Structure and composition of the aragonitic crossed lamellar layers in six species of Bivalvia and Gastropoda

The microstructures, the chemical composition and the soluble organic matrices of the aragonitic crossed lamellar layers of the shells of six species of molluscs have been studied. The microstructures and chemical contents are similar, whereas the quantities of organic matrices are variable. All the soluble matrices are glycoproteins, with low S contents. Their molecular weights, the protein-sugar ratios and acidities are variable. Neither a gastropod nor a bivalve pattern is recognized. The diversity of the organic matrices probably plays a main role in the fossilization processes of mollusc shells.     

Crystalline arrangement and nanostructure of aragonitic crossed lamellar layers of the Meretrix lusoria shell

The crystallographic microstructure of Meretrix lusoria shells was investigated using scanning electron microscopy (SEM), X-ray diffractometry (XRD), and transmission electron microscopy (TEM). Crystallite sizes were determined by XRD analysis as 72 nm, which was quite similar to the 70 nm as measured by SEM. The shell comprised aggregates of hexagonal plates of aragonite (500 nm wide, 70 nm high) and organic matter. These plates were fourth-order units of an aragonitic crossed order lamellar structure. Subsequent TEM images showed the hexagonal plates’ nanostructure. The electron diffraction pattern of the fourth-order units revealed a consistent orientation of the hexagonal plates. The fourth-order lamellae (hexagonal crystallites) were piled up in the [0 0 1] direction to produce slender prisms (third-order lamellae), arranged mutually parallel, thereby forming a broad tablet (second-order lamellae). The second-order lamellae were piled up in different directions to form the first-order lamellae. The orientation level obtained from XRD and SEM images showed that the crossed lamellar layer was piled up curvilinearly, forming semi-circular growth lines. X-ray diffraction patterns of the cross-sections of the middle layer (vertical and parallel to the growth line) showed that the c axes of aragonite have a disposition of about 20° to the growth direction.     

Structure and composition of the septal nacreous layer of Nautilus macromphalus L. (Mollusca, Cephalopoda)

The nacreous layer of Mollusca is the best-known aragonitic structure and is the usual model for biomineralization. However, data are based on less than 10 species. In situ observations of the septal nacreous layer of the cephalopod Nautilus shell has revealed that the tablets are composed of acicular laths. These laths are composed of round nanograins surrounded by an organic sheet. No hole has been observed in the decalcified interlamellar membranes. A set of combined analytical data shows that the organic matrices extracted from the nacreous layer are glycoproteins. In both soluble and insoluble matrices, S amino acids are rare and the soluble organic matrices have a higher sulfated sugar content than the insoluble matrices. It is possible that the observed differences in the structure and composition of the nacreous layers of the outer wall and septa of the Nautilus shell have a dual origin: evolution and functional adaptation. However, we have no appropriate data as yet to answer this question.     

Phylogeny of the Cocculinoidea (Mollusca, Gastropoda)

Abstract. The superfamily Cocculinoidea is a group of marine, deep-water, limpet-like gastropods. Recent speculation surrounding their affinities has concentrated on their placement within the Gastropoda. However, phylogenetic relationships within the Cocculinoidea, especially the monophyly of families and genera within the group, remain poorly understood. Phylogenetic analysis of 31 morphological characters for 15 cocculinoidean taxa and 2 outgroups resulted in a single most parsimonious tree, length=70, CI=0.62, and RI=0.71. Monophyly of the Cocculinoidea, Cocculinidae, and the genera Cocculina and Coccopigya was supported; Paracocculina and Coccocrater were found to be paraphyletic. Character optimization demonstrates that many characters often cited as diagnostic of various taxa, are often homoplastic and/or synapomorphies at different hierarchical levels.     

