The Structure of a Sessile,Stalkless Crinoid (Holopus rangii)

Specimens of Holopus rangii (order Cyrtocrinida) were collected by submersible at depths of several hundred meters in the Caribbean and fixed for light and electron microscopy. The presence of an anus is confirmed. However, the chambered organ and glandular axial organ peculiar to crinoids are lacking. The gut lumen sometimes includes partially digested prey items up to several hundred micrometers in diameter, and we propose that Holopus may feed raptorially by rapidly closing its arms over demersal zooplankton. Electron microscopy of the arm reveals a radial nerve and a radial haemal channel, which light microscopy previously failed to demonstrate. The cuticle includes bacteria that are probably symbiotic. The ten brachial nerves of the aboral nervous system unite pairwise to form five calyx nerves. The calyx nerves, one in each radial position, are connected by a pentagonal, interradial commissure and then continue to the attached end of the body where they end blindly without forming an aboral nerve center. The absence of the aboral nerve center and related internal organs strengthens the argument that no basal ossicles are included in the skeleton of the calyx and suggests that Holopus may have evolved from stalked cyrtocrinid ancestors by saltatory loss of major body parts.     

Development of Cyathidium foresti (Echinodermata: Crinoidea, Cyrtocrinida) from early attached larvae to adult-like juveniles

To date, knowledge about ontogenetic development of crinoids has been exclusively based on comatulid species, since early stages of other crinoid taxa have not been available so far. The authors now present a scanning electron microscopical and light microscopical study on a developmental series of young sessile individuals of the cyrtocrinid Cyathidium foresti. This species displays a developmental type of its own. In some aspects, the early stages resemble the early attached larva of comatulids (e.g. vestibulum, enteric sac, somatocoelomic cavity) but differ clearly in other (e.g. far oral position of hydrocoelomic primordium, pattern of podia formation, early splitting of the roof, absence of chambered organ). Therefore a specific term is proposed for this new kind of larva: cyathidula. Older juveniles are quite similar to adults; the developmental course is direct. Consequently the group of holopodid crinoids to which Cyathidium and Holopus belong, can be concluded to originate phylogenetically from neotenic larvae.     

Molecular phylogeny of the crab genus Brachynotus (Brachyura: Varunidae) based on the 16S rRNA gene

The crab genus Brachynotus de Haan, 1833 is restricted to the intertidal and shallow subtidal of the Mediterranean and northeastern Atlantic. It is presently recognized to consist of four species, of which three (B. foresti, B. gemmellari and B. sexdentatus) are endemic to the Mediterranean. The fourth species, B. atlanticus, is found along the Atlantic coasts of northern Africa and southern Europe, but also extends into the western Mediterranean. This high level of endemism suggests that speciation within Brachynotus is strongly correlated with the geography and geology of the Mediterranean Sea. A molecular phylogeny based on the mitochondrial large subunit (16S) rRNA gene indicates that the four species of Brachynotus form a monophyletic group within Atlantic Varunidae. The DNA sequence data also show that the genus Brachynotus can be subdivided into two species groups, one comprising B. atlanticus and B. foresti, and the other one B. gemmellari and B. sexdentatus. While B. atlanticus and B. foresti are clearly genetically distinct, B. gemmellari and B. sexdentatus are identical in the studied region of the 16S rRNA gene, suggesting a recent separation or continuing gene flow.     

Linear growth in spines from Acanthaster planci (L.) involving growth lines and periodic pigment bands

We report the first observations of a linear growth pattern in aboral spine ossicles of adult Acanthaster planci (L.). This is unlike the spine development of other echinoderms. Growth in aboral spine ossicles of A. planci is essentially by addition of stereom at the base and the spine's growth history is preserved along its length. There are numerous growth lines perpendicular to the long axis of the ossicle. These are clearly evident in longitudinal spine sections and apparently caused by frequent growth episodes. There are periodic pigment bands which are parallel to the growth lines and evident on the surface of the ossicle. Basal growth of the spine ossicle and the nature of the growth lines were confirmed by tetracycline staining. Size/frequency analyses of a population of A. planci from Davies Reef (GBR) found spine ossicle growth, but not body diameter growth, over the six month period between sampling dates. The additional pigment banding in spine ossicles of 4 individuals recaptured after 6 months suggests that pigment bands are laid down seasonally. If pigment band cyclicity is validated, it offers a simple method for ageing adults of A. planci in field populations.     

