The simplicity of males: dwarf males of four species of Osedax (Siboglinidae; Annelida) investigated by confocal laser scanning microscopy

Hooked on zombie worms? Genetic blueprints of bristle formation in Osedax japonicus (Annelida)

BACKGROUND

This study sheds light on the genetic blueprints of chaetogenesis (bristle formation), a complex biomineralization process essential not only for the diverse group of bristle worms (annelids) but also for other spiralians. We explore the complex genetic mechanisms behind chaetae formation in Osedax japonicus, the bone-devouring deep-sea worm known for its unique ecological niche and morphological adaptations.

RESULTS

We characterized the chaetal structure and musculature using electron microscopy and immunohistochemistry, and combined RNAseq of larval stages with in-situ hybridization chain reaction (HCR) to reveal gene expression patterns integral to chaetogenesis. Our findings pinpoint a distinct surge in gene expression during the larval stage of active chaetogenesis, identifying specific genes and cells involved.

CONCLUSIONS

Our research underscores the value of studying on non-model, "aberrant" organisms like Osedax, whose unique, temporally restricted chaetogenesis provided insights into elevated gene expression across specific larval stages and led to the identification of genes critical for chaetae formation. The genes identified as directly involved in chaetogenesis lay the groundwork for future comparative studies across Annelida and Spiralia, potentially elucidating the homology of chaetae-like chitinous structures and their evolution.

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.

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.

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.

New insights into the morphology and evolution of the ventral pharynx and jaws in Histriobdellidae (Eunicida, Annelida)

The jaw apparatus in several annelid families represents a powerful tool for systematic approaches and evolutionary investigations. Nevertheless, for several taxa, this character complex has scarcely been investigated, and complete comparative analyses of all annelid jaws are lacking. In our comprehensive study, we described the fine structure of the jaw apparatus and the ventral pharyngeal organ (VPO) in Histriobdella homari - a minute ectocommensal of lobsters putatively belonging to the Eunicida - using different comparative morphological approaches, including SEM, TEM, CLSM and subsequent 3D reconstruction. The H. homari jaw apparatus is composed of ventral paired mandibles and dorsal symmetrical maxillae consisting of numerous dental plates, ventral carriers and an unpaired dorsal rod, and the general assemblage and arrangement of the different parts are highly comparable to those of other eunicid families. The jaw ultrastructure of histriobdellids resembles that of the families Dorvilleidae and (juvenile) Onuphidae. Furthermore, our data reveal that in the process of development of the jaw apparatus, the mandibles, maxillae II and unpaired dorsal rod are formed first, and the remaining maxillae and ventral carriers appear later. Notably, the muscular apparatus differs from that in Dorvilleidae and Onuphidae in terms of the number and arrangement of muscle fibers encompassing the jaws - not only because of the very small size of Histriobdella but also because histriobdellid maxillary protraction occurs due to straightening of the dorsal rod and thus requires a different muscular scaffold. Based on our investigations, the arrangement of the muscular apparatus of the jaws, the presence of paired ventral carriers and the dorsal rod, and the morphology of the ventral pharyngeal organ represent a histriobdellid autapomorphy. Our datasets form a basis for further comparative analyses to elucidate the evolution of Eunicida and jaw-bearing Annelida.

Application of Autofluorescence for Confocal Microscopy to Aid in Archaeoparasitological Analyses

Confocal laser scanning microscopy (CLSM) was used to examine archaeoparasitological specimens from coprolites associated with La Cueva de los Muertos Chiquitos (CMC) located near present-day Durango, Mexico. The eggs for 4 different types of parasites recovered from CMC coprolites were imaged using CLSM to assist with identification efforts. While some of the parasite eggs recovered from CMC coprolites were readily identified using standard light microscopy (LM), CLSM provided useful data for more challenging identifications by highlighting subtle morphological features and enhancing visualization of parasite egg anatomy. While other advanced microscopy techniques, such as scanning electron microscopy (SEM), may also detect cryptic identifying characters, CLSM is less destructive to the specimens. Utilizing CLSM allows for subsequent examinations, such as molecular analyses, that cannot be performed following SEM sample preparation and imaging. Furthermore, CLSM detects intrinsic autofluorescence molecules, making improved identification independent of resource and time-intensive protocols. These aspects of CLSM make it an excellent method for assisting in taxonomic identification and for acquiring more detailed images of archaeoparasitological specimens.