The mechanism of shell elevation in Haliotis (Mollusca: Gastropoda) and a consideration of the evolution of the hydrostatic skeleton in Mollusca

Early in molluscan evolution, the development of a conical shell with shell or pedal retractor muscles led to the need of a mechanism for the extension of the foot or the raising of the shell. The forces generated during pedal retraction and extension have been studied in Haliotis midae , an easily obtainable and conveniently large archaeogastropod. In the mantle cavity, cephalopedal venous sinus and ventricle pressure pulses were observed during pedal retraction elicited by the shadow withdrawal reflex, but were never present during extension. However, pressure pulses were recorded in the proximal region of the columellar (or shell) muscle, both during retraction and pedal extension. Sections of this region of the muscle show a three dimensional network of muscle fibres, consisting of retractor fibres passing down to the foot and circumferential and radial fibres. Contraction of the two latter sets of fibres would bring about extension of the retractors, without the use of a discrete hydrostatic skeleton, and appears to be the principal mechanism of pedal extension. Similar muscular structures, here termed the muscular antagonistic system, have been observed in the columellar muscle of other gastropods and in the cephalopod mantle. In contrast, this system has not been observed in the proximal region of the pedal retractors of bivalves or scaphopods, for the pedal haemocoel, which allows muscular antagonism in the manner of a classical hydrostatic skeleton, has developed in association with the burrowing habit. The significance of the muscular antagonistic system in molluscan evolution is discussed.     

Phylogeography and phyloecology of dorid nudibranchs (Mollusca, Gastropoda)

Dorid nudibranchs exhibit a number of anatomical and physiological adaptations that reflect a complex evolutionary history. The lack of a fossil record means that all available information on the evolution of this group comes from phylogenetic evidence. Deep imbalances in the phylogeny of dorid nudibranchs indicates that this group has probably undergone random extinction events and subsequent speciation of derived lineages. Sister-group relationships between eastern Pacific, Atlantic and tropical Indo-Pacific taxa [(eastern Pacific, Atlantic) Indo-Pacific], repeated throughout several lineages of dorid nudibranchs, provide solid evidence of two consecutive vicariant events: (1) the closure of communication between the tropical Indo-Pacific region and the Atlantic and eastern Pacific, which began during the Oligocene–Miocene transition and was completed with the formation of the East Pacific Barrier, and (2) the rise of the Panama isthmus. The absence of solid dates for the effective isolation of the eastern Pacific and the central Pacific does not allow estimations of the time of diversification of dorid nudibranchs. Phylogenetic evidence indicates that omnivorism and de novo synthesis of chemical defences are probably the plesiomorphic conditions in dorid nudibranchs. It is also likely that all sponge-feeding cryptobranch dorids have a common ancestor, but other cases of sponge feeding in phanerobranch dorids have arisen independently. The numerous instances in which de novo synthesis was replaced by sequestration of chemicals from the prey are evidence of a great metabolic versatility in dorid nudibranchs.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 83 , 551–559.     

Fluctuation in shell composition in Nautilus (Cephalopoda, Mollusca): evidence from cathodoluminescence

Nautilusr pompilius and N. macromphalus shells show, under cathodoluminescence microscopy. a zoned luminescence of yellow to green for the former and blue to blue-green for the latter. Cathodoluminescence results from fluctuations in the amount of manganese present in the aragonite. There is a relationship between this phenomenon and growth lines. Luminescence intensity increases with ontogeny. Variations in the metabolic activity appear to be linked with the manganese content of the shell. Environmental factors may have an effect by way of their repercussion on the metabolic activity of Nautilus . The Mn²+ amount may be influenced by the physico-chemical pattern of the surrounding water but it appears also to be controlled at the species level.     

Biochemical variation and shell shape in populations of the fresh-water snail Lymnaea peregra (Mollusca, Gastropoda, Pulmonata) from south-west Ireland

Lymnaea peregra was collected at sites, predominantly from the Caha plateau, in south-west Ireland. These included sites investigated by previous workers and found to contain populations with a distinctive pattern of variation in shell-shape. Normally spired populations and non-spired ( involuta, sub-involuta ) could occur in adjacent loughs in the same watershed. The biochemical variation in some of these populations was investigated using horizontal starch-gel zone electrophoresis. Fourteen biochemical loci were investigated. Although circumscribed by difficulties in collecting and low sample numbers, the investigation showed that the pattern of variation in shell shape was still present and was not correlated or associated with the biochemical variation. While this did not support hypotheses of geographic race formation or incipient speciation, it was in agreement with the current view of variation and speciation in fresh-water pulmonates.     