Mechanisms of rapid morphogenetic movements in the metamorphosis of the bryozoan Bugula neritina (Cheilostomata,Cellularioidea): II. The role of dynamic assemblages of microfilaments in the pallial epithelium

The rapid morphogenetic movements that internalize the transitory larval epithelium and reorient the presumptive adult epidermis during the metamorphosis of the cellularioid cheilostome bryozoan, Bugula neritina, have been examined by light and electron microscopy and analyzed by experimentation with cytochalasin B (CB) and MgC1₂. The pallial epithelium is gradually drawn out over the aboral hemisphere as the larval ciliated epithelium (the corona and the pyriform organ) involutes. At the end of coronal involution the oral margin of the pallial epithelium constricts and the aboral hemisphere is pulled down against the everted sac. Ultrastructural and experimental evidence indicates that an equatorial contractile ring composed of a temporal alignment of CB-sensitive 5.5 nm microfilaments is responsible for the constriction of the oral margin of the pallial epithelium. This morphogenetic movement, in conjunction with the compression of the aboral hemisphere, juxtaposes the pallial epithelium with the oral epithelium of the everted sac. The pallial epithelium adheres to the neck and wall regions of the everted sac and begins a progressive contraction at its aboral margin, pulling the wall epithelium up over the aboral hemisphere. Ultrastructural examination reveals that the pallial cells contain apical bands of microfilaments and associated vesicles at this stage of metamorphosis. The position and time of appearance of the microfilaments in the pallial epithelium support the hypothesis that they generate the force for wall elevation. Histological and experimental data indicate that the compression of the aboral hemisphere at the umbrella stage and the final retraction of the apical disc are muscle-mediated morphogenetic movements. The constriction of the umbrellar margin and the elevation of the wall epithelium, on the other hand, appear to be caused by two distinct populations of microfilaments that assemble in different regions of the pallial epithelium at specific times during metamorphosis.     

An ultrastructural study of larval settlement in the sea anemone Urticina crassicornis (Cnidaria,Actiniaria)

Laboratory-reared larvae of the sea anemone Urticina (= Tealia) crassicornis have been examined by electron microscopy prior to and following settlement on algal substrata. At 18 days postfertilization, the free-swimming planula larva measures about 600 μm long. A stomodaeal invagination occurs at the narrow end of the larva and connects with a solid mass of endoderm in the core region. The endoderm possesses septa with well-developed myonemes and is situated subjacent to a thin sheet of mesoglea. The uniformly ciliated ectoderm that constitutes the outer layer of the larva contains: (1) spirocysts, (2) nematocysts, (3) mucus, (4) three types of membrane-bound granules, (5) a basiepithelial nerve plexus, and (6) a few nongranular cells that may represent sensory neurons. Within several minutes after the introduction of the algal substratum, the planula characteristically directs its broadened aboral end toward the alga and secretes a refractile sheet of material. As the aboral end attaches to the substratum, the larva becomes noticeably shorter along its oral-aboral axis, presumably owing to the contractions of myonemes that are located within the endodermal septa. All three types of granules and the ectodermal mucoid substances are exocytosed during settlement, but spirocysts and nematocysts characteristically remain undischarged. Ovoid, PAS⁺ granules are believed to be at least partly responsible for adhesion, since these granules are concentrated at the aboral end prior to settlement and are somewhat similar in ultrastructure to putative viscid granules produced by other species. Contrary to a previous report based on light microscopy, no discrete sensory organ is evident in serial sections of the aboral ectoderm. The ability of planulae to detect suitable substrata appears to depend instead on sparsely distributed sensory cells that occur throughout the larval ectoderm.     