Morphology of the nervous system of monogonont rotifer Epiphanes senta with a focus on sexual dimorphism between feeding females and dwarf males

Background

Monogononta is a large clade of rotifers comprised of diverse morphological forms found in a wide range of ecological habitats. Most monogonont species display cyclical parthenogenesis, where generations of asexually reproducing females are interspaced by mixis events when sexual reproduction occurs between mictic females and dwarf, haploid males. The morphology of monogonont feeding females is relatively well described, however data on male anatomy are very limited. Thus far, male musculature of only two species has been described with confocal laser scanning microscopy (CLSM) and it remains unknown how dwarfism influences the neuroanatomy of males on detailed level.

Results

Here, we provide a CLSM-based description of the nervous system of both sexes of Epiphanes senta, a freshwater monogonont rotifer. The general nervous system architecture is similar between males and females and shows a similar level of complexity. However, the nervous system in males is more compact and lacks a stomatogastric part.

Conclusion

Comparison of the neuroanatomy between male and normal-sized feeding females provides a better understanding of the nature of male dwarfism in Monogononta. We propose that dwarfism of monogonont non-feeding males is the result of a specific case of heterochrony, called “proportional dwarfism” as they, due to their inability to feed, retain a juvenile body size, but still develop a complex neural architecture comparable to adult females. Reduction of the stomatogastric nervous system in the males correlates with the loss of the entire digestive tract and associated morphological structures.

Electronic supplementary material

The online version of this article (10.1186/s12983-019-0334-9) contains supplementary material, which is available to authorized users.

Dicyemida and Orthonectida: Two Stories of Body Plan Simplification

Two enigmatic groups of morphologically simple parasites of invertebrates, the Dicyemida (syn. Rhombozoa) and the Orthonectida, since the 19th century have been usually considered as two classes of the phylum Mesozoa. Early molecular evidence suggested their relationship within the Spiralia (=Lophotrochozoa), however, high rates of dicyemid and orthonectid sequence evolution led to contradicting phylogeny reconstructions. Genomic data for orthonectids revealed that they are highly simplified spiralians and possess a reduced set of genes involved in metazoan development and body patterning. Acquiring genomic data for dicyemids, however, remains a challenge due to complex genome rearrangements including chromatin diminution and generation of extrachromosomal circular DNAs, which are reported to occur during the development of somatic cells. We performed genomic sequencing of one species of Dicyema, and obtained transcriptomic data for two Dicyema spp. Homeodomain (homeobox) transcription factors, G-protein-coupled receptors, and many other protein families have undergone a massive reduction in dicyemids compared to other animals. There is also apparent reduction of the bilaterian gene complements encoding components of the neuromuscular systems. We constructed and analyzed a large dataset of predicted orthologous proteins from three species of Dicyema and a set of spiralian animals including the newly sequenced genome of the orthonectid Intoshia linei. Bayesian analyses recovered the orthonectid lineage within the Annelida. In contrast, dicyemids form a separate clade with weak affinity to the Rouphozoa (Platyhelminthes plus Gastrotricha) or (Entoprocta plus Cycliophora) suggesting that the historically proposed Mesozoa is a polyphyletic taxon. Thus, dramatic simplification of body plans in dicyemids and orthonectids, as well as their intricate life cycles that combine metagenesis and heterogony, evolved independently in these two lineages.

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.

Convergent evolution of the ladder-like ventral nerve cord in Annelida

BACKGROUND

A median, segmented, annelid nerve cord has repeatedly been compared to the arthropod and vertebrate nerve cords and became the most used textbook representation of the annelid nervous system. Recent phylogenomic analyses, however, challenge the hypothesis that a subepidermal rope-ladder-like ventral nerve cord (VNC) composed of a paired serial chain of ganglia and somata-free connectives represents either a plesiomorphic or a typical condition in annelids.

