Homology and Evolution of the Chaetae in Echiura (Annelida)

Echiura is traditionally regarded as a small phylum of unsegmented spiralian worms. Molecular analyses, however, provide unquestionable evidence that Echiura are derived annelids that lost segmentation. Like annelids, echiurans possess chaetae, a single ventral pair in all species and one or two additional caudal hemi-circles of chaetae in two subgroups, but their evolutionary origin and affiliation to annelid chaetae are unresolved. Since annelids possess segmental pairs of dorsal (notopodial) and ventral (neuropodial) chaetae that are arranged in a row, the ventral chaetae in Echiura either represent a single or a paired neuropodial group of chaetae, while the caudal circle may represent fused rows of chaetae. In annelids, chaetogenesis is generally restricted to the ventral part of the notopodial chaetal sac and to the dorsal part of the neuropodial chaetal sac. We used the exact position of the chaetal formation site in the echiuran species, Thalassema thalassemum (Pallas, 1766) and Echiurus echiurus (Pallas, 1767), to test different hypotheses of the evolution of echiurid chaetae. As in annelids, a single chaetoblast is responsible for chaetogenesis in both species. Each chaeta of the ventral pair arises from its own chaetal sac and possesses a lateral formation site, evidencing that the pair of ventral chaetae in Echiura is homologous to a pair of neuropodia that fused on the ventral side, while the notopodia were reduced. Both caudal hemi-circles of chaetae in Echiurus echiurus are composed of several individual chaetal sacs, each with its own formative site. This finding argues against a homology of these hemi-circles of chaetae and annelids’ rows of chaetae and leads to the hypothesis that the caudal chaetal rings evolved once within the Echiura by multiplication of ventral chaetae.     

A new species of <Emphasis Type="Italic">Laonice</Emphasis> (Spionidae,Polychaeta, Annelida) from Bellingshausen Sea (West Antarctica)

During the Antarctic summers of 2002–2003 and of 2005–2006, the Spanish BENTART cruises were conducted to the Bellingshausen Sea (Western Antarctica), aiming to study its benthic communities, from depths ranging from 100 to 2,000 m. To achieve it, 30 stations were selected; each one was surveyed in such a way that the infaunal, epifaunal and suprabenthic components of the communities were sufficiently characterized. As a part of the study, some spionid individuals were identified as belonging to a new species of the genus Laonice Malmgren, 1867. The new species belongs to a group within the genus that is characterized by the presence of more than two rows of very numerous capillary chaetae in both noto- and neuropodial fascicles of anterior part of the body. However, it can be readily distinguished from the rest of species within the group by the posterior position in which neuropodial pouches appear (chaetiger 16 or 17) and by the caruncle reaching posteriorly chaetiger 19. In addition, other remarkable features of the new species are the short and triangular occipital tentacle, the rudimentary eyes, the hooded neuropodial hooks first appearing in chaetigers 34–37 and the sabre neurochaetae first occurring in chaetigers 20–27.     

Chaetal arrangement in Orbiniidae (Annelida,Polychaeta) and its significance for systematics

The systematic position of Orbiniidae within Polychaeta is still uncertain. In order to provide additional comparative data, we investigated the chaetal arrangement in this family, which is considered valuable for polychaete systematics. Specimens of Scoloplos armiger, Orbinia latreillii, and Pettibonella multiuncinata were examined by SEM and serial sections analysed by computer aided 3D-reconstructions. The obtained data suggest that the chaetal arrangement of Orbiniidae resembles that of other sedentary polychaetes in only a few respects. Transverse rows are only present in the main, anterior part of the chaetal patches of thoracic neuropods. The position of the formative site indicates homology with the transverse rows of several sedentary polychaete taxa. The chaetal patches thus differ significantly from those known in Apistobranchidae. Independent rows with an own caudal formative site, which run along the caudoventral edge of the chaetal patches, resemble the neuropodial ventral longitudinal rows known in Spionidae and related taxa. The abdominal neuropodia of S. armiger and O. latreillii bear longitudinal rows of chaetae. These are reorientated during ontogenetic chaetiger transformation and become the transverse rows of the thoracic chaetal patches. 3D reconstruction of S. armiger revealed that the notopodial chaetal bundles are organized in rows as well. Notopodia and abdominal neuropodia bear deep reaching supportive chaetae. They are the first chaetae formed during neuropodial development and reside dorsally to the longitudinal row of capillary chaetae. Neither position nor structure indicates homology with the supportive chaetae of other sedentary polychaetes. Spionidae and related taxa are thus the only sedentary polychaetes, which specifically resemble Orbiniidae in certain aspects of their chaetal arrangement. Dedicated to Prof. Dr. Wilfried Westheide on the occasion of his 70th birthday.     

