Molecular modularity and asymmetry of the molluscan mantle revealed by a gene expression atlas

Background

Conchiferan molluscs construct a biocalcified shell that likely supported much of their evolutionary success. However, beyond broad proteomic and transcriptomic surveys of molluscan shells and the shell-forming mantle tissue, little is known of the spatial and ontogenetic regulation of shell fabrication. In addition, most efforts have been focused on species that deposit nacre, which is at odds with the majority of conchiferan species that fabricate shells using a crossed-lamellar microstructure, sensu lato.

Results

By combining proteomic and transcriptomic sequencing with in situ hybridization we have identified a suite of gene products associated with the production of the crossed-lamellar shell in Lymnaea stagnalis. With this spatial expression data we are able to generate novel hypotheses of how the adult mantle tissue coordinates the deposition of the calcified shell. These hypotheses include functional roles for unusual and otherwise difficult-to-study proteins such as those containing repetitive low-complexity domains. The spatial expression readouts of shell-forming genes also reveal cryptic patterns of asymmetry and modularity in the shell-forming cells of larvae and adult mantle tissue.

Conclusions

This molecular modularity of the shell-forming mantle tissue hints at intimate associations between structure, function, and evolvability and may provide an elegant explanation for the evolutionary success of the second largest phylum among the Metazoa.

Towards high-performance bioinspired composites

Biological composites have evolved elaborate hierarchical structures to achieve outstanding mechanical properties using weak but readily available building blocks. Combining the underlying design principles of such biological materials with the rich chemistry accessible in synthetic systems may enable the creation of artificial composites with unprecedented properties and functionalities. This bioinspired approach requires identification, understanding, and quantification of natural design principles and their replication in synthetic materials, taking into account the intrinsic properties of the stronger artificial building blocks and the boundary conditions of engineering applications. In this progress report, the scientific and technological questions that have to be addressed to achieve this goal are highlighted, and examples of recent research efforts to tackle them are presented. These include the local characterization of the heterogeneous architecture of biological materials, the investigation of structure-function relationships to help unveil natural design principles, and the development of synthetic processing routes that can potentially be used to implement some of these principles in synthetic materials. The importance of replicating the design principles of biological materials rather than their structure per se is highlighted, and possible directions for further progress in this fascinating, interdisciplinary field are discussed.

Matrix-mediated biomineralization in marine mollusks: a combined transmission electron microscopy and focused ion beam approach

The teeth of the marine mollusk Acanthopleura hirtosa are an excellent example of a complex, organic, matrix-mediated biomineral, with the fully mineralized teeth comprising layers of iron oxide and iron oxyhydroxide minerals around a calcium apatite core. To investigate the relationship between the various mineral layers and the organic matrix fibers on which they grew, sections have been prepared from specific features in the teeth at controlled orientations using focused ion beam processing. Compositional and microstructural details of heterophase interfaces, and the fate of the organic matrix fibers within the mineral layers, can then be analyzed by a range of transmission electron microscopy (TEM) techniques. Energy-filtered TEM highlights the interlocking nature of the various mineral phases, while high-angle annular dark-field scanning TEM imaging demonstrates that the organic matrix continues to exist in the fully mineralized teeth. These new insights into the structure of this complex biomaterial are an important step in understanding the relationship between its structural and physical properties and may help explain its high strength and crack-resistance behavior.

Histochemical and ultrastructural characterization of the posterior esophagus of Bulla striata (Mollusca, Opisthobranchia)

The posterior esophagus of Bulla striata, running from the gizzard to the stomach, was investigated with light and electron microscopy to obtain new data for a comparative analysis of the digestive system in cephalaspidean opisthobranchs. In this species, the posterior esophagus can be divided into two regions. In the first, the epithelium is formed by columnar cells with apical microvilli embedded in a cuticle. Many epithelial and subepithelial secretory cells are present in this region. In both, electron-lucent secretory vesicles containing filaments and a peripheral round mass of secretory material fill the cytoplasm. These acid mucus-secreting cells may also contain a few dense secretory vesicles. In the second part of the posterior esophagus, the cuticle is absent and the epithelium is ciliated. In this region, epithelial cells may contain larger lipid droplets and glycogen reserves. Subepithelial secretory cells are not present, and in epithelial secretory cells the number of dense vesicles increases, but most secretory cells still contain some electron-lucent vesicles. These cells secrete a mixture of proteins and acid polysaccharides and should be considered seromucous. The secretory cells of the posterior esophagus are significantly different from those previously reported in the anterior esophagus of this herbivorous species.

Interactive 3D volume rendering in biomedical publications

We present three examples of interactive, 3D volume rendering models embedded in a PDF publication. The examples are drawn from three different morphological methods - confocal microscopy, serial sectioning and microcomputed tomography - performed on members of the phylum Mollusca. A description of the entire technical procedure from specimen preparation to embedding of the visual model including 3D labels in the document is provided. For comparison, volume rendering with standard visualization software, and surface rendering incorporated in the 3D PDF figures, are provided. The principal advantages and disadvantages of the techniques and models are discussed. Volume rendering for serial sections is relatively work-intensive, while confocal data have limitations in terms of 3D presentation. Volume renderings are normally downsampled in resolution to achieve a reasonable PDF file size, however intentional information is largely retained. We conclude that volume rendering of 3D data sets is a valuable technique and should become standard in PDF versions of biomedical publications.

Tooth use and wear in three iron-biomineralizing mollusc species

Chitons and limpets harden their teeth with biominerals in order to scrape algae from hard rock surfaces. To elucidate relationships between tooth structure and function, light and electron microscopy were used to examine naturally worn teeth in three species of mollusc with iron-mineralized teeth and to analyze the grazing marks left by members of these species feeding on wax. For the two chiton species, teeth wore down progressively from the medial to the lateral edge of the cusp, while for the limpet, wear was more evenly distributed across the edges of each cusp. In chitons, this pattern of wear matched the medially biased morphology of the cusps in their protracted position and relates to what is known about the mineral composition and substructure of the teeth. The patterns of progressive tooth wear for each of these species, together with the distinct grazing marks left by each species on the wax substrate, indicate that the teeth are designed to remain functionally effective for as long as possible, and have proved to be a valuable means of rationalizing the internal architecture of the teeth at a range of spatial scales. This information is critical for ongoing studies aimed at understanding the interactions between the organic matrix and mineral components of these teeth.