The Fine Structure of the Stalk of the Pentacrinoid Larva of a Feather Star,Comanthus japonica (Echinodermata: Crinoidea)

The stalk of the pentacrinoid larva of a feather star (Comanthus japonica) is described for the first time by transmission electron microscopy. One end of the stalk bears the calyx and the other end is cemented to the substrate by attachment cement consisting of a meshwork of 5 nm filaments. The stalk is supported by a scries of skeletal ossicles pierced by a central canal: short intercolumnal ligaments connect adjacent skeletal ossicles and central through-going ligaments run the length of the central canal. At the end of the stalk nearest the calyx, the chambered organ and the closely associated axial organ are histologically similar to those of adult crinoids. Presumed neurosecretory neurons are associated with the intercolumnal ligaments, and the following kinds of nerves run down the central canal: (1) a large stalk nerve in each of the five interradii; (2) smaller coelomic nerves in each of the five radii in association with the epithelium of tubular aboral extensions of the chambered organ; (3) a very small nerve associated with the aboral extension of the axial organ in the stalk axis. This axial organ extension is surrounded by a haemal channel. Because of the small size of the stalk, none of the nerves or the haemal channel were described in previous light microscopic studies. The discussion gives special attention to the controversial motility of the pentacrinoid stalk.     

Drilling predation on the clypeasteroid echinoid Echinocyamus pusillus from the Mediterranean Sea (Giglio,Italy)

Drilling predation of cassid gastropods (tonnacean) on echinoids is common in marine environments but is rarely documented. Tests of the minute clypeasteroid Echinocyamus pusillus OF Müller (1776) were collected from the Mediterranean Sea (Isola del Giglio, Italy). Besides general morphology, features pertaining to the morphology and distribution of predatory boreholes were examined. Furthermore, borehole frequencies among different samples sites were compared. Cassid gastropods are assumed to be the originators of the boreholes. A total of 1061 tests were analysed for drilling rates with 15.3% drilled with predominantly single boreholes. The borehole morphology is strongly affected by the microstructure of the skeleton; the borehole outline is irregular if drilled within areas where ambulacral pores are present. Vertical borehole morphology is influenced by stereom density. The size frequencies of non-drilled and drilled specimens show significant differences. Comparisons of borehole size with test size show only a low correlation between predator and prey size. The distribution of boreholes shows a high site selectivity of the predator for the aboral side of the test and the petalodium.     

Anatomical features of the euryalid snake star Astrobrachion constrictum (Ophiuroidea: Asteroschematidae)

Abstract. The morphology of Astrobrachion constrictum , a representative of the little known Euryalida, was examined by light and electron microscopy. The tegument is smooth and lacks calcified elements. Many features of A. constrictum were found to conform to the usual pattern of ophiuroid anatomy. The skeletal elements are generally composed of uniform labyrinthic stereom. The radial shields, however, consist of overlapping stereom plates. The 5 oral shields serve as madreporites, each with a U-shaped canal leading into an axial sinus. A 3-lobed axial organ is associated with each madreporite and may play some sensory or secretory role.     

Attachment and metamorphosis of the cheilo-ctenostome bryozoan bugula neritina (linné)

The structure, attachment and subsequent metamorphosis of larvae of the marine bryozoan Bugula neritina were studied by light and electron microscopy. Two points of larval anatomy are of special significance to proper interpretation of the metamorphosis:

• 1 Two cytologically similar blastemal tissues, each laden with free ribosomes, occur as parts of the apical organ complex. The upper blastema directly contacts the larval surface, forming the non-ciliated rows of the apical organ. The lower blastema is internal and is oral to and contiguous with the upper blastema.
• 2 The epidermal tissues of the larva are joined in the following sequence, beginning at the aboral pole: a. apical organ complex; b. apical-connecting cell; c. infolded pallial sinus epithelium; d. vesicular-connecting cell; e. aboral vesicular epithelium; f. corona; g. oral vesicular epithelium; and i., j., and k. internal sac neck, wall and roof regions.