RESULTS

Using a comparative approach by combining phylogenomic analyses with morphological methods (immunohistochemistry and CLSM, histology and TEM), we compiled a comprehensive dataset to reconstruct the evolution of the annelid VNC. Our phylogenomic analyses generally support previous topologies. However, the so far hard-to-place Apistobranchidae and Psammodrilidae are now incorporated among the basally branching annelids with high support. Based on this topology we reconstruct an intraepidermal VNC as the ancestral state in Annelida. Thus, a subepidermal ladder-like nerve cord clearly represents a derived condition.

CONCLUSIONS

Based on the presented data, a ladder-like appearance of the ventral nerve cord evolved repeatedly, and independently of the transition from an intraepidermal to a subepidermal cord during annelid evolution. Our investigations thereby propose an alternative set of neuroanatomical characteristics for the last common ancestor of Annelida or perhaps even Spiralia.

High diversity in neuropeptide immunoreactivity patterns among three closely related species of Dinophilidae (Annelida)

Neuropeptides are conserved metazoan signaling molecules, and represent useful markers for comparative investigations on the morphology and function of the nervous system. However, little is known about the variation of neuropeptide expression patterns across closely related species in invertebrate groups other than insects. In this study, we compare the immunoreactivity patterns of 14 neuropeptides in three closely related microscopic dinophilid annelids (Dinophilus gyrociliatus, D. taeniatus and Trilobodrilus axi). The brains of all three species were found to consist of around 700 somata, surrounding a central neuropil with 3-5 ventral and 2-5 dorsal commissures. Neuropeptide immunoreactivity was detected in the brain, the ventral cords, stomatogastric nervous system, and additional nerves. Different neuropeptides are expressed in specific, non-overlapping cells in the brain in all three species. FMRFamide, MLD/pedal peptide, allatotropin, RNamide, excitatory peptide, and FVRIamide showed a broad localization within the brain, while calcitonin, SIFamide, vasotocin, RGWamide, DLamide, FLamide, FVamide, MIP, and serotonin were present in fewer cells in demarcated regions. The different markers did not reveal ganglionic subdivisions or physical compartmentalization in any of these microscopic brains. The non-overlapping expression of different neuropeptides may indicate that the regionalization in these uniform, small brains is realized by individual cells, rather than cell clusters, representing an alternative to the lobular organization observed in several macroscopic annelids. Furthermore, despite the similar gross brain morphology, we found an unexpectedly high variation in the expression patterns of neuropeptides across species. This suggests that neuropeptide expression evolves faster than morphology, representing a possible mechanism for the evolutionary divergence of behaviors.

Phylogeny and biogeography of the scaleless scale worm Pisione (Sigalionidae, Annelida)

Pisione is a scaleless group of small scale worms inhabiting sandy bottoms in shallow marine waters. This group was once considered rare, but now 45 described species can be characterized, among others, by their paired, segmental copulatory organs (one to multiple external pairs), which display a complexity of various accessory structures. The evolutionary significance of these unique organs was suggested in the late 1960s, but has been heavily debated since the late 1990s and remains controversial. In the present paper, we study the internal relationships within Pisione, employing combined phylogenetic analyses of both molecular and morphological data from 16 terminals of Pisione, as well as two terminals of Pisionidens, and eight additional scale worms as outgroups. Our taxon sampling covers all geographical areas where the genus has been reported, as well as most of their morphological and copulatory variability, including representatives of the "africana," "remota," "crassa," and "papuensis" groups, established previously by Yamanishi. We hereby provide a first insight into the relationships of the genus, testing previously proposed hypotheses on the evolutionary significance of male copulatory structures within Pisione, while attempting to understand patterns of distribution. The phylogenetic analyses using maximum likelihood and Bayesian methods consistently recovered two large clades spanning the East Atlantic (including the Mediterranean) and the Indo-Pacific-West Atlantic, respectively. Character optimization on our trees revealed a high degree of homoplasy in both non-reproductive and sexual characters of Pisione, with buccal acicula found to be the sole apomorphy among the morphological features assessed herein, with none defining the biogeographical subclades within. Overall, our comparative analyses highlight the high degree of morphological variation in this widely distributed genus, rejecting previous assertions of an increasing number and complexity of copulatory structures across the genus.