Apoptosis and histopathological lesions in parasite-infected species of bivalve molluscs

Apoptosis (programmed cell death) is an important mechanism for preservation of a healthy and balanced immune system in vertebrates. Little is known, however, about how apoptosis processes regulate invertebrate immune system. Thus, the present histopathological study was performed to search for the level of apoptosis on hemocytes of three bivalve species namely, Macoma edentula, Hiatula rupelliana and Gastrana fraglis in the Timsah Lake infected with branchial Rickettsia-like parasites and ciliated parasites in the digestive gland. Microscopically, special to elongated intracytoplasmic Rickettsia-like colonies were observed in the base of gill filaments of the clams. Histologically, the distribution and shapes of apoptotic cells were classified into three main types. The aggregation of apoptotic cells were observed in the apical and the basal parts of the ciliated cells lining the gill epithelium. These results provide a first insight into apoptotic processes in mollusc immune cells.

[Cytoplasmic syncytial interneuronal connection in molluscs]

The absolute criteria developed by the authors have been presented; they allow revealing cytoplasmic syncytial connections between processes of nerve cells in vivo and in vitro at the light microscopy level by using classical methods and time lapse videoshooting in the phase contrast. With aid of electron microscopy, metastable membrane contacts and their perforations, cytoplasmic syncytial interneuronal pores, and fusion of nerve processes are demonstrated. In the culture of isolated molluscan neurons, the process of formation of syncytial connection between processes of the same neuron or of different neurons is reproduced. Processes of one neuron, which have syncytial connection with another neuron, are shown to remain viable after death of its neuronal soma. The cytoplasmic varicosities formed on processes of one neuron are able to overcome the place of syncytial contact with processes of another neuron and to move to the body of the latter. A hypothesis is put forward that the cytoplasmic syncytial connection between nerve processes is formed under the condition of the absence of their glial sheaths.

Biomimetic characterisation of key surface parameters for the development of fouling resistant materials

Material science provides a direct route to developing a new generation of non-toxic, surface effect-based antifouling technologies with applications ranging from biomedical science to marine transport. The surface topography of materials directly affects fouling resistance and fouling removal, the two key mechanisms for antifouling technologies. However, the field is hindered by the lack of quantified surface characteristics to guide the development of new antifouling materials. Using a biomimetic approach, key surface parameters are defined and quantified and correlated with fouling resistance and fouling removal from the shells of marine molluscs. Laser scanning confocal microscopy was used to acquire images for quantitative surface characterisation using three-dimensional surface parameters, and field assays correlated these with fouling resistance and fouling release. Principle component analysis produced a major component (explaining 54% of total variation between shell surfaces) that correlated with fouling resistance. The five surface parameters positively correlated to increased fouling resistance were, in order of importance, low fractal dimension, high skewness of both the roughness and waviness profiles, higher values of isotropy and lower values of mean surface roughness. The second component (accounting for 20% of variation between shells) positively correlated to fouling release, for which higher values of mean waviness almost exclusively dictated this relationship. This study provides quantified surface parameters to guide the development of new materials with surface properties that confer fouling resistance and release.

Lipids from the nacreous and prismatic layers of two Pteriomorpha mollusc shells

Mollusc shells are acellular biominerals, in which macromolecular structures are intimately associated with mineral phases. Most studies are devoted to proteins, despite sugars have been described. Lipids were extracted from the calcite prismatic and aragonite nacreous layer of two mollusc shells. Fourier Transform Infrared Spectrometry shows that lipids are present in both samples, but they are not similar. Thin layer chromatography confirms that lipids are different in the two studied layers, so that it may be suggested they are species-dependant. Although not yet deciphered, their role in biomineralization and fossilisation processes is probably important.

Granular chitin in the epidermis of nudibranch molluscs

Chitin is usually found in stiff extracellular coatings typified by the arthropod exoskeleton, and is not associated with the soft, flexible mollusc skin. Here, we show, however, that chitin in nudibranch gastropods (Opisthobranchia, Mollusca) occurs as intracellular granules that fill the epidermal cells of the skin and the epithelial cells of the stomach. In response to nematocysts fired by tentacles of prey Cnidaria, the epidermal cells of eolid nudibranchs (Aeolidacea) release masses of chitin granules, which then form aggregates with the nematocyst tubules, having the effect of insulating the animal from the deleterious action of the Cnidaria tentacles. Granular chitin, while protecting the animal, does not interfere with the suppleness and flexibility of the skin, in contrast to the stiffness of chitin armor. The specialized epidermis enables nudibranchs to live with and feed on Cnidaria.

The dynamics of nacre self-assembly

We show how nacre and pearl construction in bivalve and gastropod molluscs can be understood in terms of successive processes of controlled self-assembly from the molecular- to the macro-scale. This dynamics involves the physics of the formation of both solid and liquid crystals and of membranes and fluids to produce a nanostructured hierarchically constructed biological composite of polysaccharides, proteins and mineral, whose mechanical properties far surpass those of its component parts.

Ultrastructural aspects of the 'statocyst' of Xenoturbella (Deuterostomia) cast doubt on its function as a georeceptor

The "statocyst" in the enigmatic worm Xenoturbella is a structure containing motile flagellated cells. It is situated inside the subepidermal membrane complex (between epidermis and muscular layers) in the anterior end of the body. The motile cells contain a lipophilic refractile body ("statolith"), and a series of vesicles from small dense core vesicles presumably formed from the refractile body to large vesicles with dense aggregates of filamentous tubules that become exocytized through secretion. It is unlikely that the statocyst is a georeceptor (true statocyst); maybe it has an endocrine function.

Amorphous layer around aragonite platelets in nacre

We reveal that the aragonite CaCO3 platelets in nacre of Haliotis laevigata are covered with a continuous layer of disordered amorphous CaCO3 and that there is no protein interaction with this layer. This finding contradicts classical paradigms of biomineralization, e.g., an epitaxial match between the structural organic matrix and the formed mineral. This finding also highlights the role of physicochemical effects in morphogenesis, complementing the previously assumed total control by biomolecules and bioprocesses, with many implications in nanotechnology and materials science.