The initial stages of metamorphosis involve a complex sequence of morphogenetic movements, including:

• 1 eversion of the internal sac, permanently attaching the larva to the substrate;
• 2 inrolling of the aboral vesicular epithelium, corona, oral vesicular and ciliated epithelia, and neck region of the internal sac into the larval interior; concomitantly the pallial sinus epithelium evaginates;
• 3 loss of connection between the invaginated tissues and the surface;
• 4 fusion of the pallial sinus epithelium with the wall region of the internal sac, maintaining the integrity of the body surface;
• 5 retraction of the apical organ complex and invagination of the pallial sinus epithelium with the simultaneous elevation of the internal sac wall region to the aboral pole.

At the conclusion of these events the preancestrular surface is covered by the wall and roof regions of the internal sac. Cells of the wall region form the epidermis of the body wall except for the attachment disc and secrete a cuticular exoskeleton that is secondarily calcified; the attachment disc is formed by the roof region of the internal sac. Internally, the ectodermal upper blastema differentiates into the lophophore and digestive tract of the ancestrular polypide, while the lower blastema forms the lining of the lophophoral coelom and the splanchnic (but not the somatic) lining of the visceral coelom. The visceral somatic peritoneum is formed from cells that may originate from the mesodermally derived pigmented cells of the larva to which they are similar in pigmentation and cytology. Such a composite derivation of a coelomic lining has not been described previously.     

Early development, pattern, and reorganization of the planula nervous system in Aurelia (Cnidaria, Scyphozoa)

We examined the development of the nervous system in Aurelia (Cnidaria, Scyphozoa) from the early planula to the polyp stage using confocal and transmission electron microscopy. Fluorescently labeled anti-FMRFamide, antitaurine, and antityrosinated tubulin antibodies were used to visualize the nervous system. The first detectable FMRFamide-like immunoreactivity occurs in a narrow circumferential belt toward the anterior/aboral end of the ectoderm in the early planula. As the planula matures, the FMRFamide-immunoreactive cells send horizontal processes (i.e., neurites) basally along the longitudinal axis. Neurites extend both anteriorly/aborally and posteriorly/orally, but the preference is for anterior neurite extension, and neurites converge to form a plexus at the aboral/anterior end at the base of the ectoderm. In the mature planula, a subset of cells in the apical organ at the anterior/aboral pole begins to show FMRFamide-like and taurine-like immunoreactivity, suggesting a sensory function of the apical organ. During metamorphosis, FMRFamide-like immunoreactivity diminishes in the ectoderm but begins to occur in the degenerating primary endoderm, indicating that degenerating FMRFamide-immunoreactive neurons are taken up by the primary endoderm. FMRFamide-like expression reappears in the ectoderm of the oral disc and the tentacle anlagen of the growing polyp, indicating metamorphosis-associated restructuring of the nervous system. These observations are discussed in the context of metazoan nervous system evolution.     

Embryonic development and metamorphosis of the scyphozoan <Emphasis Type="Italic">Aurelia</Emphasis>

We investigated the development of Aurelia (Cnidaria, Scyphozoa) during embryogenesis and metamorphosis into a polyp, using antibody markers combined with confocal and transmission electron microscopy. Early embryos form actively proliferating coeloblastulae. Invagination is observed during gastrulation. In the planula, (1) the ectoderm is pseudostratified with densely packed nuclei arranged in a superficial and a deep stratum, (2) the aboral pole consists of elongated ectodermal cells with basally located nuclei forming an apical organ, which is previously only known from anthozoan planulae, (3) endodermal cells are large and highly vacuolated, and (4) FMRFamide-immunoreactive nerve cells are found exclusively in the ectoderm of the aboral region. During metamorphosis into a polyp, cells in the planula endoderm, but not in the ectoderm, become strongly caspase 3 immunoreactive, suggesting that the planula endoderm, in part or in its entirety, undergoes apoptosis during metamorphosis. The polyp endoderm seems to be derived from the planula ectoderm in Aurelia, implicating the occurrence of “secondary” gastrulation during early metamorphosis.     