Comparison of neuromuscular development in two dinophilid species (Annelida) suggests progenetic origin of Dinophilus gyrociliatus

BACKGROUND

Several independent meiofaunal lineages are suggested to have originated through progenesis, however, morphological support for this heterochronous process is still lacking. Progenesis is defined as an arrest of somatic development (synchronously in various organ systems) due to early maturation, resulting in adults resembling larvae or juveniles of the ancestors. Accordingly, we established a detailed neuromuscular developmental atlas of two closely related Dinophilidae using immunohistochemistry and CLSM. This allows us to test for progenesis, questioning whether i) the adult smaller, dimorphic Dinophilus gyrociliatus resembles a younger developmental stage of the larger, monomorphic D. taeniatus and whether ii) dwarf males of D. gyrociliatus resemble an early developmental stage of D. gyrociliatus females.

RESULTS

Both species form longitudinal muscle bundles first, followed by circular muscles, creating a grid of body wall musculature, which is the densest in adult D. taeniatus, while the architecture in adult female D. gyrociliatus resembles that of prehatching D. taeniatus. Both species display a subepidermal ganglionated nervous system with an anterior dorsal brain and five longitudinal ventral nerve bundles with six sets of segmental commissures (associated with paired ganglia). Neural differentiation of D. taeniatus and female D. gyrociliatus commissures occurs before hatching: both species start out forming one transverse neurite bundle per segment, which are thereafter joined by additional thin bundles. Whereas D. gyrociliatus arrests its development at this stage, adult D. taeniatus condenses the thin commissures again into one thick commissural bundle per segment. Generally, D. taeniatus adults demonstrate a seemingly more organized (= segmental) pattern of serotonin-like and FMRFamide-like immunoreactive elements. The dwarf male of D. gyrociliatus displays a highly aberrant neuromuscular system, showing no close resemblance to any early developmental stage of female Dinophilus, although the onset of muscular development mirrors the early myogenesis in females.

CONCLUSION

The apparent synchronous arrest of nervous and muscular development in adult female D. gyrociliatus, resembling the prehatching stage of D. taeniatus, suggests that D. gyrociliatus have originated through progenesis. The synchrony in arrest of three organ systems, which show opposing reduction and addition of elements, presents one of the morphologically best-argued cases of progenesis within Spiralia.

Molecular regionalization in the compact brain of the meiofaunal annelid Dinophilus gyrociliatus (Dinophilidae)

BACKGROUND

Annelida is a morphologically diverse animal group that exhibits a remarkable variety in nervous system architecture (e.g., number and location of longitudinal cords, architecture of the brain). Despite this heterogeneity of neural arrangements, the molecular profiles related to central nervous system patterning seem to be conserved even between distantly related annelids. In particular, comparative molecular studies on brain and anterior neural region patterning genes have focused so far mainly on indirect-developing macrofaunal taxa. Therefore, analyses on microscopic, direct-developing annelids are important to attain a general picture of the evolutionary events underlying the vast diversity of annelid neuroanatomy.

RESULTS

We have analyzed the expression domains of 11 evolutionarily conserved genes involved in brain and anterior neural patterning in adult females of the direct-developing meiofaunal annelid Dinophilus gyrociliatus. The small, compact brain shows expression of dimmed, foxg, goosecoid, homeobrain, nk2.1, orthodenticle, orthopedia, pax6, six3/6 and synaptotagmin-1. Although most of the studied markers localize to specific brain areas, the genes six3/6 and synaptotagmin-1 are expressed in nearly all perikarya of the brain. All genes except for goosecoid, pax6 and nk2.2 overlap in the anterior brain region, while the respective expression domains are more separated in the posterior brain.

CONCLUSIONS

Our findings reveal that the expression patterns of the genes foxg, orthodenticle, orthopedia and six3/6 correlate with those described in Platynereis dumerilii larvae, and homeobrain, nk2.1, orthodenticle and synaptotagmin-1 resemble the pattern of late larvae of Capitella teleta. Although data on other annelids are limited, molecular similarities between adult Dinophilus and larval Platynereis and Capitella suggest an overall conservation of molecular mechanisms patterning the anterior neural regions, independent from developmental and ecological strategies, or of the size and configuration of the nervous system.