Molecular phylogenetic and isotopic evidence of two lineages of chemoautotrophic endosymbionts distinct at the subdivision level harbored in one host-animal type: The genusAlviniconcha(Gastropoda: Provannidae)

The hydrothermal-vent gastropod Alviniconcha hessleri from the Alice Springs deep-sea hydrothermal field in the Mariana Back-Arc Basin in the Western Pacific houses an intracellular bacterial endosymbiont in its gill. Although enzymatic analysis has revealed that the endosymbiont is a sulfur-oxidizing chemoautotroph using the Calvin-Benson cycle for the fixation of carbon dioxide, the phylogenetic affiliation of, and the trophic relationship of A. hessleri with, the chemoautotrophic endosymbiont remains undetermined. A single 16S rRNA gene sequence was obtained from the DNA extract of the gill, and phylogenetic analysis placed the source organism within the lineage of the gamma subdivision of the Proteobacteria that consists of many chemoautotrophic endosymbionts of marine invertebrates. Fluorescence in situ hybridization analysis showed the bacterium densely colonizing the gill filaments. The fatty acid profile of the symbiont-free mantle contains the high level of the 16:1 fatty acid originating from the endosymbiont, which indicates that the endosymbiont cells are digested by, and incorporated into, the host. Compound-specific carbon isotopic analysis revealed that fatty acids from the gastropod tissues are all (13)C-depleted relative to the gastropod biomass. This fractionation pattern is consistent with chemoautotrophy based on the Calvin-Benson cycle and subsequent fatty-acid biosynthesis from (13)C-depleted acetyl coenzyme A. The results from the present study are clearly different from those from our previous study for A. aff. hessleri from the Indian Ocean that harbors a chemoautotrophic endosymbiont belonging to the epsilon subdivision of the Proteobacteria, which mediates the reductive tricarboxylic acid cycle for carbon fixation. Thus, it is concluded here that two lineages of chemoautotrophic bacteria, phylogenetically distinct at the subdivision level, occur as the primary endosymbiont in one host-animal type, which is unknown for the other metazoans.

Nacre/bone interface changes in durable nacre endosseous implants in sheep

Raw nacre implants persist even after 9 months of implantation into bone tissue in sheep. However the nacre surface undergoes a limited biodegradation process. Smooth-surfaced nacre implants were seen to become microporous after implantation. The results of these long-term, in vivo studies show that the overall process involves bone-resorbing cells, relies on a two-phase mechanism and may correspond to a regulation process. The rate of surface change depends on the bone implantation site and the nacre/bone interaction. The in vivo biodegradability of nacre is a highly variable parameter. The size and shape of the implanted nacre and the cellular environment of the implant are key factors in determining the biodegradation kinetics of the nacre in a living system.

Anti-tubulin labeling reveals ampullary neuron ciliary bundles in opisthobranch larvae and a new putative neural structure associated with the apical ganglion

This investigation examines tubulin labeling associated with the apical ganglion in a variety of planktotrophic and lecithotrophic opisthobranch larvae. Emphasis is on the ampullary neurons, in which ciliary bundles within the ampulla are strongly labeled. The larvae of all but one species have five ampullary neurons and their associated ciliary bundles. The anaspid Phyllaplysia taylori, a species with direct development and an encapsulated veliger stage, has only four ampullary neurons. The cilia-containing ampulla extends to the pretrochal surface via a long, narrow canal that opens to the external environment through a very small pore (0.1 microm diameter). Cilia within the canal were never observed to project beyond the opening of the apical pore. The ampullary canals extend toward and are grouped with the ciliary tuft cells and remain in this location as planktotrophic larvae feed and grow. If, as has been reported, the ciliary tuft is motile, the pores may be continually bathed in fresh seawater. Such an arrangement would increase sensitivity to environmental chemical stimuli if the suggested chemosensory function of these neurons is correct. In general, ciliary bundles of newly hatched veligers are smaller in planktotrophic larvae than in lecithotrophic larvae. In planktotrophic larvae of Melibe leonina, the ciliary bundles increase in length and width as the veligers feed and grow. This may be related to an increase in sensitivity for whatever sensory function these neurons fulfill. An unexpected tubulin-labeled structure, tentatively called the apical nerve, was also found to be associated with the apical ganglion. This putative nerve extends from the region of the visceral organs to a position either within or adjacent to the apical ganglion. One function of the apical nerve might be to convey the stimulus resulting from metamorphic induction to the visceral organs.

Microstructural characacterization of shell components in the mollusc Physa sp

Shells of the freshwater, pulmonate snail Physa (Mollusca, Gasteropoda), ranging from 0.5 to 10 mm in length, were studied using scanning microscopy, x-ray analysis, and infrared spectroscopy. Results obtained suggest that the shell is composed of aragonite, which occurs in several distinct crystalline forms. A selective distribution of crystalline forms (hexagonal plates, prisms, rhombohedra, and spherulites) occurred along specific sites of the shell. A variable distribution of the forms was also detected in adult shells and in protoconchs of developing embryos. Qualitative elemental analysis, using an energy-dispersive spectrometer, corroborates the presence of calcium, phosphorus and sulphur ions.

Hydrothermal vent gastropods from the same family (Provannidae) harbour e- and gamma-proteobacterial endosymbionts

The discovery of new hydrothermal vent systems in the back-arc basins of the Western Pacific revealed chemosynthesis-based faunal communities distinct from those of other vents. These vents are dominated by two related gastropods (Alviniconcha spp. and Ifremeria nautilei) that harbour symbiotic bacteria in their gills. We used comparative 16S ribosomal RNA (rRNA) gene sequencing and in situ hybridization with rRNA-targeted probes to characterize the bacterial symbionts of Alviniconcha sp. and I. nautilei from the Manus Basin in the Western Pacific. The analyses revealed that these two gastropod species, although affiliated with the same family, harbour phylogenetically distant chemosymbionts, suggesting independent origins of these endosymbioses. The I. nautilei endosymbiont clusters with sulfur-oxidizing bacteria within the gamma-Proteobacteria, as is the case for all previously characterized endosymbionts from a wide diversity of host taxa harbouring thioautotrophic prokaryotes. In contrast, the Alviniconcha endosymbiont is affiliated with sulfur-oxidizing bacteria within the epsilon-Proteobacteria. These results show that bacteria from the epsilon-Proteobacteria are also capable of forming endosymbiotic associations with marine invertebrates from chemosynthetic environments. More generally, the endosymbiotic lifestyle is now shown to be distributed throughout all recognized classes of the Proteobacteria.

