The metatrochophore of a deep-sea hydrothermal vent vestimentiferan (Polychaeta: Siboglinidae)

Postembryonic development and male paedomorphosis in Osedax (Siboglinidae, Annelida)

Most species of the bone-devouring marine annelid, Osedax, display distinct sexual dimorphism with macroscopic sedentary females rooted in bones and free-living microscopic dwarf males. The paedomorphic male resembles the non-feeding metatrochophore larva in size, presence of eight pairs of chaetae, and a head ciliation potentially representing a residual prototroch. The male development may thus uniquely reiterate and validate the theoretical heterochrony process "progenesis", which suggests that an accelerated sexual maturation and early arrest of somatic growth can lead to a miniaturized and paedomorphic adult. In this study, we describe the postembryonic larval and juvenile organogenesis of Osedax japonicus to test for a potential synchronous arrest of somatic growth during male development. Five postembryonic stages could be distinguished, resembling day one to five in the larval development at 10°C: (0D) first cleavage of fertilized eggs (embryos undergo unequal spiral cleavage), (1D) pre-trochophore, with apical organ, (2D) early trochophore, + prototroch, brain, circumesophageal connectives and subesophageal commissure, (3D) trochophore, + telotroch, four ventral nerves, (4D) early metatrochophore, + protonephridia, dorsal and terminal sensory organs, (5D) metatrochophore, + two ventral paratrochs, mid-ventral nerve, posterior trunk commissure, two dorsal nerves; competent for metamorphosis. The larval development largely mirrors that of other lecithotrophic annelid larvae but does not show continuous chaetogenesis or full gut development. Additionally, O. japonicus larvae exhibit an unpaired, mid-dorsal, sensory organ. Female individuals shed their larval traits during metamorphosis and continue organogenesis (including circulatory system) and extensive growth for 2-3 weeks before developing oocytes. In contrast, males develop sperm within a day of metamorphosis and display a synchronous metamorphic arrest in neural and muscular development, retaining a large portion of larval features post metamorphosis. Our findings hereby substantiate male miniaturization in Osedax to be the outcome of an early and synchronous offset of somatic development, fitting the theoretical process "progenesis". This may be the first compelling morpho-developmental exemplification of a progenetic origin of a microscopic body plan. The presented morphological staging system will further serve as a framework for future examination of molecular patterns and pathways determining Osedax development.

Nuchal organs in the trochophore of Siboglinum fiordicum (Annelida, Siboglinidae)

Nuchal organs are epidermal sensory structures present in most annelids. Based on one of the interpretations, they serve in larval settlement. Siboglinids lack nuchal organs in adult and larval stages, however, larvae of some siboglinids inhabiting seeps and hydrothermal vents are capable of swimming up to 100 km away from their home hydrothermal field to colonize a new one. One question that remains is, what organ are siboglinid larvae using to search and locate suitable substrates? To determine if any nuchal organs are present in siboglinid larvae, we studied the head and sensory apparatus in successive larval stages in a frenulate, Siboglinum fiordicum (Webb, 1963), using transmission electron microscopy and immunocytochemistry. In the early trochophore stage, we found an unpaired dorsal organ lying proximal to the posterior prototroch. This organ consists of trochoblast- and "covering" cells. Trochoblasts exhibited serotonin-like immunoreactivity and likely correspond to ciliated supporting cells, where cilia and microvilli project into the olfactory chamber. The "covering" cells are characterized by the presence of large nuclei with numerous pores and thick processes that project into the olfactory chamber, forming the contacts with the trochoblast projections. We have shown for the first time the presence of a nuchal-like organ in annelids as early as the trochophore stage. The presence of this organ in siboglinid trochophores while they are still in the inside the female tube suggests that this structure might be associated with functions other than settlement, such as communication or initiation of the departure from her tube.

A crown-group cnidarian from the Ediacaran of Charnwood Forest, UK

Cnidarians are a disparate and ancient phylum, encompassing corals and jellyfish, and occupy both the pelagic and benthic realms. They have a rich fossil record from the Phanerozoic eon lending insight into the early history of the group but, although cnidarians diverged from other animals in the Precambrian period, their record from the Ediacaran period (635–542 million years ago) is controversial. Here, we describe a new fossil cnidarian—Auroralumina attenboroughii gen. et sp. nov.—from the Ediacaran of Charnwood Forest (557–562 million years ago) that shows two bifurcating polyps enclosed in a rigid, polyhedral, organic skeleton with evidence of simple, densely packed tentacles. Auroralumina displays a suite of characters allying it to early medusozoans but shows others more typical of Anthozoa. Phylogenetic analyses recover Auroralumina as a stem-group medusozoan and, therefore, the oldest crown-group cnidarian. Auroralumina demonstrates both the establishment of the crown group of an animal phylum and the fixation of its body plan tens of millions of years before the Cambrian diversification of animal life.