Isolation and characterization of 11 polymorphic microsatellite markers for the marine gastropod Concholepas concholepas (Brugière, 1789)

Because of its long‐lived planktonic stage, the marine gastropod Concholepas concholepas is expected to exchange larvae over large distances. However, discrepancies between expected and realized dispersal have been documented in marine invertebrates. To investigate relationships between potential and effective (i.e. gene flow) dispersal, we developed 11 microsatellite markers and investigate their usefulness by analysing two populations distant by c. 4000 km. The 11 loci were found to be highly polymorphic in both populations, with 12–51 alleles according to the locus. This polymorphism is strong enough to allow fine‐scale population analyses including larval studies and paternity analyses.     

Function of the flexible periostracal hairs in Trichotropis cancellata (Mollusca, Gastropoda)

Abstract. The marine snail Trichotropis cancellata possesses hairy projections of periostracum (outer shell layer) whose function is unknown. Although rigid shell projections in molluscs have been studied extensively, the selective advantage of flexible extensions of periostracum is less clear. None of the functions proposed previously for periostracum (e.g., protection from erosion and boring) are promoted when it is drawn into hair-like projections. We investigated hypothetical functions that may be served by flexible periostracal hairs, including predator deterrence, alteration of flow vectors to promote feeding or affect turbulence dynamics during freefall, and providing a differential substratum for epibionts. Our laboratory results indicate that crabs, Cancer oregonensis , and sea stars, Pycnopodia helianthoides , consumed snails with the periostracum removed more often than snails with an intact hairy periostracum. However, in both predatory species, some individuals showed no significant preference, and another crab species ( Cancer productus ) did not strongly discriminate based on the shell periostracum. Field studies showed no difference in the rate of predation on hairy- versus smooth-shelled snails. The hairs did not alter flow around the shells consistently in laboratory flume experiments. Additionally, hairy- and smooth-shelled kleptoparasitic snails grew at rates that were statistically indistinguishable, while hairy, suspension-feeding snails grew more slowly. The hairs did not impact the orientation of a snail after a falling event or the time to righting after a fall. The presence of the hairs did deter settlement by barnacles. We conclude that the hairy periostracum acts as a slight deterrent to crab and sea star predators and as a stronger deterrent to the settlement of large calcareous epibionts, such as barnacles, that would increase the weight the snail must bear and potentially increase drag.     

Conchometrics, systematics and distribution of Melanopsis (Mollusca: Gastropoda) in the Levant

In this conchometric study, the systematics and distribution of the freshwater gastropod Melanopsis in the Levant are described. Of the ten species found, three are widespread, two have narrow distributions and five are known only from their type locality. Five smooth‐shelled species are recognized (buccinoidea, ammonis, dircaena, khabourensis and meiostoma). Within M. buccinoidea, Jordan Valley populations may belong to a separate subspecies. M. ammonis is clearly differentiated from buccinoidea of the nearby Jordan Valley, but less so from buccinoidea of more distant sites; differences between ammonis and meiostoma are significant but not diagnostic. Five species have ribbed shells. Within costata, four subspecies are recognized (in the northern Orontes, upper Jordan, Sea of Galilee, and in southernmost parts of the Levant). Throughout the Levant, M. buccinoidea frequently hybridizes with M. costata and fossil evidence suggests that these species have been hybridizing for the last 1.5 Myr. M. saulcyi differs from M. costata in its narrower shell and shorter, bumpier ribs and frequently hybridizes with M. buccinoidea throughout the Levant. Specimens from Homs differ from those of the Jordan Valley in their higher figurativity index, fewer ribs and lower rib density. M. germaini differs from M. costata in its more numerous ribs, M. pachya in its shorter ribs, and M. infracincta in its bumpy shell in which each rib has huge tubercles, with a pronounced ridge flanking the columella. Our conclusion that there are ten species in the Levant differs from previous studies that suggested only two subspecies of one species (or superspecies). This difference could stem from (1) our use of nonstandard as well as standard conchometrics, (2) a reappraisal of the importance of the shell vs. the radula in intrageneric systematics, and (3) differences of opinion on the subspecies concept. © 2005 The Linnean Society of London, Zoological Journal of the Linnean Society, 2005, 144 , 229?260.     