Endolithic fungi in marine ecosystems

Fungi are an important constituent of microbial endolithic assemblages in marine ecosystems. As euendoliths, they penetrate limestone, mollusk shells and other carbonate substrates, where they can exploit mineralized organic matter, attack their hosts, or engage in symbiotic relationships. They leave specific boring traces, which can be identified in the fossil record and described as trace fossils. Their distribution is independent of light and extends from the intertidal ranges to abyssal oceanic depths. Important, but insufficiently studied, is the role of aggressive endolithic fungi in skeletons of corals where they are ubiquitous and globally distributed. In healthy growing reef corals, the relationship between the coral coelenterate, endolithic algae and fungi is in a state of equilibrium, but can turn detrimental to coral health when reefs are exposed to environmental stress.

Self-organisation of nacre in the shells of Pterioida (Bivalvia: Mollusca)

Changes in the morphology and orientation of crystals have been tracked across the nacreous layer in pterioid bivalves, by SEM and X-ray diffraction. Early crystals nucleated on the organic membrane usually covering the outer prismatic layer. They formed polycrystalline aggregates of various shapes, first at prism boundaries and progressively invading the prisms in a centripetal fashion. Their c-axes were perpendicular to the substrate, but the a- and b-axis were variously arranged. As they grew, nacreous crystals became individualized and acquired a common crystallographic orientation also in the a-b plane (with the b-axis in the direction of shell growth). Initial lamellae had well delineated growth fronts, which later changed to diffuse. The increasing orientation was explained by a competition model between nacreous plates within the growing lamellae, which was particularly effective in lamellae with delineated fronts. This competition between adjacent crystals could not take place in gastropod nacre because, contrary to the stepped mode of growth of bivalve nacre, vertical stacking predominated. Our model is an alternative to the heteroepitaxial model where the organic matrix supposedly acted as an orienting template, but is nevertheless compatible with the mineral bridge hypothesis that implied epitaxial growth of stacking nacreous tablets.

Heavy metal bioavailability and effects: II. Histopathology-bioaccumulation relationships caused by mining activities in the Gulf of Cádiz (SW, Spain)

The relationship between bioaccumulation of heavy metals (Zn, Cd, Pb and Cu) and histological lesions in different tissues of organisms is assessed in three different areas located in the southwest of Spain in the Gulf of Cádiz (Ría of Huelva, Guadalquivir estuary and Bay of Cádiz) affected and non-affected by mining activities. Data included in these relationships were obtained along the years 2000 and 2001 to address the impact of the Aznalcóllar mining spill on the Guadalquivir estuary. The bioaccumulation and the histological lesions measured in this seasonal study in the Gudalquivir estuary were linked to derive tissue quality guidelines (TQGs) by means of a multivariate analysis approach (MAA). Sediments collected in the same areas of study were used to expose organisms during the survey carried out in autumn 2001 and to address the relationship between bioaccumulation and histological lesions under laboratory conditions and related to chemicals bound to sediments. Lesions show that the organisms collected in the ría of Huelva and exposed to their sediments were severe, intermediate in the Guadalquivir estuary and absent in the Bay of Cádiz. Results show that the Guadalquivir estuary trends to recover its initial status quo previous to the mining spill. The link between chemical concentration and the lesions measured in the same tissues using MAA permits to derive tissue quality guidelines for two organisms, oysters (Crassostrea angulata) and clams (Scrobicularia plana) collected in the Guadalquivir estuary and associated with the heavy metals from the mining spill (Zn and Cd). The TQG values expressed as concentrations (mgkg-1--dry weight) not associated with biological effects are for oysters, Zn, 8603, Cd, 3.42; and for clams Zn, 800, Cd, 2.6.

Foregut glands of Solenogastres (Mollusca): Anatomy and revised terminology

In the molluscan class Solenogastres, different types of foregut glands vary in number, structure, and location within the foregut. The present article describes their anatomy and cytology and intends to clarify their confused terminology. Pharyngeal glands, esophageal glands, and the more complex dorsal and ventrolateral foregut glands can be distinguished. The ventrolateral foregut glands (ventral foregut glandular organs, ventral salivary glands of auct.), in the literature subdivided previously into four types, are revisited here in the context of current vertebrate gland terminology. The results of recent investigations are added to earlier ones, and a classification system for these multicellular glands is proposed. This system is based on cytological characters of glandular cells (intra- or extraepithelial), characters of the associated musculature (inner or outer musculature), location of the gland relative to the pharynx epithelium (endoepithelial or exoepithelial), characters of the gland openings (paired or unpaired), morphology of the gland duct (simple or branched), and some additional features like the arrangement of glandular cells along the gland ducts. Gross morphology and anatomy of ventrolateral foregut glands constitute useful taxonomic characters in determining higher taxa (family level), and finer details of the anatomy and cytology are useful in determining lower levels (genus and species). Possible pathways for the evolution of the different gland types of Solenogastres in relation to foregut glands present in the other molluscan clades are presented. The importance of ventrolateral foregut gland characters for phylogenetic considerations within the Solenogastres is discussed.

Ultrastructure of spermatozoa of Bullacta exarata (philippi) and its significance on reproductive evolution and physio-ecological adaptation

The morphology and ultrastructure of Bullacta exarata spermatozoa observed by light and transmission electron microscopy are presented in this paper. The spermatozoa is composed of head with a simple acrosomal complex and an elongated nucleus, and tail with a midpiece, principal piece and an end piece. The midpiece consists of a mitochondrial ring, and the principal piece is composed of axoneme and lateral fin. The structure of B. exarata spermatozoa differs significantly from that of other gastropods, especially in the lateral fin and the principal piece, which was described scarcely before. A comparison is made between B. exarata and other gastropods, and its significance on reproductive evolution and physio-ecological adaptation is preliminarily discussed.

