Ultrastructure of nuchal organs in some marine polychaetes

Polychaetes normally possess one pair of nuchal organs at the posterior edge of the prostomium or peristomium. They have been regarded as chemosensory organs. The nuchal organs of four marine polychaete species with different habits were investigated by electron microscopy. Although the shapes of nuchal organs can vary greatly from simple ciliary bands (Scolelepis squamata, Spionidae) to retractile tongue-like, piston- or finger-shaped forms (Eteone longa, Anaitides mucosa, Phyllodocidae; Heteromastus filiformis, Capitellidae), the structural components, including the ciliated supporting cells, sensory cells, and nuchal epidermal cells, are essentially similar. The differences basically concern 1) the position of the sensory cells with relation to the ciliated supporting cells, 2) the location and structure of the nuchal nerve, and 3) the structure of the nuchal cuticle. The diverging nature of this modified cuticle is described and discussed in detail. Comparisons are made with the fine structure of nuchal organs of other polychaete species. Similarities of cellular components of nuchal organs are found not only in the four species studied here but also in all nuchal organs investigated so far. This is hypothesized to be due to the fact that the polychaete stem species already possessed nuchal organs with the respective cell types. Differences in the number and distribution of cellular components and in the overall shape of nuchal organs are thought to have evolved in correlation with the equipment of other cephalic appendages and with different habits and modes of nutrition.     

Ultrastructural investigations on the cephalic and metameric nuchal organs of Spio cf. filicornis (Polychaeta, Spionidae)

The nuchal organs of Spio cf. filicornis from northern Europe have been studied by scanning and transmission electron microscopy. Spio cf. filicornis is the first species in which metameric nuchal organs are described. The nuchal organs consist of a distinct cephalic nuchal complex followed by metameric structures for a variable number of chaetigers. Their microanatomy corresponds to the general structural plan of nuchal organs: these are ciliated supporting cells and bipolar sensory cells with sensory cilia traversing an olfactory chamber. The organs are overlaid by a secondary paving-stone-like cover and innervated by longitudinally elongated paired nuchal nerves. The findings clearly favour the hypothesis that the paired metameric ciliated structures found in some Spionidae are in fact homologous with the prostomial nuchal organs characteristic of Polychaeta.     

Ultrastructural studies of dorsal ciliated organs in Spionidae (Annelida: Polychaeta)

The distribution and structural components of dorsal ciliated organs (dco) in 15 species of the Spionidae were studied by scanning- and transmission electron microscopy. Based on the distribution patterns of dco, the investigated species are divided into four non-systematic groups: (I) paired anterior dco, (II) paired dco extending posteriorly for several chaetigers, (III) paired anterior dco in combination with unpaired, sexually dimorphic, metameric dco, and (IV) paired anterior dco in combination with paired, metameric dco. Previous ultrastructural studies have only included species possessing organs of groups I and III. In the present investigation the ultrastructure of dco found in Laonice bahusiensis and Spio cf. filicornis (species with dco of groups II and IV) is studied in an attempt to consider their homology. Apart from the metameric dco of group III, similarities of the cellular components of the dco indicate a homology to nuchal organs.     

Ultrastructure of the nuchal organs in the polychaete Platynereis dumerilii (Annelida,Nereididae)