Comparative immunohistochemistry of the cephalic sensory organs in Opisthobranchia (Mollusca,Gastropoda)

The structure and function of the central nervous systems of opisthobranch gastropods have been studied extensively. However, the organisation and function of the peripheral nervous system are poorly understood. The cephalic sensory organs (CSOs) are known to be chemosensory structures in the head region of opisthobranchs. In the present study, we used immunohistochemical methods and confocal laserscanning microscopy to comparatively examine the CSOs of different opisthobranchs, namely Acteon tornatilis, Aplysia punctata, Archidoris pseudoargus and Haminoea hydatis. We wanted to characterise sensory epithelia in order to infer the function of sensory structures and the organs they constitute. Immunoreactivity against the three antigens tyrosine hydroxylase, FMRFamide and serotonin was very similar in the CSOs of all investigated species. Tyrosine hydroxylase-like immunoreactivity was detected primarily in subepidermal sensory cell bodies, which were much more abundant in the anteriorly situated CSOs. This observation indicates that these cells and the respective organs may be involved in contact chemoreception and mechanoreception. The dominant features of FMRFamide-like immunoreactivity, especially in the posterior CSOs, were tightly knotted fibres which reveal glomerulus-like structures. This suggests an olfactory role for these organs. Serotonin-like immunoreactivity was detected in an extensive network of efferent fibres, but was not found within any peripheral cell bodies. Serotonin-like immunoreactivity was found in the same glomerulus-like structures as FMRFamide-like immunoreactivity, indicating a function of serotonin in the efferent control of olfactory inputs. Besides this functional implication, this study could also add some knowledge on the doubtful homology of the CSOs in opisthobranch gastropods.     

Vacuolar-type ATPase in the accessory boring organ of Nucella lamellosa (Gmelin) (Mollusca : Gastropoda): role in shell penetration

The structure and function of the accessory boring organ (ABO) of muricid gastropods has been described in numerous studies, and the ABO of Nucella lamellosa was found to be similar to those of other muricid species. The active cap region of the ABO is composed of tall, mitochondria-rich cells with distinct brush borders at their apicies, surrounding a hemolymph-containing central sinus. Using antibodies specific for vacuolar-type ATPase (V-ATPase), enzyme immunoreactivity was found to be limited to the brush border of the epithelial cells. Electron immunohistochemistry revealed that V-ATPase immunoreactivity resides in the plasma membranes of the microvilli. Immunodot blotting using yeast V-ATPase as a positive control confirmed the specificity of the reactions. SDS-PAGE of membrane suspensions from the ABO revealed protein bands of the requisite molecular weight for V-ATPase subunits. Western blots suggest that antibodies raised against mammalian V-ATPase subunits recognize subunits of the molluscan V-ATPase. The molecular weights of these identified subunits are similar to those in mammals. The V-ATPase-specific inhibitor bafilomycin A1 inhibited ATPase activity in samples of ABO homogenate by about 10% relative to control, providing further evidence for the presence of V-ATPase. Specific V-ATPase activity was about 67 picomoles of inorganic phosphate per microgram of protein per minute in the homogenate. Collectively this evidence strongly suggests that a vacuolar-type proton transporting ATPase is present in the brush border of the accessory boring organ of Nucella lamellosa, and is responsible for acidifying secretions from this gland. Similarities between the ABO, osteoclasts, and the mantle of freshwater bivalves also suggest that the mechanism for decalcification of calcareous substrates is conserved.     

Evaluation of radiography,ultrasonography and endoscopy for detection of shell lesions in live abalone Haliotis iris (Mollusca: Gastropoda)

Radiography, ultrasonography and endoscopy were examined for their efficacy as non-destructive techniques for the detection of shell lesions in the marine gastropod Haliotis iris Gmelin. X-rays provided 69% correct diagnoses, with detection being restricted to those lesions which were mineralised. Ultrasound also showed potential to reliably detect lesions (83% correct diagnoses), but only where the lesions demonstrated a clear 3-dimensional relief. Lesion dimensions were underestimated using ultrasound. Endoscopy, applied to anaesthetised individuals, provided the most accurate method (92% correct diagnoses) for lesion detection and, although invasive, had no discernible effect on survival of the abalone 8 mo after screening.     