Fouling deterrence on the bivalve shell Mytilus galloprovincialis: a physical phenomenon?

The physical nature of fouling deterrence by the mussel Mytilus galloprovincialis was investigated using high-resolution biomimics of the bivalve surface. The homogeneous microtextured surface of M. galloprovincialis (1.94 +/- 0.03 microm), the smooth surface of the bivalve Amusium balloti (0 microm), and moulds of these surfaces (biomimics) were compared with controls of smooth (0 microm) and sanded moulds, (55.4 +/- 2.7 microm) and PVC strips (0 microm) in a 12-week field trial. The shell and mould of M. galloprovincialis were fouled by significantly fewer species and had significantly less total fouling cover than the shell and mould of A. balloti over a 12-week period. However, the major effects were between surfaces with and without microtopography. Surface microtopography, be it structured as in the case of M. galloprovincialis shell and mould, or random as in the case of the sanded mould, had a lower cover of fouling organisms than treatments without microtopography after 6 weeks. There was also no difference between the effect of the M. galloprovincialis mould and the sanded mould. The strong fouling deterrent effects of both these surfaces diminished rapidly after 6 to 8 weeks while that of M. galloprovincialis shell remained intact for the duration of the experiment suggesting factors in addition to surface microtopography contribute to fouling deterrence.

Articulated Palaeozoic fossil with 17 plates greatly expands disparity of early chitons

Modern chitons (Mollusca: Polyplacophora) possess a highly conserved skeleton of eight shell plates (valves) surrounded by spicules or scales, and fossil evidence suggests that the chiton skeleton has changed little since the first appearance of the class in the Late Cambrian period (about 500 million years before present, Myr bp). However, the Palaeozoic problematic taxon Multiplacophora, in spite of having a more complex skeleton, shares several derived characters with chitons. The enigmatic status of the Multiplacophora is due in part to the fact that its members had an exoskeleton of numerous calcium carbonate valves that usually separated after death. A new articulated specimen from the Carboniferous period (about 335 Myr bp) of Indiana reveals that multiplacophorans had a dorsal protective surface composed of head and tail valves, left and right columns of overlapping valves (five on each side), and a central zone of five smaller valves, all surrounded by an annulus of large spines. Here we describe and name the articulated specimen and present evidence that multiplacophorans were chitons. Thus the highly conserved body plan of living chitons belies the broad disparity of this clade during the Palaeozoic era.

Conch shell structure and its effect on mechanical behaviors

The shell of pink conch is a kind of natural well-designed composite with excellent mechanical properties, which provides us information for material design. In this paper the microstructures of pink conch shell are characterized by using SEM and TEM. The microscopic analysis indicates that the pink conch shell is with crossed-lamellar microstructure and the angle between two second-order lamellae is 70-90 degrees. The cracking and fracture morphologies indicate that the crack deflection, bridge and fiber pullout are the main toughening mechanisms. Bamboo lamellae are employed to make the simple bio-mimetic model materials. In the model material the rotated angle between the fibers of each glued lamella varies from 0 degrees to 90 degrees. Three-point-bending is employed to test the properties of models material, such as the elastic modulus, the flexural strength and the fracture strain energy density. It is found that the fracture strain energy density of the specimens with the rotated angle between the fibers of the layers about 60 degrees exhibits the maximum value, which is close to the angle of the two second-order lamellae of the conch shell.

Light production by the arm tips of the deep-sea cephalopod Vampyroteuthis infernalis

The archaic, deep-sea cephalopod Vampyroteuthis infernalis occurs in dark, oxygen-poor waters below 600 m off Monterey Bay, California. Living specimens, collected gently with a remotely operated vehicle (ROV) and quickly transported to a laboratory ashore, have revealed two hitherto undescribed means of bioluminescent expression for the species. In the first, light is produced by a new type of organ located at the tips of all eight arms. In the second, a viscous fluid containing microscopic luminous particles is released from the arm tips to form a glowing cloud around the animal. Both modes of light production are apparently linked to anti-predation strategies. Use of the tip-lights is readily educed by contact stimuli, while fluid expulsion has a much higher triggering threshold. Coelenterazine and luciferase are the chemical precursors of light production. This paper presents observations on the structure and operation of the arm-tip light organs, the character of the luminous cloud, and how the light they produce is incorporated into behavioral patterns.

Original molluscan radula: comparisons among Aplacophora, Polyplacophora, Gastropoda, and the Cambrian fossil Wiwaxia corrugata

As the original molluscan radula is not known from direct observation, we consider what the form of the original radula may have been from evidence provided by neomenioid Aplacophora (Solenogastres), Gastropoda, Polyplacophora, and the Cambrian fossil Wiwaxia corrugata (Matthews). Conclusions are based on direct observation of radula morphology and its accessory structures (salivary gland ducts, radular sac, anteroventral radular pocket) in 25 species and 16 genera of Aplacophora; radula morphogenesis in Aplacophora; earliest tooth formation in Gastropoda (14 species among Prosobranchia, Opisthobranchia, and Pulmonata); earliest tooth formation in four species of Polyplacophora; and the morphology of the feeding apparatus in W. corrugata. The existence of a true radula membrane and of membranoblasts and odontoblasts in neomenioids indicates that morphogenesis of the aplacophoran radula is homologous to that in other radulate Mollusca. We conclude from p redness of salivary gland ducts, a divided radular sac, and a pair of anteroventral pockets that the plesiomorphic state in neomenioids is bipartite, formed of denticulate bars that are distichous (two teeth per row) on a partially divided or fused radula membrane with the largest denticles lateral, as occurs in the genus Helicoradomenia. The tooth morphology in Helicoradomenia is similar to the feeding apparatus in W. corrugata. We show that distichy also occurs during early development in several species of gastropods and polyplacophorans. Through the rejection of the null hypothesis that the earliest radula was unipartite and had no radula membrane, we conclude that the original molluscan radula was similar to the radula found in Helicoradomena species.