Abstract. We examined the nuchal organs of adults of the nereidid polychaete Platynereis dumerilii by means of scanning and transmission electron microscopy. The most prominent features of the nuchal organs are paired ciliary bands located dorsolaterally at the posterior margin of the prostomium. They are composed of primary sensory cells and multiciliated supporting cells, both covered by a thin cuticle. The supporting cells have motile cilia that penetrate the cuticle and are responsible for the movement of water. Subapically, they have a narrowed neck region; the spaces between the neck regions of these supporting cells comprise the olfactory chamber. The dendrites of the sensory cells give rise to a single modified cilium that crosses the olfactory chamber; numerous thin microvillus-like processes, presumably extending from the sensory cells, also traverse the olfactory chamber. At the periphery of the ciliated epithelium runs a large nervous process between the ciliated supporting cells. It consists of smaller bundles of sensory dendrites that unite to form the nuchal nerve, which leaves the ciliated epithelium basally and runs toward the posterior part of the brain, where the perikarya of the sensory cells are located in clusters. The ciliated epithelium of the nuchal organs is surrounded by non-ciliated, peripheral epidermal cells. Those immediately adjacent to the ciliated supporting cells have a granular cuticle; those further away have a smooth cuticle. The nuchal organs of epitokous individuals of P. dumerilii are similar to those described previously in other species of polychaetes and are a useful model for understanding the development of nuchal organs in polychaetes.     

Ultrastructural investigation of the nuchal organs of Pygospio elegans (Polychaeta)

Summary The differentiation of the dorsal organs as well as the structure of the nuchal organs and their relation to the central nervous system in adult Pygospio elegans were studied by electron microscopy and compared to the nuchal organs of the larvae. The nuchal organs are represented by paired ciliary bands on the dorsal side of the first setiger, delimiting a median caruncle that is completely filled with epidermal and nervous tissue. They are composed of ciliated supporting cells and bipolar primary sensory cells constituting the nuchal ganglia, which are integrated into the brain. Microvillus-like processes of the ciliated cells give rise to a secondary covering layer over the sensory epithelium. The size of the nuchal organs is a sexually dimorphic feature.Dorsal organ formation is concomitant with the onset of sexual maturation in the male sex only. They appear as metameric ciliary bands on the dorsal side of the anterior body region and consist of ciliated cells accompanied by lateral accumulations of tubular gland cells. In the gametogenic segments they are structurally associated with the male genital pores and may be involved in reproduction. The results refute previous theories that dorsal organs are sensory and have a common origin to nuchal organs.Abbreviations ac anterior commissure of the brain - ace anterior circumesophageal connective - bb basal body - bl basal lamina - c cuticle - ca caruncle - cc ciliated cell - ci sensory cilium - co microvillar cover - d septate desmosome - db dorsal blood vessel - dn dorsal nerve cord - ea efferent axons - ec epidermal cell - eg elementary granules - g Golgi complex - i filamentous inclusion - lm longitudinal muscles - ly lysosome - mc motile cilia - mv microvillus - n neuron - ng nuchal ganglion - nn nuchal nerve - nu nucleus - oc olfactory chamber - pa palp - pc posterior commissure of the brain - pce posterior circumesophageal connective - rer rough endoplasmic reticulum - sI setiger I - sb sensory bulb - sc sensory cell - sd sensory dendrite - ser smooth endoplasmic reticulum - tf tonofilament bundle - v clear vesicles - za zonula adherens     

Photoreceptive organs in larvae of Spionidae (Annelida) and Sipuncula

A pair of spherical unpigmented ocelli in addition to pigmented eyes have been observed in the anterior part of the prostomium in larvae of more than 40 species of Spionidae examined alive with light microscopy. Ocelli become visible in larvae at the one- to three-segment stage, increase in size as growth proceeds, and probably disintegrate in the course of metamorphosis because they were not observed in adults and settled juveniles. One pair of transparent, spherical bodies is also found in the anterior part of the head of planktotrophic pelagosphera larvae of Sipuncula. These bodies are in a similar position and have a similar appearance and size to unpigmented ocelli in Spionidae larvae. A pair of epidermal invaginations, densely covered with short cilia, is also observed antero-laterally in the head in pelagospheras of one species. These invaginations appear similar to the nuchal organs present in many polychaete larvae. Photoreceptive organs so far reported for sipunculan larvae comprise only pigmented eyes. Unpigmented ocelli and nuchal organs have never been reported in pelagospheras. Further ultrastructural investigations on sipunculan larvae are encouraged to clarify the composition, function and morphogenesis of transparent, spherical bodies and ciliated invaginations in the anterior part of the head. Such investigations may help to better understand the nature of photoreceptive structures and nuchal organs in Sipuncula, and also contribute to phylogenetic hypotheses regarding relationships of the Sipuncula and Annelida.     