The Magelonidae (Annelida: Polychaeta) from the Seychelles, with the description of three new species

Three new species of Magelona are described from the Seychelles: M. conversa, M. falcifera and M. gemmata. Magelona conversa belongs to a 'M. mirabilisgroup', having a rounded prostomium and specialised chaetae on chaetiger 9, but differs from all other members in having long prechaetal neuropodial lamellae on chaetigers 1–8. Magelona falcifera bears large sickle-shaped hooded hooks in the abdomen and thus approaches the genus Meredithia, but lacks prostomial horns. Magelona gemmata belongs to a 'M. longicornis group' in having distinct prostomial frontal horns, and a thorax with lanceolate postchaetal lamellae in the notopodia and ventral neuropodial lobes. The distinctively swollen bud-like tips on the notopodial lamellae of chaetiger 9 are an unique feature. The status of MeredithiaHernández-Alcántara & Solís-Weiss, 2000 and current magelonid terminology are discussed.     

Parapodial glandular organs in Spiophanes (Polychaeta: Spionidae)—studies on their functional anatomy and ultrastructure

Parapodial glandular organs (PGOs) of Spiophanes (Polychaeta: Spionidae) were studied using light and electron microscopy. These organs are found in parapodia of the mid body region, starting on chaetiger 5 and terminating with the appearance of neuropodial hooks (chaetiger 14 or 15 in adult individuals). Large PGOs in anterior chaetigers display different species‐specific types of openings whereas small PGOs in posterior parapodia of the mid body region always open in a simple vertical slit. Each PGO is composed of three main complexes: (1) the glandular sac with several distinct epithelia of secretory cells and secretory cell complexes and the reservoir filled with fibrous material, (2) the gland‐associated chaetal complex (including the region of chaetoblasts and follicle cells, follicular canals, two chaetal collector canals, the combined conducting canal, the chaetal spreader including the opening of the glandular organ with associated type‐1 secretory cells, and the gland‐associated chaetae), and (3) a bilayered musculature surrounding the gland. A considerable number of different cell types are involved in the secretory activity, in the guidance of the gland‐associated chaetae, and in the final expulsion of the fibrous secretion at the opening slit. Among these different cell types the type‐5 secretory cells of the proximal glandular complex with their cup‐shaped microvilli emanating thick microfibrils into the lumen of the glandular sac are most conspicuous. Secretory cells with cup‐shaped microvilli being involved in the production of β‐chitin microfibrils have so far only been reported from some representatives of the deep‐sea inhabiting Siboglinidae (Polychaeta). We suggest that the gland‐associated chaetae emerging from inside the PGOs of Spiophanes are typical annelid chaetae formed by chaetoblasts and follicle cells. Functional morphology implies the crucial role of PGOs in tube construction. Furthermore, the PGOs are discussed in consideration of phylogenetic aspects. J. Morphol., 2012. © 2011 Wiley Periodicals, Inc.     

Cell interactions in the generation of chaetae pattern inDrosophila

Summary We have already shown that theachaetae-scute complex (AS-C) ofDrosophila is regulated by two genes,hairy andextramacrochaetae. Using mutants in these genes, we have analysed how different levels of expression of AS-C affect the pattern of chaetae. The results indicate that the spatial distribution of chaetae results from cell interactions, probably by a mechanism of lateral inhibition. The results are discussed in view of the different theories of pattern formation.     