[RNA polymerase II and pre-mRNA splicing factors in diplotene oocyte nuclei of the giant African gastropod Achatina fulica]

The nuclear distribution of pre-mRNA splicing factors (snRNPs and SR-protein SC35) and unphosphorylated from of RNA polymerase II (Pol II) was studied using fluorescent and immunoelectron cytochemistry in diplotene oocytes of the gastropod Achatina fulica. Association of Pol II and splicing factors with oocyte nuclear structures was analysed. The antibodies against splicing factors and Pol II were shown to label perichromatin fibrils at the periphery of condensed chromatin blocks as well as those in interchromatin regions of nucleoplasm. The revealed character of distribution of snRNPs, SC35 protein, and Pol II, together with the decondensed chromatin and absence of karyosphere, enable us to suggest that oocyte chromosomes maintain their transcriptional activity at the diplotene stage of oogenesis. In A. fulica oocytes, sparse nuclear bodies (NBs) of a complex morphological structure were revealed. These NBs contain snRNPs rather than SC35 protein. NBs are associated with a fibrogranular material (FGM), which contains SC35 protein. No snRNPs were revealed in this material. Homology of A. fulica oocyte nuclear structures to Cajal bodies and interchromatin granule clusters is discussed.

An ultrastructural study of connective tissue in mollusc integument III. Cephalopoda

We studied structure and ultrastructure of the subepidermal connective tissue (SEC) of the integument of three cephalopods (Sepia officinalis, Octopus vulgaris and Loligo pealii). In all species, three distinct regions of the SEC were recognised: (a) an outer zone (OZ) that included the dermal-epidermal junction, and consisted of a thin layer of connective tissue containing muscles, (b) an extensive middle zone (MZ) containing a compact network of collagen fibres and numerous cells, (c) an inner zone (IZ) of loose connective tissue that merged with muscular fascia. This arrangement differs from that in bivalves and gastropods and recalls vertebrate integument. The dermal-epidermal junction of cephalopods differed from that of bivalves, gastropods and mammals in that the epidermal cells did not possess hemidesmosomes, and their intermediate filaments terminated directly in the plasmamembrane. The thick (120-500 nm) basal membrane (BM) had a superficial zone containing a regular array of granules; a lamina densa composed of a compact network of small filaments and granules; and an IZ distinguished by expansions of granular material protruding into underlying structures. Collagen fibres contained fibroblast-derived cytoplasmic thread, running through their centres and were surrounded by granular material that joins them to adjacent fibres. The collagen fibrils were of medium diameter (30-80 nm) had the typical ultrastructure of fibrillar collagens, and were surrounded by abundant interfibrillar material. The hypodermis was loose, with a network of small bundles of collagen fibrils. Cephalopod integument appears to represent a major evolutionary step distinguishing this class of molluscs.

Soluble organic matrices of the calcitic prismatic shell layers of two Pteriomorphid bivalves. Pinna nobilis and Pinctada margaritifera

The calcitic prisms of the shells of two bivalves, Pinna and Pinctada, are considered simple prisms according to some morphological and mineralogical characteristics. Scanning electron microscopic and atomic force microscopic studies show that the microstructures and nanostructures of these two shells are different. Pinna prisms are monocrystalline, whereas Pinctada prisms are not. Moreover, intraprismatic membranes are present only in the Pinctada prisms. The soluble organic matrices extracted from these prisms are acidic, but their bulk compositions differ. Ultraviolet and infrared spectrometries, fluorescence, high pressure liquid chromatography, and electrophoresis show that the sugar-protein ratios and the molecular weights are different. Sulfur is mainly associated with acidic sulfated sugars, not with amino acids, and the role of acidic sulfated sugars is still underestimated. Thus, the simple prism concept is not a relevant model for the biomineralization processes in the calcitic prismatic layer of mollusk shells.

Structural organization of the nervous system of the mantle cavity wall and organs in prosobranch mollusks

A variety of common histological stains was used, along with the Golgi and Colognier silver impregnation methods and electron microscopy, to perform a comparative study of the morphological characteristics of receptor and nerve cells and their interactions in the nervous system of the wall of the mantle cavity and mantle derivatives - gills, siphon, and osphradia - of the marine prosobranch gastropod mollusks Buccinum undatum and Littorina littorea. The results are discussed along with our own previously obtained data on the organization of the osphradial sensory organs and the afferent elements of the mantle cavity wall in other prosobranch mollusks - the terrestrial Pomatia elegans and the freshwater Viviparus contectus and Pomacea paludosa. Using the nervous system of the complex of mantle organs of prosobranch mollusks as examples, the structural features and evolutionary trends of the afferent part of the visceral nervous system of gastropods are discussed.

[Localization of NADPH diaphorase in the central nervous system of the bivalve mollusc Cristaria tuberculata]

By histochemical and electron histochemical methods, NADPH-diaphorase was discovered in neurons and their processes of all ganglia of the central nervous system (CNS) of Cristaria tuberculata. Small cells predominated among neurons containing NADPH-diaphorase. Ultrastructural localization of the enzyme was detected on the perinuclear membrane and membranes of endoplasmic reticulum, cytoplasmic granules and cytosol. In the majority of cells, the reaction product was commonly found in cytoplasmic granules-cytosomes. We studied peculiarities of synaptic contacts between nitrogen oxide synthesizing neurons and their processes. In active synaptic areas, a sediment was discovered on the internal surface of pre- and postsynaptic membranes.

[Somatic polyploidy in neurons of the gastropod mollusca. III. Mitosis and endomitosis in the postnatal development of neurons in the Succinea snail central nervous system]

General morphology of chromatin, the number of chromosomes and chromocenters in normal condition and at the increase of bivalent cation (Ca2+, Mg2+) concentration were studied with the purpose to reveal mechanisms of polyploidization of neuron nuclei in the snail Succinea lauta (Gastropoda, Pulmonata). The morphology of nuclei was studied on squashed preparations. Normal diploid mitoses are described in the cerebral ganglia. A possibility is supposed that part of neurons or neuroblasts in the central nervous system (CNS) of succineid snail may divide mitotically. It has been shown that the basic mechanism of neuron postnatal growth is endomitotic polyploidization of nuclei. The transition from ordinary mitosis to polyploid cycles occurs via restitutional (polyploidizing) mitosis (4c2n-->4c4n). The next endocycles are carried out by means of classic endomitosis up to reaching the highest ploidy levels--4096n--16,384n. The study of general morphology of chromatin and chromocenters at normal condition and at artificial compactization enabled us to exclude any probability of polyteny in the CNS of lauta.