Ultrastructure of the nuchal organ and cerebral organ in Onchnesoma squamatum (Sipuncula, Phascolionidae)

 The ultrastructure of the nuchal organ and cerebral organ is described for the first time in a species of the Sipuncula, Onchnesoma squamatum. The nuchal organ is an unpaired structure lying outside and dorsal to the tentacular crown; furrows give the organ a paired appearance. The cerebral organ is an unciliated pad anterior to the nuchal organ. The nuchal organ consists of ciliated supporting cells, non-ciliated supporting cells and bipolar primary sensory cells. The cerebral organ is composed of unciliated supporting cells and numerous bipolar sensory cells. This clearly favours the hypothesis that this structure has a sensory function in adults rather than being a vestige of a larval organ. The sensory cells are similar in both organs and exhibit features indicative of chemoreception. Since the density of the sensory cells is low in the nuchal organ, an exclusively sensory function is questioned. There is some evidence that the two organs represent a functional unit. The present findings do not support the view that the nuchal organs of Sipuncula and ”Polychaeta” are homologous, but instead suggest that they are convergent structures. Accepted: 18 September 1996     

Ultrastructural investigation of the nuchal organs ofPygospio elegans (Polychaeta). I. Larval nuchal organs

The structural differentiation of the nuchal organs during the post-embryonic development ofPygospio elegans is described. The sensory organs are composed of two cell types: ciliated cells and bipolar primary sensory cells, constituting the nuchal ganglion, which is associated with both the sensory epithelium and the brain. Since the sensory neurons are largely integrated into posterolateral parts of the cerebral ganglion, the nuchal organs are primary presegmental structures. The microvilli of the ciliated cells form a cover over the cuticle with a presumed protective function. An extracellular space extends between cuticle and sensory epithelium. The distal dendrites of the sensory cells terminate in sensory bulbs, bearing one modified sensory cilium each that projects into the olfactory chamber, embedded within the secretion of the ciliated cells. During development, the nuchal organs increase in size. This is accompanied by a shift in position, an expansion of the sensory area, and secretory activity of the ciliated cells. The nuchal ganglion differentiates into three nuchal centres forming three distinct sensory areas around the ciliated region. Each nuchal complex reveals two short nuchal nerves comprising the sensory axons, which enter the posterior circumesophageal connective. The sensory cells lying in the brain exhibit neurosecretory activity; the sensory cilia enlarge their surface area by dilating and branching. Nuchal organs accomplish the basic structural adaptions of chemoreceptors and show structural analogies to arthropod olfactory sensilla; thus, there is every reason to suppose chemoreceptor function.     

Ultrastructure of presumptive light sensitive ciliary organs in larvae of Poecilochaetidae,Trochochaetidae, Spionidae,Magelonidae (Annelida) and its phylogenetic significance

Larvae of Poecilochaetus serpens, Trochochaeta multisetosum and Polydora ciliata possess almost identical unpigmented, ciliary, presumptive light sensitive organs within the prostomium. The data corroborate hypotheses on the close relationship of Poecilochaetidae, Trochochaetidae and Spionidae and are even congruent with inclusion of Poecilochaetidae and Trochochaetidae within Spionidae. The organs in P. serpens, T. multisetosum and P. ciliata are composed of one monociliary receptor cell, one supportive cell and several associated flask shaped bipolar sensory cells. The receptor cell cilium enters the supportive cell cavity through a thin pore, dilates and then branches into a high number of disordered projections. The associated sensory cells bear one or occasionally two cilia, which run horizontally beneath or within the cuticle. The supportive cell cavity is not sealed by any cell contact from the subcuticular extracellular space. The organs in Magelona mirabilis are composed of a single supportive cell, but several receptor cells. No further sensory cells are associated. Each receptor cell sends one cilium into an own invagination of the supportive cell, and the ciliary branches are highly ordered. The examined organs in P. serpens, T. multisetosum and P. ciliata exhibit a unique organization amongst polychaetes. The organs of M. mirabilis are most probably homologous. A homology to ciliary organs of Protodrilida is conceivable. In the lineage leading to Protodrilida, primary larval organs may have been integrated into the adult body organization by heterochrony.     