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.

Neural architecture of Galathowenia oculata Zach, 1923 (Oweniidae, Annelida)

BACKGROUND

Oweniids are marine tubeworms burrowing in muddy sediments that in current phylogenies form an early branching lineage within Annelida. Little is known about their general morphology, in particular the nervous system. Here we provide an immunocytochemical investigation of the nervous system of Galathowenia oculata in order to discuss putative ancestral neuronal features in Oweniidae.

RESULTS

Adult Galathowenia oculata have neither a supraesophageal ganglion nor ganglia associated with the ventral nerve cord. Instead, there is a dorsal brain commissure in the head collar that is engulfed by a cellular cortex. Accordingly, we herein term this neural structure "medullary brain commissure". The anterior margin of the head collar exhibits numerous neurites that emerge from the brain commissure. The dorsolateral folds are innervated by the ventrolateral neurite bundles extending from the circumesophageal connectives. In the anterior uniramous and biramous segments immunoreactive somata are distributed evenly along the ventral nerve cord and arranged metamerically in the posterior-most short segments. One dorsal and two pairs of lateral neurite bundles extend longitudinally along the body. Numerous serially arranged circular neurite bundles were labeled in anteriormost long segments. Metameric arrangement of the circular neurite bundles stained against FMRFamide and acetylated α-tubulin is revealed in posterior short segments. For the first time immunoreactive somata arranged in clusters are reported within the pygidium in oweniids.

CONCLUSIONS

Due to the lack of head appendages and a sedentary mode of life, G. oculata exhibits a single dorsal commissure (versus a brain with four commissures in most annelids). A "medullary brain commissure" is known so far only in Oweniidae and Echiura. Lack of ganglia and metamery in the ventral nerve cord of the anteriormost segments might be the result of the elongation of these segments. In the short posterior segments the metamery of immunoreactive somata and circular neurite bundles is conserved. We hypothesize that the unpaired ventral nerve cord in adult oweniids might be a result of an initially paired ventral nerve cord that fuses during development, a condition not uncommon within Annelida.

Bone-Eating Worms Spread: Insights into Shallow-Water Osedax (Annelida, Siboglinidae) from Antarctic, Subantarctic, and Mediterranean Waters

Osedax, commonly known as bone-eating worms, are unusual marine annelids belonging to Siboglinidae and represent a remarkable example of evolutionary adaptation to a specialized habitat, namely sunken vertebrate bones. Usually, females of these animals live anchored inside bone owing to a ramified root system from an ovisac, and obtain nutrition via symbiosis with Oceanospirillales gamma-proteobacteria. Since their discovery, 26 Osedax operational taxonomic units (OTUs) have been reported from a wide bathymetric range in the Pacific, the North Atlantic, and the Southern Ocean. Using experimentally deployed and naturally occurring bones we report here the presence of Osedax deceptionensis at very shallow-waters in Deception Island (type locality; Antarctica) and at moderate depths near South Georgia Island (Subantarctic). We present molecular evidence in a new phylogenetic analysis based on five concatenated genes (28S rDNA, Histone H3, 18S rDNA, 16S rDNA, and cytochrome c oxidase I-COI-), using Maximum Likelihood and Bayesian inference, supporting the placement of O. deceptionensis as a separate lineage (Clade VI) although its position still remains uncertain. This phylogenetic analysis includes a new unnamed species (O. 'mediterranea') recently discovered in the shallow-water Mediterranean Sea belonging to Osedax Clade I. A timeframe of the diversification of Osedax inferred using a Bayesian framework further suggests that Osedax diverged from other siboglinids during the Middle Cretaceous (ca. 108 Ma) and also indicates that the most recent common ancestor of Osedax extant lineages dates to the Late Cretaceous (ca. 74.8 Ma) concomitantly with large marine reptiles and teleost fishes. We also provide a phylogenetic framework that assigns newly-sequenced Osedax endosymbionts of O. deceptionensis and O. 'mediterranea' to ribospecies Rs1. Molecular analysis for O. deceptionensis also includes a COI-based haplotype network indicating that individuals from Deception Island and the South Georgia Island (ca. 1,600 km apart) are clearly the same species, confirming the well-developed dispersal capabilities reported in other congeneric taxa. In addition, we include a complete description of living features and morphological characters (including scanning and transmission electron microscopy) of O. deceptionensis, a species originally described from a single mature female, and compare it to information available for other congeneric OTUs.