Experimental and comparative morphology of radula renewal in pulmonates (Mollusca,Gastropoda)

Summary The continuous renewal of the pulmonate radula and the histology and regeneration of its concomitant epithelia were studied by light and electron microscopy, autoradiography and electron microprobe analysis. The two species investigated show histological differences and the results were compared with those of a preceding study on a prosobranch radula. The radula is an intricate cuticular structure of the foregut. Only the fully grown part, which is active during feeding, lies in the buccal cavity while it is constantly renewed by the coordinated cooperation of specialized cells forming the radular sheath. The end of the sheath is occupied by cells which produce the organic matrix of the radula. In taeniogloss prosobranchs, seven multicellular cushions of small odontoblasts lie at the end of the sheath and produce the seven teeth of each cross-row. In pulmonates, the multidenticular radula is generated by numerous groups of a few voluminuous cells. Despite these histological differences, prosobranchs and pulmonates generate the radula matrix by microvilli, cytoplasmatic protrusions and apocrine secretions. The epithelia of the radular sheath contribute to the transport, tanning and mineralization of the radula. The concomitant epithelia are replaced in limited proliferation zones at the end of the radular sheath and their cells migrate anteriorly to the buccal cavity. The ultrastructure of the sheath cells and the alterations which they undergo in connection with their functions are discussed. The proliferation zone of the superior epithelium is strictly confined and the cells move together with the radula forward. In prosobranchs, the cells of the superior epithelium begin to degenerate in the middle of the radular sheath and the entire epithelium is simply extruded into the buccal cavity. In pulmonates, the opening of the radular sheath is closed by the cuticular collostylar hood which is generated by a distinct epithelium which is proved to be stationary. When leaving the proliferation zone, the superior epithelium differentiates into supporting cells and mineralizing cells; the latter cause the hardening of the radular teeth and already degenerate in the middle of the sheath. The whole superior epithelium degenerates at the border to the collostylar hood-epithelium. In Lymnaea the degeneration zone is strictly confined whereas in Cepaea the collostylar hood and its generating epithelium extend into the radular sheath and the degeneration zone ranges over a distance of 3–5 rows of teeth. The proliferation zone of the inferior epithelium extends over the posterior half of the radular sheath, but the replacement rate is much lower than in the superior epithelium. Although the inferior epithelium carries the radula, it migrates slower than the radula. Obviously the radula has to be transported actively by apical protrusions of the cells, which penetrate into the radular membrane. At the opening of the radular sheath the inferior epithelium generates the adhesive layer and degenerates. During feeding, the adhesive layer has to maintain the firm mechanical connection between radula and distal radular epithelium. Autoradiographic experiments demonstrate that the distal radular epithelium is stationary. Nevertheless, the radula is known to advance to its degeneration zone. Special attention is paid to this problem. We strongly suspect that the transport of the adhesive layer and the radula is based on pseudopodial movements of apical protrusions characteristic for the distal radular epithelium. These protrusions interdigitate with the lower face of the adhesive layer. The mechanical connection has to be maintained and so the respective structures (tonofilaments and hemi-desmosomes) have to be continually renewed. This needs a high amount of energy and obviously results in the conspicuous concentration of mitochondria near the apical surface.Abbreviations al adhesive layer - ax axon - bc buccal cavity - bce buccal cavity epithelium - bl basal layer - bla basal labyrinth - bm basal membrane - bp basal plate - bpc basal plate cell - c cilia - ch collostylar hood - che collostylar hood-epithelium - cl cuticular layer - col collostyle - cr cell remnant - cts connective tissue sheath - d desmosome - dl upper layer - dre distal radular epithelium - dz degeneration zone - fe front edge - g granula - gol dictyosome - hd hemidesmosome - hl haemolymph - ie inferior epithelium - j jaw - ma tooth matrix - mc mineralizing cell - mem membranoblast - mfb microfibrills - mfl microfilaments - mgb multigranular body - mi mitochondria - mit mitosis - ml middle layer - mt microtubuli - mv microvilli - mw membrane whirl - n nucleus - nc necrotic cluster - nf nerve fibres - nsg neurosecretory granula - o odontophor - od odontoblast - odg odontoblast group - pod pre-odontoblast - rb residual body - rer rough endoplasmatic reticulum - rm radular membrane - rt radula teeth - sc supporting cell - se superior epithelium - sj septate junction - sro subradular organ - ss secretion substance - tf tonofilaments - tsm supramedian tensor muscle - tw terminal web - v vacuole - ves vesicle     