Ultrastructure of the nuchal organ in the interstitial polychaete Stygocapitella subterranea (Parergodrilidae)

The nuchal organs of Stygocapitella subterranea are paired narrow pits. They are lined by unciliated cells at the opening and by ciliated cells at the basal parts. The primary sensory cells (6–8) are arranged in a single patch at the bottom of the nuchal pit. The nuclei of the sensory cells are located in the posterior portion of the brain. Their dendrites form the nuchal nerve which is sheathed by the ciliated cells. Each sensory cell bears up to 4 modified sensory cilia and several microvilli extending into the olfactory chamber. The sensory cilia show various patterns of axonemal organization and have no rootlets. The olfactory chamber is covered by a cuticular matrix. Another primary sensory cell lies at the opening of the nuchal pit. It bears cilia which penetrate the cuticle but are enveloped by the epicuticle. Retractor muscles insert caudally on the organ. The nuchal organ of S. subterranea shows similarities to those of opheliids but exhibits several features not to be found in other nuchal organs.     

Sense organs in polychaetes (Annelida)

Polychaetes possess a wide range of sensory structures. These form sense organs of several kinds, including the appendages of the head region (palps, antennae, tentacular cirri), the appendages of the trunk region and pygidium (parapodial and pygidial cirri), the nuchal organs, the dorsal organs, the lateral organs, the eyes, the photoreceptor-like sense organs, the statocysts, various kinds of pharyngeal papillae as well as structurally peculiar sensory organs of still unknown function and the apical organs of trochophore larvae. Moreover, isolated or clustered sensory cells not obviously associated with other cell types are distributed all over the body. Whereas nuchal organs are typical for polychaetes and are lacking only in a few species, all other kinds of sensory organs are restricted to certain groups of taxa or species. Some have only been described in single species till now. Sensory cells are generally bipolar sensory cells and their cell bodies are either located peripherally within the epidermis or within the central nervous system. These sensory cells are usually ciliated and different types can be disinguished. Structure, function and phylogenetic importance of the sensory structures observed in polychaetes so far are reviewed. For evaluation of the relationships of the higher taxa in Annelida palps, nuchal organs and pigmented ocelli appear to be of special importance.     

Ultrastructure of Nuchal Organs in Polychaetes (Annelida) — New Results and Review

Nuchal organs are epidermal sensory structures present in most polychaetes. They are situated at the posterior edge of the prostomium and may extend posteriorly onto the peristomium. Although there is considerable external variation, they all consist of ciliated supporting cells, bipolar primary sensory cells and retractor muscles. They are innervated directly from the brain by paired nerves. The sensory cells are usually monociliated; their sensory processes lie in subcuticular spaces, the olfactory chambers. Structural variability is to be observed in the location of the sensory cells, the course of the nuchal nerve, position of nuchal ganglia as well as in cytological features of sensory and supporting cells. These differences provide useful characters for phylogenetic considerations to establish supraspecific taxa within the phylogenetic system of the Annelida. Special emphasis is laid on the problem of whether the nuchal organs represent an autapomorphy of the Polychaeta or the Annelida and thus whether the lack of nuchal organs in Clitellata is primary or secondary. As is discussed, the probability of a loss of the nuchal organs in Clitellata is higher, which favours the second hypothesis: that nuchal organs are part of the ground pattern of the Annelida and very likely are an autapomorphy of this group.     