A dwarf male reversal in bone-eating worms

Darwin hypothesized that sexes in a species should be similar unless sexual selection, fecundity selection, or resource partitioning has driven them apart. Male dwarfism has evolved multiple times in a range of animals, raising questions about factors that drive such extreme size dimorphism. Ghiselin noted that dwarf males are more common among smaller marine animals, and especially among sedentary and sessile species living at low densities, where mates are difficult to find, or in deep-sea environments with limited energy sources. These benefits of male dwarfism apply well to Osedax (Annelida: Siboglinidae), bone-eating marine worms. Osedax males, notable for extreme sexual size dimorphism (SSD), are developmentally arrested larvae that produce sperm from yolk reserves. Harems of dwarf males reside in the lumen of the tube surrounding a female. Herein, we describe Osedax priapus n. sp., a species that deviates remarkably by producing males that anchor into, and feed on, bone via symbiont-containing "roots," just like female Osedax. Phylogenetic analyses revealed O. priapus n. sp. as a derived species, and the absence of dwarf males represents a character reversal for this genus. Some dwarf male features are retained due to functional and morphological constraints. Since O. priapus n. sp. males are anchored in bone, they possess an extensible trunk that allows them to roam across the bone to contact and inseminate females. Evolutionary and ecological implications of a loss of male dwarfism are discussed.

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.

How to get into bones: proton pump and carbonic anhydrase in Osedax boneworms

Osedax are gutless siboglinid worms that thrive on vertebrate bones lying on the ocean floor, mainly those of whales. The posterior body of female Osedax penetrates into the bone forming extensions known as 'roots', which host heterotrophic symbiotic bacteria in bacteriocytes beneath the epidermis. The Osedax root epithelium presumably absorbs bone collagen and/or lipids, which are metabolized by the symbiotic bacteria that in turn serve for Osedax's nutrition. Here, we show that Osedax roots express extremely high amounts of vacuolar-H(+)-ATPase (VHA), which is located in the apical membrane and in cytoplasmic vesicles of root and ovisac epithelial cells. The enzyme carbonic anhydrase (CA), which catalyses the hydration of CO2 into H(+) and HCO3(-), is also expressed in roots and throughout Osedax body. These results suggest Osedax roots have massive acid-secreting capacity via VHA, fuelled by H(+) derived from the CA-catalysed hydration of CO2 produced by aerobic metabolism. We propose the secreted acid dissolves the bone carbonate matrix to then allow the absorption of bone-derived nutrients across the skin. In an exciting example of convergent evolution, this model for acid secretion is remarkably similar to mammalian osteoclast cells. However, while osteoclasts dissolve bone for repairing and remodelling, the Osedax root epithelium secretes acid to dissolve foreign bone to access nutrients.

An anatomical description of a miniaturized acorn worm (hemichordata, enteropneusta) with asexual reproduction by paratomy

The interstitial environment of marine sandy bottoms is a nutrient-rich, sheltered habitat whilst at the same time also often a turbulent, space-limited, and ecologically challenging environment dominated by meiofauna. The interstitial fauna is one of the most diverse on earth and accommodates miniaturized representatives from many macrofaunal groups as well as several exclusively meiofaunal phyla. The colonization process of this environment, with the restrictions imposed by limited space and low Reynolds numbers, has selected for great morphological and behavioral changes as well as new life history strategies.Here we describe a new enteropneust species inhabiting the interstices among sand grains in shallow tropical waters of the West Atlantic. With a maximum body length of 0.6 mm, it is the first microscopic adult enteropneust known, a group otherwise ranging from 2 cm to 250 cm in adult size. Asexual reproduction by paratomy has been observed in this new species, a reproductive mode not previously reported among enteropneusts. Morphologically, Meioglossus psammophilus gen. et sp. nov. shows closest resemblance to an early juvenile stage of the acorn worm family Harrimaniidae, a result congruent with its phylogenetic placement based on molecular data. Its position, clearly nested within the larger macrofaunal hemichordates, suggests that this represents an extreme case of miniaturization. The evolutionary pathway to this simple or juvenile appearance, as chiefly demonstrated by its small size, dense ciliation, and single pair of gill pores, may be explained by progenesis. The finding of M. psammophilus gen. et sp. nov. underscores the notion that meiofauna may constitute a rich source of undiscovered metazoan diversity, possibly disguised as juveniles of other species.