A new fossil cnidarian, Auroralumina attenboroughi, from the Ediacaran of Charnwood Forest, UK, described as showing mosaic anthozoan and medusozoan characters, is the oldest yet-known crown-group cnidarian.

Structure and Origin of the Vestimentiferan Trophosome (Annelida, Siboglinidae)

The anatomical and histological structure of the trophosome of the giant vestimentiferan Riftiapachyptila has been studied. The trophosome consists of longitudinally oriented cords. The cords of the trophosome intertwine, form diverticula and anastomose with each other. Each cord has an axial blood vessel inside, which is connected to afferent vessels on the surface of the cord by radial capillaries. Based on the data on the structure and development of the trophosome, it is suggested that the evolutionary precursor of the trophosome was a blood network connecting the ventral and dorsal vessels. The cells of the coelomic lining on the surface of the vessels grew and gave rise to the parenchymal tissue of the trophosome. At the same time, the trophosome developed from two sources, namely: due to the coelomic lining on the surface of the vessels of the intestinal plexus and due to the coelomic lining on the surface of the vessels of the circulatory plexus of the body wall.

Sensory cells and the organization of the peripheral nervous system of the siboglinid Oligobrachia haakonmosbiensis Smirnov, 2000

Background

The nervous system of siboglinids has been studied mainly in Osedax and some Vestimentifera, while data in Frenulata – one of the four pogonophoran main branches – is still fragmentary. In most of the studies, the focus is almost always on the central nervous system, while the peripheral nervous system has traditionally received little attention. In contrast to other annelids, the structure and diversity of sensory structures in siboglinids are still quite undescribed. Meanwhile, the peripheral nervous system, as well as sensory elements, are extremely evolutionarily labile, and information about their organization is of high importance to understand lifestyles and behavior as well as main trends that lead siboglinids to their peculiar organization.

Results

The structure of the peripheric nervous system, sensory elements, and neuromuscular relationships of Oligobrachia haakonmosbiensis were studied using both scanning electron and confocal laser microscopy. A significant number of monociliary sensory cells, as well as sensory complexes located diffusely in the epithelium of the whole body were revealed. The latter include the cephalic tentacles, sensory cells accumulations along the dorsal furrow and ciliary band, areas of the openings of the tubiparous glands, and papillae. The oval ciliary spot located on the cephalic lobe at the base of the tentacles can also be regarded as a sensory organ. Most of the detected sensory cells show immunoreactivity to substance P and/or acetylated α-tubulin. FMRFamide- and serotonin-like immunoreactivity are manifested by neurons that mainly innervate tentacles, muscles, body wall epithelium, skin glands, tubiparous glands, and papillae. In the larva of O. haakonmosbiensis, monociliary sensory elements were revealed in the region of the apical organ, along the body, and on the pygidium.

Conclusions

The diversity of sensory structures in O. haakonmosbiensis comprises epidermal solitary sensory cells, sensory spots around tubiparous glands openings, and putative sensory organs such as cephalic tentacles, an oval ciliary spot on the cephalic lobe, the dorsal furrow, and papillae. Sensory structures associated with papillae and tubiparous glands play presumable mechanosensory functions and are associated with regulation of tube building as well as anchorage of the worm inside the tube. Sensory structures of the dorsal furrow are presumably engaged in the regulation of reproductive behavior. An overall low level of morphological differentiation of O. haakonmosbiensis peripheral nervous system is not typical even for annelids with the intraepithelial nervous system. This can be considered as a plesiomorphic feature of its peripheral plexus’s organization, or as evidence for the neotenic origin of Siboglinidae.