Histochemical evidence of β‐chitin in parapodial glandular organs and tubes of Spiophanes (Annelida,Sedentaria: Spionidae), and first studies on selected Annelida

A generic character of the genus Spiophanes (Annelida, Sedentaria: Spionidae) is the presence of parapodial glandular organs. Parapodial glandular organs in Spiophanes species include secretory cells with cup‐shaped microvilli, similar to those present in deep‐sea inhabiting vestimentiferans and frenulate Siboglinidae. These cells are supposed to secrete β‐chitin for tube‐building. In this study, transverse histological and/or ultrathin sections of parapodial glandular organs and tubes of Spiophanes spp. as well as of Glandulospio orestes (Spionidae) and Owenia fusiformis (Oweniidae) were examined. Fluorescent markers together with confocal laser scanning microscopy, and Raman spectroscopy were used to detect chitin in the parapodial glandular organs of Spiophanes and/or in the glands of Owenia and Glandulospio. Tubes of these taxa were tested for chitin to elucidate the use of it for tube‐building. The examinations revealed a distinct labelling of the gland contents. Raman spectroscopy documented the presence of β‐chitin in both gland types of Spiophanes. The tubes of Spiophanes were found to have a grid‐like structure that seems to be built with this β‐chitin. Tests of tubes of Dipolydora quadrilobata (Spionidae) for chitin were negative. However, the results of our study provide strong evidence that Spiophanes species, O. fusiformis and probably also G. orestes produce chitin and supposedly use it for tube‐building. This implies that the production of chitin and its use as a constituent part of tube‐building is more widespread among polychaetes as yet known. The histochemical data presented in this study support previous assumptions inferring homology of parapodial glandular organs of Spionidae and Siboglinidae based on ultrastructure. Furthermore, transmission electron microscopy‐based evidence of secretory cells with nail‐headed microvilli in O. fusiformis suggests homology of parapodial grandular organs across annelids including Sipuncula. J. Morphol. 276:1433–1447, 2015. © 2015 Wiley Periodicals, Inc.     

Lateral organs in sedentary polychaetes (Annelida) – Ultrastructure and phylogenetic significance of an insufficiently known sense organ

Lateral organs are sense organs visible as densely ciliated pits or papillae between the noto‐ and the neuropodia in certain taxa of sedentary polychaetes. Ultrastructural studies in about 10 species of the following taxa Maldanidae, Opheliidae, Orbiniidae, Paraonidae, Magelonidae, Spionidae, Poecilochaetidae and Terebellidae have been designed to evaluate whether these organs are homologous among polychaetes. In spite of great external diversity, the investigations revealed an overall ultrastructural similarity. Differences between species investigated mainly concern the size of the organs as well as the number and arrangement of cells. The organs comprise supportive cells and uniciliated penetrative sensory cells. Their dendrites are closely arranged and thus their cilia may resemble multiciliated cells. There are two types of sensory cells: one type possesses no or mainly thin microvilli of which usually only a few reach the cuticular surface, and in the other type the cilium is consistently surrounded by 10 strong microvilli, which form a pore‐like opening in the cuticle. Further differences occur in the structure of the rootlet system. Basally, a retractor muscle attaches to the organ. The systematic significance of these organs within Annelida is discussed with respect to the conflicting phylogenetic hypotheses explaining the relationships of annelid taxa.     