The anatomy and histology of the nervous system and excretory system of the maldanid polychaetes Clymenella torquata and Euclymene oerstedi

The nervous system of the maldanid polychaetes Clymenella torquata (Leidy) and Euclymene oerstedi (Claparede) (= Caesicirrus neglectus [Arwidsson, '11-'12]) retains its primitive association with the epidermis. It shows only slight metamerism in the presence of larger collections of neurones opposite the parapodia and of larger nerves at the segmental boundaries. Multicellular giant fibers are present in the ventral nerve cord; giant neurones which show a characteristic pattern of distribution in each species are also present. The cerebral ganglia supply nerves to the prostomial wall, nuchal grooves and the wall of the buccal cavity, and a pair of large nerves from the circumpharyngeal connectives also appear to join the buccal system. The organs of special sense are the elongated prostomial nuchal grooves, and prostomial ocelli in Euclymene but not in Clymenella. Statocysts are absent. Four pairs of nephromixia are present. They lie in the aseptate anterior trunk, in chaetigers 5–9 of Clymenella, and 6–10 of Euclymene. The nephridiopores lie at the ventral ends of the neuropodia of chaetigers 6–9 and 7–10, respectively. Each nephromixium consists of coelomostome, tubule and contractile bladder. The wall of the tubule and bladder consists of both excretory and ciliated cells. Most of the cytoplasm of the latter forms a bounding layer at the outer surface. The cytoplasm of the excretory cells contains lipid material and appears to synthesize lipofuscin. The tips of the excretory cells swell, fill with granules, and break off in the form of vesicles which are periodically expelled in clouds from the nephridiopores. Glycogen is present, especially in the ciliated cells of the tubule and coelomostome. Granules of a lipoid nature accumulate in (or between) cells of the nephridia, epidermis, and some regions of the gut, and may be excretory. Lipid granules also appear to be synthesized by coelomocytes which eventually end up in masses in the ventrolateral coelomic cavities of the tail. The nephridia act as gonoducts, but show no seasonal variation in either size or histological structure.     

Chaetal arrangement and chaetogenesis of hooded hooks in Lumbrineris (Scoletoma) fragilis and Lumbrineris tetraura (Eunicida,Annelida)

As the structure and arrangement of chaetae are highly specific for annelid species and higher taxonomic entities, we assume that rather conservative information guarantees formation of specific chaetae. Each chaeta of an annelid is formed within an ectodermal invagination, and the modulation of the apical microvilli pattern of the basalmost cell of this invagination determines the structure of the chaeta. Any hypothesis of the homology of chaetae could thus be tested by examining the process of chaetal formation. Investigations into the ultrastructure and formation of hooded hooks in different capitellids and spionids revealed that these chaetae can be homologized. The hood of each of their hooded hooks is formed by elongation of two rings of microvilli peripheral to the chaetal anlage, which give rise to the inner and outer layers of the hood. The hood layers are well separated and surround an empty space. Superficially similar hooded hooks are described for certain Eunicida. Presently available cladistic analyses suggest that the hooded hooks of eunicidans evolved independently of those in Capitellidae and Spionidae. Compared with the latter two families, we therefore expected to find differences in chaetogenesis of the hooded hooks in the eunicids Lumbrineris (Scoletoma) fragilis and Lumbrineris tetraura (Lumbrineridae). This was the case. In these eunicidans, the hood was formed by the bisected apical wall of the chaetoblast right after the mid‐apical section of the chaeta had been sunk deeply into the chaetoblast during its formation. The apical wall generated a brush of microvilli that preformed the hood. Because the microvilli of the hood showed some accelerated differentiation, they soon merged with those of the slowly growing setal shaft to form the broad manubrium of the hooded hook in lumbrinerids. Our study confirms the predicted differences in chaetogenesis of the superficially similar hooded hooks of capitellids and spionids compared with those of eunicids.     