Novel insights on obligate symbiont lifestyle and adaptation to chemosynthetic environment as revealed by the giant tubeworm genome

The mutualism between the giant tubeworm Riftia pachyptila and its endosymbiont Candidatus Endoriftia persephone has been extensively researched over the past 40 years. However, the lack of the host whole genome information has impeded the full comprehension of the genotype/phenotype interface in Riftia. Here we described the high-quality draft genome of Riftia, its complete mitogenome, and tissue-specific transcriptomic data. The Riftia genome presents signs of reductive evolution, with gene family contractions exceeding expansions. Expanded gene families are related to sulphur metabolism, detoxification, anti-oxidative stress, oxygen transport, immune system, and lysosomal digestion, reflecting evolutionary adaptations to the vent environment and endosymbiosis. Despite the derived body plan, the developmental gene repertoire in the gutless tubeworm is extremely conserved with the presence of a near intact and complete Hox cluster. Gene expression analyses establishes that the trophosome is a multi-functional organ marked by intracellular digestion of endosymbionts, storage of excretory products and haematopoietic functions. Overall, the plume and gonad tissues both in contact to the environment harbour highly expressed genes involved with cell cycle, programmed cell death, and immunity indicating a high cell turnover and defence mechanisms against pathogens. We posit that the innate immune system plays a more prominent role into the establishment of the symbiosis during the infection in the larval stage, rather than maintaining the symbiostasis in the trophosome. This genome bridges four decades of physiological research in Riftia, whilst simultaneously provides new insights into the development, whole organism functions and evolution in the giant tubeworm.

Myogenesis of Siboglinum fiordicum sheds light on body regionalisation in beard worms (Siboglinidae, Annelida)

BACKGROUND

Many annelids, including well-studied species such as Platynereis, show similar structured segments along their body axis (homonomous segmentation). However, numerous annelid species diverge from this pattern and exhibit specialised segments or body regions (heteronomous segmentation). Recent phylogenomic studies and paleontological findings suggest that a heteronomous body architecture may represent an ancestral condition in Annelida. To better understand the segmentation within heteronomous species we describe the myogenesis and mesodermal delineation of segments in Siboglinum fiordicum during development.

RESULTS

Employing confocal and transmission electron microscopy we show that the somatic longitudinal musculature consists of four separate strands, among which ventrolateral one is the most prominent and is proposed to drive the search movements of the head of the late metatrochophore. The somatic circular musculature lies inside the longitudinal musculature and is predominantly developed at the anterior end of the competent larva to support the burrowing behaviour. Our application of transmission electron microscopy allows us to describe the developmental order of the non-muscular septa. The first septum to form is supported by thick bundles of longitudinal muscles and separates the body into an anterior and a posterior region. The second group of septa to develop further divides the posterior body region (opisthosoma) and is supported by developing circular muscles. At the late larval stage, a septum reinforced by circular muscles divides the anterior body region into a forepart and a trunk segment. The remaining septa and their circular muscles form one by one at the very posterior end of the opisthosoma.

CONCLUSIONS

The heteronomous Siboglinum lacks the strict anterior to posterior sequence of segment formation as it is found in the most studied annelid species. Instead, the first septum divides the body into two body regions before segments are laid down in first the posterior opisthosoma and then in the anterior body, respectively. Similar patterns of segment formation are described for the heteronomous chaetopterid Chaetopterus variopedatus and serpulid Hydroides elegans and may represent an adaptation of these annelids to the settlement and transition to the sedentarian-tubiculous mode of life.

Trophosome in the Vestimentiferan Tubeworm Ridgeia piscesae Jones 1985 (Annelida, Siboglinidae) Develops from Cells of the Coelomic Lining

The paper reports the study of the anatomy of early juvenile individuals of the vestimentiferan tubeworm Ridgeia piscesae (Annelida, Siboglinidae). Adult vestimentiferans lack the digestive tract but have the trophosome, whose cells are inhabited by chemoautotrophic bacteria. It has been shown, in 280- to 300-µm early juvenile individuals, that the trophosome develops from cells of the coelomic lining on the gut surface and on the lateral body walls. The observed proto-trophosome structure suggests that the bacteria are first captured by the coelomic cells of the body wall and then transferred to the coelomic cells located on the gut surface.

The neuroanatomy of the siboglinid Riftia pachyptila highlights sedentarian annelid nervous system evolution