[Localization of NADPH diaphorase in central nervous system of the chiton Leptochiton assimilis in normal conditions and during hypoxia]

By light and electron microscope histochemical and cytochemical methods, the localization and activity of NADPH-diaphorase (NADPH-d) were studied in the central nervous system (CNS) of the chiton in control and after hypoxia. After acute hypoxia, the enzymatic activity increased in all regions of CNS. At a chronic hypoxia, the activity of NADPH-d decreased to remain, however, higher than in control. Ultrastructural studies confirmed the availability of structural changes in neurons, and shifts in the activity of NADPH-d in control and in experimental mollusks. The elevated enzymatic activity revealed in this study may be due to the fact that these mollusks have been evolutionary adapted to a periodical oxygen deficiency.

Microstructure of Monoplacophora (Mollusca) shell examined by low-voltage field emission scanning electron and atomic force microscopy

The shell of Micropilina arntzi (Mollusca: Monoplacophora), a primitive molluscan class, was examined by using field emission scanning electron microscopy (FESEM) at low voltage and atomic force microscopy (AFM). The use of these two techniques allowed the observation of fine details of Micropilina arntzi shell and contributed to bring new features concerning the study of molluscan shell microtexture. Imaging with low-voltage FESEM provided well-defined edge contours of shell structures, while analyzing the sample with AFM gave information about the step height of stacked internal structures as well as the dimension of the particles present in their surface at a nanometric level. The shell microstructure of Monoplacophora species presents different patterns and may be a taxonomic implication in the systematic studies of the group.

A single mitochondrial lineage is shared by morphologically and allozymatically distinct freshwater Corbicula clones

Despite that the exotic invasion and rapid range expansion of Asian freshwater Corbicula into new environments have been of intensive research topic in freshwater ecology, the genetic structures of freshwater Corbicula in its native range remain poorly understood. In this study, the genetic structures of two Korean freshwater Corbicula clonal lineages were characterized by cross-referencing the nuclear genomic structures with mtDNA sequence analysis. In spite of substantial genetic differences (Nei's D = 0.363-0.372) and a pronounced level of fixed allelic distinctions (in six of 20 allozyme loci) between Corbicula lineages, no lineage-specific mtDNA differentiation was observed. The evident disjunction between mtDNA sequences and nuclear genomes is a compelling evidence for the existence of interspecific nuclear hybrid genome structures, comprising different combinations of paternal and maternal contributions. This unusual novel finding is the first case demonstrating that morphologically and allozymatically distinct, yet mitochondrially identical clonal lineages exist in the genus Corbicula. However, we could not find the ancestral species for these two clonal lineages in the present study, and the answer for this question must wait until the genetic structure of Asian Corbicula taxa is fully characterized.

Mollusc larval shell formation: amorphous calcium carbonate is a precursor phase for aragonite

The larval shells of the marine bivalves Mercenaria mercenaria and Crassostrea gigas are investigated by polarized light microscopy, infrared spectroscopy, Raman imaging spectroscopy, and scanning electron microscopy. Both species contain similar shell ultrastructures. We show that larval shells contain amorphous calcium carbonate (ACC), in addition to aragonite. The aragonite is much less crystalline than non-biogenic aragonite. We further show that the initially deposited mineral phase is predominantly ACC that subsequently partially transforms into aragonite. The postset juvenile shell, as well as the adult shell of Mercenaria also contains aragonite that is less crystalline than non-biogenic aragonite. We conclude that ACC fulfills an important function in mollusc larval shell formation. It is conceivable that ACC may also be involved in adult shell formation.

Mollusc larval shell formation: amorphous calcium carbonate is a precursor phase for aragonite

The larval shells of the marine bivalves Mercenaria mercenaria and Crassostrea gigas are investigated by polarized light microscopy, infrared spectroscopy, Raman imaging spectroscopy, and scanning electron microscopy. Both species contain similar shell ultrastructures. We show that larval shells contain amorphous calcium carbonate (ACC), in addition to aragonite. The aragonite is much less crystalline than non-biogenic aragonite. We further show that the initially deposited mineral phase is predominantly ACC that subsequently partially transforms into aragonite. The postset juvenile shell, as well as the adult shell of Mercenaria also contains aragonite that is less crystalline than non-biogenic aragonite. We conclude that ACC fulfills an important function in mollusc larval shell formation. It is conceivable that ACC may also be involved in adult shell formation.

Magnetic anisotropy of the radula of chiton Acanthochiton rubrolinestus LISCHKE

The magnetic anisotropy of the whole radula, the major lateral radula teeth, and magnetic material in the major lateral radula teeth of the chiton Acanthochiton rubrolinestus LISCHKE have been studied by a magnetic torque meter and superconducting quantum interference device (SQUID) magnetometer. The length and width axes of the teeth are the easily magnetized axes, while the thickness axis is difficult to magnetize. The width and thickness axes of the radula are the easily magnetized axes, and the length axis is difficult to magnetize. The measurement results of the whole radula and the major lateral radula teeth agree well with each other. The magnetic anisotropy of the magnetic material is given as well as a possible distribution of the magnetic material in the major lateral radula teeth.