Fine structure of the dorsal arm plate of Ophiocoma wendti: Evidence for a photoreceptor system (Echinodermata,Ophiuroidea)

Summary Three structures in the dermis of the dorsal arm plate (DAP) of the brittlestar, Ophiocoma wendti, appear to comprise a photoreceptor system. The upper surface of the DAP bears transparent, knob-like, microscopic structures which are expanded peripheral trabeculae (EPT) of the calcite stereom. The EPT are part of the photoreceptor system and can facilitate light transmission through the DAP by decreasing light refraction, reflection and absorption that occur at stereom/stroma interfaces. Bundles of nerve fibres located below the EPT are a second component of the system, and may function as primary photoreceptors. The intensity of light impinging on the putative sensory tissue is regulated by the diurnal activity cycle of chromatophores, the third element of the system. During the day the chromatophores cover the EPT and thereby shade the nerve fibres. At night they retract into inter-trabecular channels, uncovering the EPT and thereby exposing the nerve fibres to transmitted light. Thus, the transparent stereom may play a role in photoreception, in addition to its generally recognized skeletal function. Although ciliated cells that may be sensory are present in the epidermis of Ophiocoma wendti, they do not appear to be photoreceptors. Functional analogues of the brittlestar photoreceptor system in other echinoderms are discussed, emphasizing the relationship between photosensitivity and the transparency of the stereom in several classes of Echinodermata.     

Stereom morphogenesis and differentiation during regeneration of adambulacral spines of Asterias rubens (Echinodermata,Asteroida)

Summary The very first mineral deposits appearing in regenerating fractured adambulacral spines of Asterias rubens are minute polyhedrons that cover the surface of fractured trabeculae. Polyhedrons fuse together forming a fold from which a microspine differentiates. Microspines develop into long linear trabeculae which send out lateral processes at regular length intervals. Lateral processes from adjacent trabeculae fuse together, bridging the trabeculae and giving the regenerate the typical meshwork structure of stereom. Most of the regenerate is built up according to this growth pattern which ensures its longitudinal growth. Simultaneously, the initial fascicular stereom of the stub sends out short radial processes which branch into upward and downward directed subprocesses. The latter fuse with their equivalents located above or below, building up longitudinal rows of stereom meshes. These rows then bridge together by additional branched or unbranched lateral processes, so forming a new stereom layer which progressively covers the whole stub. Up to three new layers of stereom are formed in this way at the stub periphery. These become continuous with the stereom layers of the regenerate by fusion of reciprocal subprocesses, so ensuring the continuity between the stub and the regenerate. In both structures the first stage of mineralization results in an open stereom. Stereom thickening occurs in a second stage of mineralization (that is chronologically separated from the formation of the open stereom) and results in the differentiation of the original stereom fabrics (i.e. fascicular stereom). Regeneration of removed spines starts with the formation of a new spine base made of labyrinthic stereom. The development of the latter mostly relies on short branched and unbranched processes which fuse with each other or with predifferentiated meshes. After completion of its base, the regenerating spine lengthens and thickens similarly to the regenerating fractured spines. The diversity of the stereom growth processes observed in the present work may be reduced to the combination of one to three elementary events, viz. the development of long linear processes, of short unbranched processes and of short branched processes. A survey of the literature allows the suggestion that the implementation of these elementary events is sufficient to describe most types of stereom morphogenesis.Senior research assistant NFSR (Belgium)     