Morphology of anterior regeneration in two spionid polychaete species: implications for feeding efficiency

Abstract. In marine soft‐sediment habitats, tissue loss by infuanal invertebrates can provide significant energetic input to higher trophic levels, have substantial impacts on individual behavior, growth, and fecundity, and resulting changes in bioturbation rates can secondarily affect community dynamics. The degree to which a community is affected by such sublethal predation depends in part on whether injured individuals can regenerate and on the speed at which they do so. Previously, we demonstrated differences in the rate of anterior segment and palp regeneration by the spionid polychaetes Pygospio elegans and Dipolydora quadrilobata. The current study examined the morphology of the anterior segment and palp regeneration in these species using scanning electron microscopy (SEM) and indirect immunohistochemistry with confocal laser scanning microscopy at 3, 6, 9, and 12 d post‐ablation. Antibodies for acetylated α‐tubulin and the neural tetrapeptide FMRFamide were used to label the regenerating nervous system. SEM revealed that the morphology of anterior tissue regeneration was similar for both species, but the ciliated food groove tended to form sooner on palps in P. elegans than on those of D. quadrilobata. In both species, palp regeneration and ciliated food groove formation were faster when only palps were removed. A shortened ciliated food groove is likely to reduce particle contact and transport efficiency in suspension and deposit feeding. Regenerating palp nerves were initially visible at 3 d following ablation of palps only, but at 6 d following ablation of five anterior segments. Following ablation of anterior segments, the regenerating nervous system was largely complete by 9 d, nuchal organs were innervated by 6 d, and processes of palp sensory cells were visible at 12 d. Contact chemoreception by sensory cells on the palps may be diminished during the early stages of regeneration, but chemoreception of waterborne cues via the nuchal organ should not.     

Observations of serotonin and FMRFamide-like immunoreactivity in palp sensory structures and the anterior nervous system of spionid polychaetes

Evidence suggests that ciliated sensory structures on the feeding palps of spionid polychaetes may function as chemoreceptors to modulate deposit-feeding activity. To investigate the probable sensory nature of these ciliated cells, we used immunohistochemistry, epi-fluorescence, and confocal laser scanning microscopy to label and image sensory cells, nerves, and their organization relative to the anterior central nervous system in several spionid polychaete species. Antibodies directed against acetylated alphatubulin were used to label the nervous system and detail the innervation of palp sensory cells in all species. In addition, the distribution of serotonin (5-HT) and FMRFamide-like immunoreactivity was compared in the spionid polychaetes Dipolydora quadrilobata and Pygospio elegans. The distribution of serotonin immunoreactivity was also examined in the palps of Polydora cornuta and Streblospio benedicti. Serotonin immunoreactivity was concentrated in cells underlying the food groove of the palps, in the palp nerves, and in the cerebral ganglion. FMRFamide-like immunoreactivity was associated with the cerebral ganglia, nuchal organs and palp nerves, and also with the perikarya of ciliated sensory cells on the palps.     

Immunohistochemical (cLSM) and ultrastructural analysis of the central nervous system and sense organs in Aeolosoma hemprichi (Annelida, Aeolosomatidae)

The Aeolosomatidae are very small limnetic or terrestrial annelids of apparently simple organisation and uncertain phylogenetic position. They have been placed either at the base of the Clitellata, as a highly derived taxon within the Clitellata closely related to the Naididae, or as their sister group within the „Polychaeta”. A combined immunohistochemical (cLSM) and ultrastructural investigation of the central nervous system and the sense organs in Aeolosoma hemprichi was undertaken to look for characters which might support one of these theories. The position of the brain within the prostomium and the organisation of the ventral nerve cord, with its intraepithelial paired longitudinal nerves lying far apart from each other and the presence of a median longitudinal nerve, are atypical for the Clitellata and clearly differ from the situation found in Naididae. Moreover, the circumoesophageal connectives are bifurcated and enter the brain as dorsal and ventral roots; this arrangement is unknown in Clitellata, in which these connectives are unbranched. An ultrastructural analysis of the ciliated pits located laterally in the furrow between prostomium and peristomium in A. hemprichi and other Aeolosomatidae show that they are in fact nuchal organs. Such presumed chemosensory organs are typical of „Polychaeta” and absent in all Clitellata. Two pairs of ciliary sense organs are present in the prostomium in front of the brain of A. hemprichi. Although similarly organised sensory structures occur in many species of the Clitellata, they differ in position and certain ultrastructural features and are known from other Annelida as well. These results clearly support the exclusion of the Aeolosomatidae from the Clitellata and do not provide any evidence for a sister-group relationship between these two taxa. Accepted: 8 February 2000     