Systematic revision of Entomobryidae (Collembola) by integrating molecular and new morphological evidence

Entomobryidae, the largest collembolan family, is traditionally classified at suprageneric level using a limited set of morphological structures, such as scales, antennal segmentation. Most tribal and subfamilial delimitations appear, however, disputable in the light of recent works. Integrating molecular and morphological evidence, we propose here a revision of the systematics of the family. In addition to traditional taxonomic characters, tergal specialized chaetae (S‐chaetae) are newly introduced, and their patterns are shown to be diversified at all levels from species to subfamilies. S‐chaetotaxic pattern on phylogenetic tree shows that evolution of S‐chaetae is not parallel between the different terga and that their patterns coincide well with the known molecular phylogeny, providing a powerful tool for the systematics of Entomobryidae. Orchesellinae sensu Soto‐Adames et al. (Annals of the Entomological Society of America, 101, 2008, 501); is divided into three subfamilies: Orchesellinae s. s., Bessoniellinae and Heteromurinae, the latter two upgraded from the original tribal level. Entomobryinae sensu Szeptycki (Morpho‐Systematic Studies on Collembola. IV. Chaetotaxy of the Entomobryidae and its Phylogenetical Significance, 1979), is no longer divided into scaled and unscaled tribes, and Lepidosira‐group is transferred from Seirinae to Entomobryinae. A key to subfamilies and tribes and a comparison with previous classifications of the Entomobryidae are provided. This study greatly improves the understanding of primary and secondary characters and erects the fundamental framework for the taxonomy of Entomobryidae.     

Two new species of Mediomastus (Annelida,Capitellidae) from Tokyo Bay,Japan

Two undescribed species of polychaetes in Mediomastus (Annelida: Capitellidae) were collected from intertidal to shallow habitats in Tokyo Bay, Japan. These are M. duobalteus sp. n. and M. hanedaensis sp. n. Mediomastus duobalteus sp. n. is distinguishable from all congeners by the following characters: 1) segments 3, 4, 8–11 stainable with methyl green, 2) thoracic capillary chaetae unilimbate, 3) abdominal capillary chaetae absent, 4) paddle-like chaetae in the thorax absent, and 5) abdominal hooded hooks not flared. Mediomastus hanedaensis sp. n. is similar to M. warrenae Green, 2002, but differs from the latter in the shapes of the thoracic capillary chaetae and the abdominal hooded hooks, and the staining pattern with methyl green. In addition, a key to all Mediomastus species is provided.     

Developmental stability in hybrids between the sibling species pair,Drosophila melanogaster and Drosophila simulans

Drosophila melanogaster and its sibling species D. simulans were hybridized in the laboratory to test the hypothesis that developmental homeostasis in hybrids between two species having no prior gene flow would be significantly reduced. Developmental stability was assessed by measuring fluctuating asymmetry for three bilateral traits: sternopleural chaetae, wing length, and fronto-orbital plus frontal chaetae. Male F₁ hybrids showed no decrease in developmental stability compared to males of parental species. Female hybrids showed significant fluctuating asymmetry compared to other flies. The results are discussed with respect to ideas about coadaptation and gene flow based upon previous studies of hybrid developmental stability.     

External structures used during attachment and sperm transfer in tubificids (Annelida,Oligochaeta)

The genital region of seven species of Tubificidae has been studied by SEM (Scanning Electron Microscopy). The form and the position of penial and spermathecal chaetae, male and spermathecal pores and other special structures have been examined. Peristodrilus montanus shows a special system to hold the partner: the penial chaetae anchor in an elaborated structure of the body wall formed between the spermathecal pores, the `anchorage bridge'. Protuberodrilus tourenqui has a long glandular porophore with the male pores at the tip, allowing contact with the spermathecal pores which are located in deep, close to the mid-ventral line of the body. The grooved and strongly curved penial chaetae of Rhyacodrilus falciformis seem to be used both for attachment and for sperm transfer, entering into the lateral spermathecal pores. The embrace of the partners, as suggested by observations on Psammoryctides barbatus, Potamothrix bavaricus, Potamothrix hammoniensis and Potamothrix heuscheri, seems to be another important mechanism to fix contact between male and spermathecal pores and allow sperm transfer. The spermathecal chaetae could be interpreted as piercing chaetae with a chemical or mechanical stimulating role. Sensitive cilia near the penial chaetae seem to be used by the three rhyacodrilines studied to find the correct anchorage place. There is a great variety of structures which appear to be used for attachment and sperm transfer in tubificids, and consequently their role in the evolution of the whole family may be profound.     