Fine structural study of the spermatogenic cycle in Pitar rudis and Chamelea gallina (Mollusca, Bivalvia, Veneridae)

A comparative ultrastructural study of spermatogenesis was performed in the bivalve molluscs Pitar rudis and Chamelea gallina (Veneridae) from Turkey. Sertoli cells appeared to be rich in glycogen, lipid droplets and germ-cell phagolysosomes. Premeiotic cells exhibited nuage and a flagellum, with the Golgi complex and the rough endoplasmic reticulum originating proacrosomal vesicles during the pachytene stage. In round spermatids, the acrosomal vesicle migrated linked to the plasma membrane. In P. rudis, the acrosomal vesicle base formed a thin expansion that attached to the nuclear apex and was associated with development of the perforatorium. The cap-shaped acrosomal vesicle then differentiated into external and internal regions, and also into a small apical light region, although some cells exhibited an apical extension of the external component. On the contrary, two lateroapical light pouches developed in C. gallina. During spermiogenesis, chromatin became fibrillar and then condensed while the nucleus turned conical shaped in P. rudis or slightly curved in C. gallina. In P. rudis, the midpiece contained glycogen and four mitochondria, although five mitochondria were sometimes observed, whereas in C. gallina the midpiece contained four mitochondria. Comparison with other members of Veneroida shows a common ectaquasperm type, but novel findings in acrosome biogenesis.

Neurogenesis in the mossy chiton, Mopalia muscosa (Gould) (Polyplacophora): evidence against molluscan metamerism

Neurogenesis in the chiton Mopalia muscosa (Gould, 1846) was investigated by applying differential interference contrast microscopy, semithin serial sectioning combined with reconstruction techniques, as well as confocal laser scanning microscopy for the detection of fluorescence-conjugated antibodies against serotonin and FMRFamide. The ontogeny of serotonergic nervous structures starts with cells of the apical organ followed by those of the cerebral commissure, whereas the serotonergic prototroch innervation, pedal system, and the lateral cords develop later. In addition, there are eight symmetrically arranged serotonergic sensory cells in the dorsal pretrochal area of the larva. FMRFamide-positive neural elements include the cerebral commissure, specific "ampullary" sensory cells in the pretrochal region, as well as the larval lateral and pedal system. In the early juvenile the cerebral system no longer stains with either of the two antibodies and the pedal system lacks anti-FMRFamide immunoreactivity. Outgroup comparison with all other molluscan classes and related phyla suggests that the cord-like, nonganglionized cerebral system in the Polyplacophora is a reduced condition rather than a primitive molluscan condition. The immunosensitivity of the pedal commissures develops from posterior to anterior, suggesting independent serial repetition rather than annelid-like conditions and there is no trace of true segmentation during nervous system development. Polyplacophoran neurogenesis and all other available data on the subject contradict the idea of a segmented molluscan stem species.

Anatomical correlates of venom production in Conus californicus

Like all members of the genus, Conus californicus has a specialized venom apparatus, including a modified radular tooth, with which it injects paralyzing venom into its prey. In this paper the venom duct and its connection to the pharynx, along with the radular sac and teeth, were examined using light and transmission electron microscopy. The general anatomy of the venom apparatus resembles that in other members of the genus, but several features are described that have not been previously reported for other species. The proximal (posterior) quarter of the venom duct is composed of a complex epithelium that may be specialized for active transport rather than secretion. The distal portion of the duct is composed of a different type of epithelium, suggestive of holocrine secretion, and the cells display prominent intracellular granules of at least two types. Similar granules fill the lumen of the duct. The passageway between the lumen of the venom duct and pharynx is a flattened branching channel that narrows to a width of 10 micro m and is lined by a unique cell type of unknown function. Granular material similar to that in the venom duct was also found in the lumen of individual teeth within the radular sac. Mass spectrometry (MALDI-TOF) demonstrated the presence of putative peptides in material derived from the tooth lumen, and all of the more prominent species were also evident in the anterior venom duct. Radular teeth thus appear to be loaded with peptide toxins while they are still in the radular sac.

Comparison of the soluble matrices of the calcitic prismatic layer of Pinna nobilis (Mollusca, Bivalvia, Pteriomorpha)

The calcitic prisms of the outer layer of the shell of Pinna nobilis, surrounded by thick organic walls, contain a soluble intracrystalline matrix. The structure and composition of the outer interprismatic walls are not well known. The current viewpoint is they are composed of an insoluble matrix. Another thick organic structure, the interlamellar sheet of the nacreous layer, is composed of insoluble and soluble matrices. The composition of two sets of soluble organic matrices from the calcitic layer of Pinna nobilis, extracted with and without the organic walls are compared. According to the various analyses (SEM and AFM, UV and FTIR spectrometry, HPLC, electrophoreses, XANES), the main characteristics of the two matrices are similar, but not identical. Thus, the organic walls contain soluble components. However, the three-layered structure of the interlamellar sheet of the nacreous layer has not been observed.

Larval and metamorphic development of the foregut and proboscis in the caenogastropod Marsenina (Lamellaria) stearnsii

The specialized, postmetamorphic feeding structures of predatory caenogastropods evolved by changes to an ancestral caenogastropod developmental program that generated a planktotrophic larval stage followed by a herbivorous postmetamorphic stage. As part of a program of comparative studies aimed at reconstructing these developmental changes, I studied the development of the postmetamorphic feeding system of Marsenina stearnsii using histological sections for light microscopy and scanning and transmission electron microscopy. The feeding system of this species has two very different designs during ontogeny. The larval system uses ciliary effectors to capture and ingest microalgae, whereas the juvenile/adult system includes a proboscis, jaws, and radular apparatus for predation on ascidian zooids. The postmetamorphic foregut begins to develop during the early larval phase, but the anlagen does not interfere with larval feeding because it develops as an increasingly elaborate outpocketing from the ventral wall of the larval esophagus. At metamorphosis, an opening is created in the anterior tip of the prospective, postmetamorphic buccal cavity and the margins of this opening anneal with the metamorphically remodeled lips of the larval mouth. This process exposes the jaws, which differentiate within the buccal cavity prior to metamorphosis. As a working hypothesis, I suggest that rupture of the buccal cavity to the outside at metamorphosis was selected as a mechanism to allow precocious development of jaws in species where jaws enhanced feeding performance by young juveniles. The larval esophagus of M. stearnsii appears to be completely destroyed at metamorphosis. Larval esophageal cells have distinctive apical characteristics (cilia, blebbed microvilli, stacks of lamellae within the glycocalyx) and no cells having this signature persist through metamorphosis. Development of the proboscis and proboscis sac, which begins prior to metamorphosis, conforms to previous descriptions of pleurembolic proboscis development, although an acrembolic proboscis has been ascribed to members of the Lamellaroidea.