Revision of the disparid Stylocrinus (Crinoidea) from the Devonian of Europe,Asia and Australia

Abstract: The discovery of new specimens and the restudy of known collections result in revision of the diagnosis and the stratigraphic distribution of the disparid crinoid genus Stylocrinus, from the Middle and Upper Devonian of Europe, Asia and Australia. The consistent development of three basal plates, the atomous arms with internally inclined edges adjoining laterally with adjacent brachials in an interlocking network and an apparently rudimentary pinnulation is recognised. The high ecophenotypic plasticity of the common species S. tabulatus negates the validity of several former subspecies and demonstrates the general morphologic variability of the aboral cup proportions. This contrasts with the low morphological spectrum of rarer stylocrinid species. With exclusion of ‘S. elimatus’ (Silurian) from Stylocrinus, the genus is limited to the Devonian. A neotype is proposed for the lost holotype of S. tabulatus. Stylocrinus prescheri sp. nov. is described from the Eifelian to Givetian of Europe and Asia. The first evidence of the gastropod grazing trace fossil Radulichnus on a crinoid aboral cup (S. tabulatus), the post‐mortem incurred ossicular‐boring of radial and basal plates as well as the post‐mortem encrusting by a rugose coral are further observations on Stylocrinus aboral cups.     

Middle Cambrian cystoid (sensu lato) stem columnals from Bornholm, Denmark

A rich material of echinoderm fragments from two Middle Cambrian stratigraphical levels on Bornholm are preserved due to phosphatization of the original calcitic stereom. Preservation of echinoderms in this way - not previously recorded from the Middle Cambrian - permits detailed analysis of the three-dimensional stereom structure. Identifiable are fragments of stylophorans and eocrinoids. Stem columnals, most likely from eocrinoids, show a wide and advanced morphological variation indicating articulation similar to that of crinoids. The material from the Exsulans Limestone/Kalby marl ( Ptychagnostus gibbus Zone) represents stem-bearing cystoids older than Akadocrinus from Bohemia. The Andrarum Limestone ( Sole-nopleura brachymetopa Zone) contains echinoderm fragments from a higher stratigraphical level, a level correlatable with that from which the oldest North American stem–bearing cystoid, Eustypocystis , has been recorded.     

Surface morphology of Probolocoryphe uca (Sarkisian, 1957) (Digenea: Microphallidae) from Kuwait Bay

The surface ultrastructure of Probolocoryphe uca (Digenea: Microphallidae), recovered from a rat experimentally fed on crabs, Nanosesarma minutum(Brachyura: Grapsidae), naturally infected with the metacercariae, was studied by scanning electron microscopy. The flukes were leaf-like, ventrally concave and pyriform or ovoid in outline. The anterior end was modified into a sucker-like organ, comprising a protrusible disc-shaped structure surrounded by single-pointed spines. This organ is probably involved in the attachment and feeding process in a manner similar to the action of the oral suckers. Apart from the sucker-like organ, the entire tegumental surface was covered with triangular spines with multi-pointed tips. Ciliated, dome-shaped papillae were observed, singly or in groups, arranged symmetrically on the sucker-like organ and around the oral and ventral suckers. Kuwait Bay constitutes a new geographical record and the crab N. minutum is a new second intermediate host record for P. uca.     

Embryonic and larval development of the sea anemone Haliplanella lineata from Japan

The development of Haliplanella lineata, following fertilization in the laboratory, was studied by light and electron microscopy. Spawned ova were spherical, magenta in color and about 120–150 µm in diameter. Cleavage was holoblastic and radial. Gastrulation occurred by immigration and invagination. Eighteen hours after fertilization, the embryo became a swimming planula larva with an apical organ and ciliary tuft at the aboral end. In the laboratory, planulae lived for about 2 weeks in the swimming state but in no case was there any settlement by larvae in this study. The structural study of planulae concentrated on the development of the aboral ectoderm, because of the functional significance of its cellular organization in larval settlement.