for the circulation of hemolymph are discussed. The gonad lies inside Dohrn’s septum, it is covered with its own basal lamina and surrounded by numerous lacunae of the hemocoel entering the septum. The gonad wall is formed with a single layer of epithelium. The same epithelial cells form the gonad stroma. The gonad cavity is not lined with the basal lamina; muscle cells are present in the gonad wall epithelium, thus rendering the lumen similar to a coelomic cavity. Freely circulating cells of two types are found in the hemocoel: small amebocytes containing electronic-dense granules that are similar to granulocytes of other arthropods, as well as hemocytes with large vacuoles of varying structure that are comparable with plasmatocytes; however some of these may be activated granulocytes.     

Ultrastructural studies of the hearts in Arenicola marina L. (Annelida: Polychaeta)

Summary The two hearts in Arenicola are capable of great dilation and contraction. The heart wall consists of myoepithelial cells resting on a basal lamina. On the luminal side of the basal lamina is a layer of collagen fibrils. No true endothelium was observed but occasional haemocytes were observed, subjacent to the collagenous layer. A few chloragogen cells are also found peripherally.The myofibrils are of a non-striated type consisting of thick and thin filaments and scattered Z-bodies. The sarcoplasmic reticulum forms a three-dimensional network. Only peripheral couplings were observed. The myofibrils are in contact with the sarcolemma on the luminal side of the cells, constituting a kind of hemidesmosome. The myoepithelial heart muscle is compared with other muscle types described in invertebrates. Supercontraction is discussed.     

Induction of the avian coelom with associated vitelline blood circulation by Rauber's sickle derived junctional endoblast and its fundamental role in heart formation

In histological sections through chicken blastoderms of different ages we describe the temporospatial relationship between junctional endoblast, the formation of blood islands (appearing first from a peripherally migrating mesoblastic blastema), and the formation of coelomic vesicles developing later in/and from a more superficially extending mesoblastic blastema (coelomic mesoblast). After unilateral removal of the Rauber's sickle-derived junctional endoblast in early streak blastoderms (stage 2-4; Vakaet [1970] Arch Biol 81:387-426) and culture to stage 11 (Hamburger and Hamilton [1951] J Morphol 88:49-92), we observed that the early formation of the coelomic cavity was locally or totally disturbed in the operated area. Besides the simultaneous absence of blood islands, the coelomic vesicles did not form normally. Instead of regularly aligned coelomic vesicles, progressively forming the coelomic cavity by fusion, some voluminous irregular cavities appeared. Thus, the extent of the coelomic cavity was greatly reduced and the operated side was considerably smaller than the unoperated side. Furthermore, in the youngest operated blastoderms the cranial portion of the involved coelomic cavity (hemipericardial cavity) exhibited rudimentary development and usually did not reach the region of the foregut endoderm. This resulted in the absence of the myoepicardium and associated endocardium at this side. In another experiment, after removal of the junctional endoblast at one side of the chicken blastoderm, a fragment of quail junctional endoblast was placed isotopically. This resulted, after further in vitro culture, in the restoration of the formation of coelomic vesicles and accompanying subjacent blood islands in the immediate neighborhood of the apposed quail junctional endoblast. Also, the pericardium and primary heart tube developed normally. Similarly, by using the quail-chicken chimera technique, we demonstrated that the splanchnic mesoderm cells of the pericardium develop in intimate association with the most cranial part of the junctional endoblast (derived from the Rauber's sickle horns). Our experiments indicate that the coelom and, in particular, the pericardium and primary heart tube form progressively (in time and space) under the inductory influence of Rauber's sickle and junctional endoblast.     

Fine structure of the epistome in Phoronis ovalis: significance for the coelomic organization in Phoronida

Abstract. The hypothesis of a common ancestry of the lophophorate taxa Brachiopoda, Bryozoa, Phoronida, and the Deuterostomia can be traced back to the late 19th century when Masterman recognized a tripartite organization of the body consisting of pro-, meso-, and metasome, along with coelomic body cavities in each compartment, as characteristic for Echinodermata, Pterobranchia, Phoronida, and Brachiopoda. This idea became quite popular under the name "archicoelomate" concept. The organization of the phoronids, and especially of their transparent actinotroch larva, has for a long time been used as a touchstone for the validity of this concept. As a coelomic lining can reliably be recognized only on the ultrastructural level, this technique has been applied for adults of Phoronis ovalis , which is assumed to be a sister species to all other phoronids. Phoronis ovalis contains only two coelomic compartments, a posterior coelom inside the trunk (metasoma), occupying the space between the trunk epidermis and the digestive epithelium, and an anterior lophophoral coelom inside and basal to the tentacular crown (mesosoma). There is no coelomic cavity inside the epistome (prosoma). This part of the body is filled with myoepithelial cells, which are continuous with the epithelial lining of the lophophore cavity. These cells form a lumenless bilayer and possess long, tiny myofilamentous processes, which are completely embedded in an extracellular matrix. A comparison with data on P. muelleri shows that there is no need to assume three different coelomic cavities in Phoronida, in contrast to the predictions of the archicoelomate concept. At least for this taxon, a correspondence to the situation in deuterostomes can hardly be found.     