Comparative morphology of serotonergic-like immunoreactive elements in the central nervous system of kinorhynchs (Kinorhyncha, Cyclorhagida)

Cycloneuralian taxa exhibit similar organ system architectures, providing informative characters of metazoan evolution, yet very few modern comparative descriptions of cellular and molecular homologies within and among those taxa are available. We immunolabeled and characterized elements of the serotonergic nervous system in the kinorhynchs Echinoderes spinifurca, Antygomonas paulae, and Zelinkaderes brightae using confocal laser scanning microscopy. Fluorescent markers targeting DNA were combined with observations of auto-fluorescent structures to guide interpretations of the internal and external anatomy in each species. Results show a common pattern of the central nervous system with a circumenteric brain divided into ring-shaped anterior and posterior neuronal somata and a central neuropil connected to a multi-stringed, longitudinal ventral nerve cord. Structural similarities and differences in the nervous systems of these species were observed and described, stressing the incomplete ring nature of the anterior region of the kinorhynch brain, the functional relationship between the brain and the movable introvert, and the number and arrangement of nerve strings and somata of the ventral nerve cord. The ventral cord ends in two ventrolateral cell bodies in E. spinifurca, and forms a terminal loop associated with a midterminal spine in A. paulae and Z. brightae. The possible functional and phylogenetic significance of these features and arrangements are discussed.

The potent respiratory system of Osedax mucofloris (Siboglinidae, Annelida)--a prerequisite for the origin of bone-eating Osedax?

Members of the conspicuous bone-eating genus, Osedax, are widely distributed on whale falls in the Pacific and Atlantic Oceans. These gutless annelids contain endosymbiotic heterotrophic bacteria in a branching root system embedded in the bones of vertebrates, whereas a trunk and anterior palps extend into the surrounding water. The unique life style within a bone environment is challenged by the high bacterial activity on, and within, the bone matrix possibly causing O₂ depletion, and build-up of potentially toxic sulphide. We measured the O₂ distribution around embedded Osedax and showed that the bone microenvironment is anoxic. Morphological studies showed that ventilation mechanisms in Osedax are restricted to the anterior palps, which are optimized for high O₂ uptake by possessing a large surface area, large surface to volume ratio, and short diffusion distances. The blood vascular system comprises large vessels in the trunk, which facilitate an ample supply of oxygenated blood from the anterior crown to a highly vascularised root structure. Respirometry studies of O. mucofloris showed a high O₂ consumption that exceeded the average O₂ consumption of a broad line of resting annelids without endosymbionts. We regard this combination of features of the respiratory system of O. mucofloris as an adaptation to their unique nutrition strategy with roots embedded in anoxic bones and elevated O₂ demand due to aerobic heterotrophic endosymbionts.

Walter Garstang: a retrospective

Although, Walter Garstang died over 60 years ago, his work is still cited--sometimes praised, but sometimes belittled. On the negative side, he often appropriated ideas of others without attribution, ignored earlier studies conflicting with his theories, and clung to notions like inheritance of acquired characters, progressive evolution, and saltation after many of his contemporaries were advancing toward the modern synthesis. Moreover, his evolutionary scenarios--especially his derivation of vertebrates from a sessile ascidian--have not been well supported by recent work in developmental genetics and molecular phylogenetics. On the positive side, Garstang firmly established several points of view that remain useful in the age of evolutionary development (evo-devo). He popularized the valid idea that adaptive changes in larvae combined with shifts in developmental timing (heterochrony) could radically change adult morphology and provide an escape from overspecialization. Moreover, his re-statement of the biogenetic law is now widely accepted: namely, that recapitulation results when characters at one stage of development are required for the correct formation of other characters at subsequent stages (his stepping stone model). In other words, ontogeny creates phylogeny because some developmental features are constraints, favoring particular evolutionary outcomes while excluding others. This viewpoint is a useful basis for advancing concepts of homology and for comparing the phylogeny of ontogenies across a series of animals to ascertain the timing and the nature of the underlying ontogenetic changes.