Comparative ultrastructure of juvenile and adult nuchal organs of an annelid (Polychaeta: Opheliidae)

Opheliid nuchal organs are composed of ciliated cells, retractor muscles, and sensory cells. The perikarya of sensory cells are located in the posterior portion of the brain, and their distal processes extend along the body wall, as the nuchal nerve, and terminate just anterior to the ciliated region. The nuchal nerve of the juvenile is composed of 30–35 dendrites; the adult nuchal nerve has 35–40 dendrites. The ends of the sensory dendrites form sensory bulbs which are clustered around the olfactory chamber, and each bulb bears a modified cilium. Sensory cilia lose their axonemes and extend as microvillous-like structures into the olfactory chamber. Supportive cells delineate approximately the posterior and dorsal portions of the chamber with sensory bulbs forming the remaining ventral and anterior portions. On the lateral aspect of the chamber, cuticular matrix extends into it, and in this area supportive cells bear microvilli which extend into the matrix. The adult nuchal organ is larger than that of the juvenile, and the sensory portion of the olfactory chamber wall is expanded. Expansion of the sensory area is apparently the result of size increase in sensory bulbs and by intrusion of supportive cells between sensory bulbs.     

Polychaete community structure of the northwestern coast of Mexico: patterns of abundance and distribution

In August 1998, 72 benthic stations were sampled off the northwestern coast of Mexico. A baseline survey of the polychaete macrofauna was carried out for the southernmost portion of the Southern California Bight (SCB) region. Spatial patterns of polychaete assemblages were characterized to determine evidence of environmental impact from point sources of wastewater discharge near the study area, and to define a standard against which to detect future change. A total of 4231 polychaetous annelids representing 38 families and 210 species were identified, which constituted 53% of the total macrofauna. Families with highest species richness were Spionidae (18 spp.), Onuphidae (15 spp.), and Terebellidae (14 spp.). Species with greatest abundances were Spiophanes duplex (490 ind.), Spiophanes fimbriata (330 ind.), Lanassa sp. D (209 ind.), Pectinaria californiensis (138 ind.), and Euclymeninae sp. A (124 ind.). Based on polychaete assemblages, communities for the southernmost SCB were found to be composed primarily of non-opportunistic species, and no numerically dominant species were found. High values of polychaete diversity (H) indicated the majority of the area of study could be considered an undisturbed environment. Principal Components Analysis (PCA) applied to environmental and faunistic parameters indicated depth, organic carbon, and sediment grain size as the main influences determining the polychaete faunal communities.     

Ultrastructural investigations on the nuchal organ of the protandric polychaete,Ophryotrocha puerilis (Polychaeta,Dorvilleidae)

Summary The nuchal organs of the protandric hermaphrodite Ophryotrocha puerilis were studied by electron microscopy. Ophryotrocha puerilis is the first species hitherto described which possesses four instead of two nuchal organs. These sensory structures are located as ciliary pits at the posterior margin of the prostomium. Histologically, the nuchal organs are composed of supporting cells with long motile cilia and bipolar sensory cells, the perikarya of which form four distinct nuchal ganglia adjoining the brain. These structural components are concentrically arranged around the central sensory area. This area is covered by a modified cuticle, whereas the cuticle above the peripheral region of the sense organ exhibits the appearance typical for polychaetes. Two types of vesicular material are produced in the basal supporting cells, a dense-cored one within the central supporting cells only and a clear irregular-shaped one in all of these cells. The first type is considered to be responsible for the formation of the modified cuticle. The significance of these most probably long-distance chemoreceptory organs and their possible role in reproductive behaviour is discussed.