Ultrastructure of the body cavities in Phylactolaemata (Bryozoa)

Only species belonging to the bryozoan subtaxon Phylactolaemata possess an epistome. To test whether there is a specific coelomic cavity inside the epistome, Fredericella sultana, Plumatella emarginata, and Lophopus crystallinus were studied on the ultrastructural level. In F. sultana and P. emarginata, the epistome contains a coelomic cavity. The cavity is confluent with the trunk coelom and lined by peritoneal and myoepithelial cells. The lophophore coelom extends into the tentacles and is connected to the trunk coelom by two weakly ciliated coelomic ducts on either side of the rectum. The lophophore coelom passes the epistome coelom on its anterior side. This region has traditionally been called the forked canal and hypothesized to represent the site of excretion. L. crystallinus lacks an epistome. It has a simple ciliated field where an epistome is situated in the other species. Underneath this field, the forked canal is situated. Compared with the other species, it is pronounced and exhibits a dense ciliation. Despite the occurrence of podocytes, which are prerequisites for a selected fluid transfer, there is no indication for an excretory function of the forked canal, especially as no excretory porus was found. J. Morphol. 2009. © 2008 Wiley‐Liss, Inc.     

Morphological analysis of the hagfish heart. II. The venous pole and the pericardium

The morphological characteristics of the venous pole and pericardium of the heart were examined in three hagfish species, Myxine glutinosa, Eptatretus stoutii, and Eptatretus cirrhatus. In these species, the atrioventricular (AV) canal is long, funnel‐shaped and contains small amounts of myocardium. The AV valve is formed by two pocket‐like leaflets that lack a papillary system. The atrial wall is formed by interconnected muscle trabeculae and a well‐defined collagenous system. The sinus venosus (SV) shows a collagenous wall and is connected to the left side of the atrium. An abrupt collagen‐muscle boundary marks the SV‐atrium transition. It is hypothesized that the SV is not homologous to that of other vertebrates which could have important implications for understanding heart evolution. In M. glutinosa and E. stoutii, the pericardium is a closed bag that hangs from the tissues dorsal to the heart and encloses both the heart and the ventral aorta. In contrast, the pericardium is continuous with the loose periaortic tissue in E. cirrhatus. In all three species, the pericardium ends at the level of the SV excluding most of the atrium from the pericardial cavity. In M. glutinosa and E. stoutii, connective bridges extend between the base of the aorta and the ventricular wall. In E. cirrhatus, the connections between the periaortic tissue and the ventricle may carry blood vessels that reach the ventricular base. A further difference specific to E. cirrhatus is that the adipose tissue associated with the pericardium contains thyroid follicles. J. Morphol. 277:853–865, 2016. © 2016 Wiley Periodicals, Inc.     

Restoring the endangered oyster mussel (Epioblasma capsaeformis) to the upper Clinch River,Virginia: an evaluation of population restoration techniques

From 2005 to 2011, the federally endangered freshwater mussel Epioblasma capsaeformis (oyster mussel) was reintroduced at three sites in the upper Clinch River, Virginia, using four release techniques. These release techniques were (1) translocation of adults (site 1, n = 1418), (2) release of laboratory‐propagated sub‐adults (site 1, n = 2851), (3) release of 8‐week‐old laboratory‐propagated juveniles (site 2, n = 9501), and (4) release of artificially infested host fishes (site 3, n = 1116 host fishes). These restoration efforts provided a unique research opportunity to compare the effectiveness of techniques used to reestablish populations of extirpated and declining species. We evaluated the relative success of these four population restoration approaches via monitoring at each release site (2011–2012) using systematic 0.25‐m² quadrat sampling to estimate abundance and post‐release survival. Abundances of translocated adult and laboratory‐propagated sub‐adult E. capsaeformis at site 1 ranged 577–645 and 1678–1700 individuals, respectively, signifying successful settlement and high post‐release survival. Two untagged individuals (29.1 and 27.3 mm) were observed, indicating that recruitment is occurring at site 1. No E. capsaeformis were found at sites where 8‐week‐old laboratory‐propagated juveniles (site 2) and artificially infested host fishes (site 3) were released. Our results indicate that translocations of adults and releases of laboratory‐propagated sub‐adults were the most effective population restoration techniques for E. capsaeformis. We recommend that restoration efforts focus on the release of larger (>20 mm) individuals to accelerate augmenting and reintroducing populations and increase the probability for recovery of imperiled mussels.     

Fine Structure of Tentacles,Arms and Associated Coelomic Structures of Cephalodiscus gracilis (Pterobranchia,Hemichordata)

The tentacles of the pterobranch Cephalodiscus, a hemisessile ciliary feeder, originate from the lateral aspects of the arms and are covered by an innervated epithelium, the majority of its cells bearing microvilli. Each side of a tentacle has two rows of ciliated cells and additional glandular cells. The coelomic spaces in the tentacles are lined by cross-striated myoepithelial cells, allowing rapid movements of the tentacles. One, possibly two, blood vessels accompany the coelomic canal. On their outer sides the arms are covered by a simple ciliated epithelium with intra-epithelial nerve fibres; the inner side is covered by vacuolar cells. On both sides different types of exocrine cells occur. The collar canals of the mesocoel are of complicated structure. Ventrally their epithelium is pseudostratified and ciliated; dorsally it is lower and forms a fold with specialized cross-striated myoepithelial cells of the coelomic lining. Arms, tentacles, associated coelomic spaces and the collar canal of the mesocoel are considered to be functionally interrelated. It is assumed that rapid regulation of the pore width is possible and even necessary when the tentacular apparatus is retracted, which presumably leads to an increase of hydrostatic pressure in the coelom.