The Osedax trophosome: organization and ultrastructure

The polychaete family Siboglinidae, which is currently construed as comprising the Frenulata, Monilifera (composed of Sclerolinum), Vestimentifera, and Osedax, has become known for its specialized symbiont-housing organ called the trophosome. This organ replaced the digestive system of the worms and is located in the elongated trunk region in Frenulata, Sclerolinum, and Vestimentifera. Currently two types of trophosomes have been described: in the taxa Frenulata and Sclerolinum the bacteriocytes originate from endoderm, and in Vestimentifera they originate from mesoderm. In Osedax, a trophosome was described as lacking (Rouse et al., 2004), but bacteriocytes are located in Osedax's characteristic root tissue. Here, we argue for a consistent name for the symbiont-housing tissue, namely trophosome, as in other siboglinids. In this study we provide morphological evidence that in Osedax the bacteriocytes are derived from somatic mesoderm. We show that the trophosome in Osedax is an apolar tissue composed of bacteriocytes and nonsymbiotic cells. As in vestimentiferans, a specific cell cycle was identified; however, in this case it is directed from the posterior to the anterior end of the worms instead of from the center toward the periphery. Comparison of all siboglinid trophosomes and re-evaluation of their body regions allows us to discuss whether the trophosomes are homologous and to hypothesize about the organization of the last common ancestor of Siboglinidae.

Confocal analysis of nervous system architecture in direct-developing juveniles of Neanthes arenaceodentata (Annelida, Nereididae)

BACKGROUND

Members of Family Nereididae have complex neural morphology exemplary of errant polychaetes and are leading research models in the investigation of annelid nervous systems. However, few studies focus on the development of their nervous system morphology. Such data are particularly relevant today, as nereidids are the subjects of a growing body of "evo-devo" work concerning bilaterian nervous systems, and detailed knowledge of their developing neuroanatomy facilitates the interpretation of gene expression analyses. In addition, new data are needed to resolve discrepancies between classic studies of nereidid neuroanatomy. We present a neuroanatomical overview based on acetylated alpha-tubulin labeling and confocal microscopy for post-embryonic stages of Neanthes arenaceodentata, a direct-developing nereidid.

RESULTS

At hatching (2-3 chaetigers), the nervous system has developed much of the complexity of the adult (large brain, circumesophageal connectives, nerve cords, segmental nerves), and the stomatogastric nervous system is partially formed. By the 5-chaetiger stage, the cephalic appendages and anal cirri are well innervated and have clear connections to the central nervous system. Within one week of hatching (9-chaetigers), cephalic sensory structures (e.g., nuchal organs, Langdon's organs) and brain substructures (e.g., corpora pedunculata, stomatogastric ganglia) are clearly differentiated. Additionally, the segmental-nerve architecture (including interconnections) matches descriptions of other, adult nereidids, and the pharynx has developed longitudinal nerves, nerve rings, and ganglia. All central roots of the stomatogastric nervous system are distinguishable in 12-chaetiger juveniles. Evidence was also found for two previously undescribed peripheral nerve interconnections and aspects of parapodial muscle innervation.

CONCLUSIONS

N. arenaceodentata has apparently lost all essential trochophore characteristics typical of nereidids. Relative to the polychaete Capitella, brain separation from a distinct epidermis occurs later in N. arenaceodentata, indicating different mechanisms of prostomial development. Our observations of parapodial innervation and the absence of lateral nerves in N. arenaceodentata are similar to a 19th century study of Alitta virens (formerly Nereis/Neanthes virens) but contrast with a more recent study that describes a single parapodial nerve pattern and lateral nerve presence in A. virens and two other genera. The latter study apparently does not account for among-nereidid variation in these major neural features.