Tracing the evolution of the siboglinid group, peculiar group of marine gutless annelids, requires the detailed study of the fragmentarily explored central nervous system of vestimentiferans and other siboglinids. 3D reconstructions of the neuroanatomy of Riftia revealed that the "brain" of adult vestimentiferans is a fusion product of the supraesophageal and subesophageal ganglia. The supraesophageal ganglion-like area contains the following neural structures that are homologous to the annelid elements: the peripheral perikarya of the brain lobes, two main transverse commissures, mushroom-like structures, commissural cell cluster, and the circumesophageal connectives with two roots which give rise to the palp neurites. Three pairs of giant perikarya are located in the supraesophageal ganglion, giving rise to the paired giant axons. The circumesophageal connectives run to the VNC. The subesophageal ganglion-like area contains a tripartite ventral aggregation of perikarya (= the postoral ganglion of the VNC) interconnected by the subenteral commissure. The paired VNC is intraepidermal, not ganglionated over most of its length, associated with the ciliary field, and comprises the giant axons. The pairs of VNC and the giant axons fuse posteriorly. Within siboglinids, the vestimentiferans are distinguished by a large and considerably differentiated brain. This reflects the derived development of the tentacle crown. The tentacles of vestimentiferans are homologous to the annelid palps based on their innervation from the dorsal and ventral roots of the circumesophageal connectives. Neuroanatomy of the vestimentiferan brains is close to the brains of Cirratuliiformia and Spionida/Sabellida, which have several transverse commissures, specific position of the giant somata (if any), and palp nerve roots (if any). The palps and palp neurite roots originally developed in all main annelid clades (basally branching, errantian and sedentarian annelids), show the greatest diversity in their number in sedentarian species. Over the course of evolution of Sedentaria, the number of palps and their nerve roots either dramatically increased (as in vestimentiferan siboglinids) or were lost.

Evolution of Sulfur Binding by Hemoglobin in Siboglinidae (Annelida) with Special Reference to Bone-Eating Worms, Osedax

Most members of Siboglinidae (Annelida) harbor endosymbiotic bacteria that allow them to thrive in extreme environments such as hydrothermal vents, methane seeps, and whale bones. These symbioses are enabled by specialized hemoglobins (Hbs) that are able to bind hydrogen sulfide for transportation to their chemosynthetic endosymbionts. Sulfur-binding capabilities are hypothesized to be due to cysteine residues at key positions in both vascular and coelomic Hbs, especially in the A2 and B2 chains. Members of the genus Osedax, which live on whale bones, do not have chemosynthetic endosymbionts, but instead harbor heterotrophic bacteria capable of breaking down complex organic compounds. Although sulfur-binding capabilities are important in other siboglinids, we questioned whether Osedax retained these cysteine residues and the potential ability to bind hydrogen sulfide. To answer these questions, we used high-throughput DNA sequencing to isolate and analyze Hb sequences from 8 siboglinid lineages. For Osedax mucofloris, we recovered three (A1, A2, and B1) Hb chains, but the B2 chain was not identified. Hb sequences from gene subfamilies A2 and B2 were translated and aligned to determine conservation of cysteine residues at previously identified key positions. Hb linker sequences were also compared to determine similarity between Osedax and siboglinids/sulfur-tolerant annelids. For O. mucofloris, our results found conserved cysteines within the Hb A2 chain. This finding suggests that Hb in O. mucofloris has retained some capacity to bind hydrogen sulfide, likely due to the need to detoxify this chemical compound that is abundantly produced within whale bones.

Neural reconstruction of bone-eating Osedax spp. (Annelida) and evolution of the siboglinid nervous system

BACKGROUND

Bone-devouring Osedax worms were described over a decade ago from deep-sea whale falls. The gutless females (and in one species also the males) have a unique root system that penetrates the bone and nourishes them via endosymbiotic bacteria. Emerging from the bone is a cylindrical trunk, which is enclosed in a transparent tube, that generally gives rise to a plume of four palps (or tentacles). In most Osedax species, dwarf males gather in harems along the female's trunk and the nervous system of these microscopic forms has been described in detail. Here, the nervous system of bone-eating Osedax forms are described for the first time, allowing for hypotheses on how the abberant ventral brain and nervous system of Siboglinidae may have evolved from a ganglionated nervous system with a dorsal brain, as seen in most extant annelids.

RESULTS

The intraepidermal nervous systems of four female Osedax spp. and the bone-eating O. priapus male were reconstructed in detail by a combination of immunocytochemistry, CLSM, histology and TEM. They all showed a simple nervous system composed of an anterior ventral brain, connected with anteriorly directed paired palp and gonoduct nerves, and four main pairs of posteriorly directed longitudinal nerves (2 ventral, 2 ventrolateral, 2 sets of dorso-lateral, 2 dorsal). Transverse peripheral nerves surround the trunk, ovisac and root system. The nervous system of Osedax resembles that of other siboglinids, though possibly presenting additional lateral and dorsal longitudinal nerves. It differs from most Sedentaria in the presence of an intraepidermal ventral brain, rather than a subepidermal dorsal brain, and by having an intraepidermal nerve cord with several plexi and up to three main commissures along the elongated trunk, which may comprise two indistinct segments.