The effects of environment upon shell construction and strength in the Rough periwinkle Littorina rudis Maton (Mollusca: Gastropoda)

The interrelationships between environment, shell construction and shell strength of the Rough periwinkle Littorina rudis are investigated in samples of shells taken from a range of shores differing in exposure and salinity level. The effects of different aspects of shell construction upon shell strength are evaluated. Shell strength is compared to chelar bite force of the predatory crab Carcinus maenas. The results are discussed in terms of a theory of factors of safety, and the observed patterns of variation in shell strength and construction with respect to environment are found to be consistent with the predictions of the theory.     

Patellogastropoda and Vetigastropoda (Mollusca,Gastropoda) from the marine Jurassic of Patagonia,Argentina

Gastropod faunas from the Early Jurassic (Late Pliensbachian–Early Toarcian) marine deposits of Chubut Province, Argentina, are described from Lomas Occidentales, Cerro La Trampa and Puesto Currumil localities, representing eight species, three of them new. These are Scurriopsis? sp., Chartronella gradata sp. nov., Calliotropis? sp., Pleurotomaria sp., Leptomaria sp., Hamusina? wahnishae sp. nov., Colpomphalus musacchioi sp. nov. and Jurassiphorus? cf. triadicus Haas. The gastropod assemblage reported here testifies paleobiogeographical connections with other coeval gastropod associations from the western Tethys. However, Chartronella, Hamusina and Jurassiphorus may represent survivors of Triassic associations, considering the ancient seaway from Peru as the most plausible hypothesis for biotic exchange of these faunas during the Late Triassic–Early Jurassic boundary. An abundant and diverse invertebrate fauna such as corals, echinoderms, cephalopods, brachiopods, bivalves and other gastropods found in association with the gastropods described here characterises a shallow marine environment for the gastropod-bearing rocks.http://www.zoobank.org/urn:lsid:zoobank.org:pub:7B8EAFC0-3AC0-4F91-97A5-22AAC6A19909     

Structure,crystallography, and morphogenesis of the cryptic shell of the terrestrial slug Limax maximus (Mollusca,gastropoda)

The structure and crystallography of the internal shell of the pulmonate gastropod slug Limax maximus were studied at the levels of light and scanning electron microscopy, revealing patterns of shell ontogeny and morphogenesis. The calcified portion of the slightly convex ovoid shell is composed of a single palisade layer of calcitic crystals. Numerous projections, 100 μm in width at the dorsal tip, are found on the dorsal surface of the shell and coincide with local nucleation sites of primordial calcium salt deposition onto the periostracum. With continued calcification these projections coalesce ventrally, forming the single crystalline shell layer. The organic portion of the shell includes the periostracum and an extensive PAS-staining conchiolin. In EDTA-etched preparations, conchiolin appears as a spongy network of fibers throughout the shell. Both horizontal and vertical components of the conchiolin are present, the former of variable thickness and occurring in an intercrystalline manner, the latter always occurring normal to the horizontal set. Macromorphogenic growth is characterized by three distinct temporal stages. Primary growth occurs radially from the umbonal region. Secondary growth is synonymous with shell thickening. Tertiary growth is characterized by both a lateral component, in which the shell extends beyond the primary growth boundaries, and a ventral component, in which the shell continues to grow in thickness. SEM of the ventral shell surface reveals a pattern of growth at the crystalmatrix interface. Proteinaceous fibers of the conchiolin occur unidirectionally in horizontal rows. Zones of incipient calcitic crystallization onto these hypostracal fiber bundles are contrasted by zones of increasing crystallization until the fibrous template (reduced hypostracum) is completely covered by crystals.