Morphological reinvestigation and phylogenetic relationship of Acanthobdella peledina (Annelida,Clitellata)

Summary In recently collected specimens of Acanthobdella peledina the nervous system, the genital organs and the coelomic organisation were reinvestigated after complete serial sections. These anatomical results are schematically represented. In addition, the integument, the chaetae and the peripheral muscle layer were investigated by electron microscopy. In general, the results confirm Livanow's classic monograph (1906), with the exception of a few details. The body apparently possesses neither a prostomium nor an achaetous buccal region (peristomium). The number of 29 true segments is concluded from the number of segmental ganglia. The five anteriormost segments, each with four pairs of hookshaped chaetae arranged around the mouth opening, are considered to be functionally equivalent to an anterior sucker. The ultrastructure of the integument and the chaetae generally conforms to the typical annelidan pattern. The muscle cells are of the typical hirudinean type. The outer male genital pore is positioned in segment 10; the female organs open in segment 11 directly behind the septum between segments 10 and 11. The main emphasis is laid on the evaluation of the position of the taxon within the Clitellata, including a discussion of the Branchiobdellida, and the cladograms presented show the Acanthobdellida to be the sister group of the Euhirudinea. Characters shared by the Branchiobdellida and Hirudinea (including A. peledina) are considered to be convergently evolved.     

Chaetae and chaetogenesis in polychaetes (Annelida)

Annelid chaetae are epidermal extracellular structures that are in general clearly visible from the exterior. Their structure is highly diverse, especially within the Polychaeta, and each species shows a specific pattern of chaetae. Chaetae have therefore gained immense significance for species determination, making them the best studied structures in polychaetes. The shape of chaetae is determined by the temporal and spatial modification of the microvilli pattern of a single cell, the chaetoblast. As chaetae are species specific, the process of their formation must be under strict control and the information needed to form certain chaetae must be highly conservative. It can be assumed that corresponding chaetogenesis is caused by commonly inherited information. Thus, comparative chaetogenesis can help to test hypotheses on the homology of certain types of chaetae and help to unravel the influence of functional constraints on the shape of chaetae. Different types of chaetae are compared here and the present state of our knowledge of their structure and formation is used to present some homology hypotheses. There are some strong arguments for a homology of uncini and certain hooks and hooded hooks. Acicula are compared to other supportive setae and the significance of the arrangement of chaeta for phylogenetic considerations is shown. Coding issues are provided in order to facilitate inclusion of information on chaetae into data matrices.     

Getting to the root of fireworms' stinging chaetae—chaetal arrangement and ultrastructure of Eurythoe complanata (Pallas, 1766) (Amphinomida)

Amphinomid species are since long known to cause urtication upon contact with the human skin. Since it has been reported that amphinomid chaetae are hollow, it has repeatedly been suggested that poison is injected upon epidermal contact. To test predictions for the structural correlate of such a stinging device we studied the structure and formation of chaetae in the fireworm Eurythoe complanata (Amphinomida). Neither the structure of the chaetae nor their formation and their position within the parapodium provide evidence for their function as hollow needles to inject poison. The chaetae even turned out to be not hollow, but containing calcareous depositions. The latter most likely cause artificial ruptures of delicate chitin lamellae in the inner of the chaeta when treated with acidic fixatives. Inorganic calcium compounds harden the chaetae and make them brittle so that they break easily. Additional information on the structure of the chaetal sac, the site of formation and the acicula do not contradict the position of the Amphinomida within Annelida as revealed by phylogenomic studies.     