CONCLUSIONS

Osedax shows closer neuroarchitectural resemblance to Vestimentifera + Sclerolinum (= Monilifera) than to Frenulata. The intraepidermal nervous system with widely separated nerve cords, double brain commissures, double palp nerves and other traits found in Osedax can all be traced to represent ancestral states of Siboglinidae. A broader comparison of the nervous system and body regions across Osedax and other siboglinids allows for a reinterpretation of the anterior body region in the group.

Dual Cellular Supporters: Multi-Layer Glial Wrapping and the Penetrative Matrix Specialized in Deep-Sea Hydrothermal Vent Endemic Scale-Worms

Hydrothermal vent organisms undergo extreme environments that may require unique innovations. The present study reports a distinct case of cellular supportive systems in the nervous systems of a scale-worm, Branchinotogluma japonica, endemic to deep-sea hydrothermal vents. We found two organizations in the tissues of these animals. First, multi-layers of glia ensheath the ventral cell bodies of the brain and ventral nerve cord, in a manner similar to that of myelin or lamellar ensheathments. Second, matrices of numerous penetrative fibers, or tonofilaments, composed of bundles of ca. 20-nm fibers, are directly connected with the basal parts of epidermal cuticles and run into the diffuse intercellular spaces of the brain neuropils and peripheral nerves. Both types of tissue might be mechanical supportive structures for the neuronal cell bodies. In addition, as a glial function, the multi-layer membranes and the epithelial support cells may be required for physicochemical homeostatic regulation to filter toxic heavy metals and for inhibiting breakdown of glial membrane integrity under strong oxidative stress imposed by hypoxia in the hydrothermal vent environment. Similar functions are known in the well-studied cases of the blood-brain barrier in mammalian brains, including in human stroke.

The reproductive system of Osedax (Annelida, Siboglinidae): ovary structure, sperm ultrastructure, and fertilization mode

Osedax is a genus of siboglinid annelids in which the females live on dead vertebrate bones on the seafloor. These females have a posterior end that lies within the bone and contains the ovarian tissue, as well as the "roots" involved with bone degradation and nutrition. The males are microscopic and live as "harems" in the lumen of the gelatinous tube that surrounds the female trunk, well away from the ovary. Females are known to spawn fertilized primary oocytes, suggesting internal fertilization. However, little is known about sperm transfer, sperm storage, or the location of fertilization, and the morphology of the female reproductive system has not been described and compared with the reproductive systems of other siboglinids. A 3D-reconstruction of the ovisac of Osedax showed ovarian tissue with multiple lobes and mature oocytes stored in a "uterus" before being released through the single oviduct. The oviduct emerges as a gonopore on the trunk and travels along the trunk to finally open to the seawater as a thin cylindrical tube among the crown of palps. Light and transmission electron microscopy of mature Osedax sperm revealed elongate heads consisting of a nucleus with helical grooves occupied by mitochondria. In contrast to other Siboglinidae, Osedax sperm are not packaged into spermatophores or spermatozeugmata, and Osedax females lack a discrete region for sperm storage. Transmission electron microscopy and fluorescence microscopy allowed detection of sperm associated with ovarian tissue of the female ovisac of four different Osedax species. This provides the first evidence for the site of internal fertilization in Osedax. A heart body was found in the circulatory system, as seen in other siboglinids and some other annelids. The possible presence of nephridia in the anterior ovisac region was also documented. These morphological features provide new insights for comparing the regionalization of Osedax females in relation to other siboglinids.

Morphology, microanatomy and sequence data of Sclerolinum contortum (Siboglindae, Annelida) of the Gulf of Mexico

Sclerolinum is a small genus of Siboglinidae (Annelida) living in an obligate mutualistic association with thiotrophic bacteria as adults. Its taxonomic position, based on morphology, has been controversial; however, molecular data point to a sister taxa relationship with vestimentiferans. 16S rRNA gene sequencing and comparative morphology revealed that the studied population from deep-sea hydrocarbon seeps of the Gulf of Mexico belongs to Sclerolinum contortum known from the Arctic Sea. Since no anatomical and microanatomical studies have been published yet, we conducted such a study on S. contortum using serial sectioning and light and transmission electron microscopy. We show that the Sclerolinum body, divided into a head, trunk, and opisthosoma, is very similar to that of the vestimentiferans, and therefore we propose that the body regions are homologous in both taxa.