Chaetal arrangement provides no support for a close relationship of Sabellidae and Sabellariidae (Annelida)

Sabellid and sabellariid polychaetes are regarded as sister groups in a number of recent phylogenetic analyses. This is based mainly on a shared specific arrangement of chaetae referred to as chaetal inversion. Remarkably, the uncini have a notopodial position in the abdomen, whereas capillary chaetae occur in the neuropodia in both taxa in contrast to the situation in putative relatives. However, in sabellids uncini and capillary chaetae change their position completely at the border between thorax and abdomen, whereas uncini are missing in the parathorax of Sabellariidae. Due to this difference the significance of the chaetal inversion for systematics has been subject to discussion for years. Serial semithin sections of parapodia of the Sabellidae Sabella pavonina, Branchiomma bombyx, Fabricia stellaris, and of the Sabellariidae Sabellaria alveolata were studied in order to obtain detailed information on their chaetal arrangement and sites of chaetal origin. SEM investigations and computer-aided 3D-reconstructions provide deep insight into the spatial organization of the rami. Though differing externally, the principal chaetal arrangement and the location of the formative sites turned out to be almost identical within the species of Sabellidae. Most chaetae are aligned in straight transverse rows with a dorsal site of origin within neuropodia and a ventral one in notopodia as is common in sedentary polychaetes. Semicircular and spiral arrangements are revealed to be modified transverse rows. Only in thoracic notopodia does an additional dorsocaudal formative site form distinct rows. The chaetal inversion in Sabellidae is additionally characterized by an abrupt change of capillary chaetae and uncini along with a sudden change of the parapodial morphology at the border between thorax and abdomen. All chaetae of S. alveolata are aligned in transverse rows with the same location of the formative sites as in sabellids and other sedentary polychaetes. However, in contrast to sabellids the chaetae are not inverted across a parathoracic abdominal border. Moreover, there is no inversion of the parapodial structure from parathorax to abdomen and the neuropodial chaetal composition changes gradually from parathorax to abdomen. The chaetal arrangement in Sabellariidae thus cannot be described as inverted along the body-axis as in Sabellidae. Evolutionary steps implied by the assumption of an inverted chaetal pattern in a supposed common ancestor are discussed. It is concluded that the specific chaetal arrangement of Sabellidae and Sabellariidae arose independently and therefore provides no support for a sistergroup relationship of sabellids and sabellariids.     

Intra- and interspecific ultrastructural character variation: The chaetation of the Microphthalmus listensis species group (Polychaeta,Hesionidae)

Summary The different chaetal types of three eidonomically very similar, closely related polychaete species (Microphthalmus carolinensis, M. nahantensis and M. listensis) were ultrastructurally investigated and compared with one another. Characteristic substructures are regularly arranged channels, the course and number of which were analysed in cross-sections. The intraspecific variability of the number of these channels is low in the individual chaetal type. Between the species, however, distinct differences exist, facilitating separation and identification of the species at the level of ultrastructure in certain chaetae.     

Two new species of Coecobrya (Collembola,Entomobryidae) from China,with an updated key to the Chinese species of the genus

Two new Coecobrya species, which were newly collected in 2014, are described from China. Coecobrya sanmingensis sp. n. from southeast China (Fujian) is the fourth 1+1 eyed species in the genus; it can be distinguished from other three species by the ciliate chaetae X and X_2-4 on the ventral side of head, the abundant chaetae on the trochanteral organ, a large outer tooth on the unguiculus, the absence of smooth manubrial chaetae, and the dorsal chaetotaxy. Coecobrya qinae sp. n. from southwest China (Yunnan) is characterized by paddle-like S-chaetae of Ant. III organ, ciliate chaetae X, X₂ and X₄ posterior to labium, medial macrochaetae on the mesothorax, and 5+5 central and 2+2 lateral macrochaetae on the fourth abdominal segment. An updated key to the Chinese species of Coecobrya is given.