Polymorphic shell banding in the dog-whelk, Nucella lapillus (Mollusca)

The shell of the Common dog-whelk (Nucella lapillus (L.)) is white and unbanded at most places around the British Isles. However, high frequencies of banding occur on the Buchan coast, around Anglesey and the Menai Straits, on the Cower Peninsula, around the Devon–Somerset border in the Bristol Channel, and especially on the north Cornish coast (reaching a peak between Newquay and Padstow). The frequency of banding is significantly less in older than younger whelks in the same locality, and this change is uncorrelated with the selection against shell shape variation that takes place on exposed shores. It is concluded that banding is a pleiotropic manifestation of physiological variation, and that a study of such variation in different morphs could indicate the importance of different physiological stresses at different stages of the life history of N. lapillus.     

The diet of the dog-whelk, Nucella lapillus (Gastropoda Prosobranchia)

The information at present available suggests that, when adult, Nucella lapillus may prey upon a considerable number of different species, but that the diet of recently hatched dog-whelks is rather more restricted. The food preferences of young Nucella have been investigated and it has been concluded that, while the diet of young and adult animals may differ to some extent, it is probably equally varied at all ages, and that cannibalism may be a frequent occurrence amongst all age groups.
It has been determined that the size of the prey influences the selective predation of Mytilus by Nucella. Dog-whelks show a marked preference for mussels in the 1 to 3 cm size range which may possibly be identified chemotactically.     

Genetic differentiation of the endemic Baikalian mollusk Baicalia carinata (Mollusca: Caenogastropoda)

In gastropod mollusk Baicalia carinata Dybowski, 1875, sampled in different sites of the Lake Baikal, comparison of 81 sequences of internal transcribed spacer 1 (ITS1) located between the genes for 18S rRNA and 5.8S rRNA, and of 100 sequences of the fragment of mitochondrial cytochrome-c-oxidase subunit 1 gene (CO1) was performed. Molecular phylogenetic analysis showed that the endemic mollusk species studied formed at least two distinct populations, Southwestern and Eastern. Statistical significance of the species subdivision into two populations was evaluated using the Mann-Whitney rank test.     

The anatomy of Addisonia (Mollusca,Gastropoda)

Summary The organization of Addisonia lateralis (Requien, 1848) and A. brophyi McLean, 1985 is described. Addisonia species have a thin, asymmetrical, cup-like shell and a very simple shell muscle. Eyes, oral lappets and epipodial tentacles are absent and the right cephalic tentacle is also used as a copulatory organ. Most characteristic is the enormously developed gill which is enlarged into the right subpallial cavity. It is composed of about 30 leaflets with skeletal rods and its epithelia are uniquely arranged. The heart is large and the single auricle is situated anteriorly left. There are two kidneys: the left is small, while the right forms large coelomic cavities and has no connection with the pericardium or the hermaphroditic genital system. Testis and ovary are separate: both have a simple duct proper (vas deferens, oviduct). They are connected to the copulatory organ by an open seminal groove; a small receptaculum is present. The mouth opening is typically triangular, with no jaws or subradular sense organs. Addisonia possesses tuft-like salivary glands, a radula diverticulum and distinct, tubular oesophageal glands. The oesophagus itself is simple. The radula and the posterior alimentary tract are unique; the stomach is completely reduced and the intestine forms a pseudostomach. The streptoneurous, hyoathroid nervous system has pedal cords with three commissures. The visceral loop is also cord-like. A single (left) osphradium is present and the small statocysts have several statocones.The peculiarities and unique combination of primitive and advanced characters in Addisonia reflect a highly enigmatic organization among the Archaeogastropoda. Possible relationships to other archaeogastropod groups are discussed.     

Tissue Hemoglobins of Gastropoda (Mollusca)

The distribution and content of tissue hemoglobins in the radular muscle, subradular cartilages, myocardium, and nerve tissue of gastropods are analyzed.     

Dart formation in Helix aspersa (Mollusca,Gastropoda)

Summary Dart formation in Helix aspersa has been investigated by SEM of isolated darts at progressive stages in their development, and by histology of dart sacs at the same times. Dart formation begins at the tip of a tubercle where a small group of epithelial cells secrete an organic material filling a small CaCO₃ cone that is the first mineralized part of the shaft. Subsequent secretory activity by an increasing area of the tubercle epithelium results in an increase in the diameter and anterior lengthening of the shaft. Continued secretion by the tubercle and dart sac epithelium produces the flare and finally the corona. A pattern of deposition is also evident in the fine structure of the mineral. In the shaft and vanes there is an inner layer of spherulitic prismatic structure which is covered by a layer of irregular patches of simple prismatic structure. The outermost layer of the shaft and vanes has a continuous simple prismatic structure. Two layers are present in the flare, an inner granular amorphous layer and an outer spherulitic prismatic layer. The corona consists of a single rarefied prismatic layer. A mechanism of dart formation is suggested that involves two types of organic matrix, calcifying and non-calcifying. Measurements of the calcium content of darts, dart sacs, and collars indicate that the hemolymph is the probable source of calcium for the dart.     

Spermiogenesis in the vermetid gastropod Dendropoma petraeum (Gastropoda,Prosobranchia)

The structure and maturation of the male gonad of the Mediterranean vermetid gastropod Dendropoma petraeum are described. Histological sections of the gonads were made throughout development and gonad activity was monitored at regular monthly intervals. During the autumn months the gonad is very small and is surrounded by a large quantity of connective tissue; it becomes more voluminous from December to August, with the highest growth peak in springtime. The stages of spermatogenesis were also observed and described.     

Growth and Longevity of the Gastropod Nucella heyseana in Vostok Bay,Sea of Japan

Features of the morphology, longevity, and growth of the gastropod mollusk Nucella heyseana collected in different seasons in 2000–2001 in Vostok Bay (Sea of Japan) were investigated. Shell surface features such as grooves and spurs of the axial sculpture have proven to be unhelpful in the estimation of individual age and growth rate of this species. Concentric rings on the outside of the operculum formed yearly in the cold season are useful for these purposes. The longevity of N. heyseana in southern Primorye is 10 years; this value is similar to the life span of mollusks from the southern Kuril Islands. Local differences in longevity, growth allometry, and age changes in the body size and mass of N. heyseana in relation to habitat conditions are discussed.     

Silver staining analysis on spermatozoa of Nucella lapillus (Gastropoda,Prosobranchia)

Summary

Sperm of Nucella lapillus was studied by electron microscopy, including the application of a cytochemical silver method. Using silver impregnation a dense precipitation of Ag granules in spermatocyte II nucleoli was seen over the fibrillar component and a slight one in the granular component. On longitudinal sections of the spermatozoon the results demonstrate that argyrophilic proteins are located in the external limiting zone of the acrosome in the anterior portion of the nucleus between the cytoplasmic and the nuclear membranes, in the posterior end of the nucleus and in the terminal portion of the middle region. These data indicate an affinity for silver in areas of the cytoplasm containing microtubules and in zones of transition.     

Basal chromodorid sperm ultrastructure (Nudibranchia, Gastropoda, Mollusca)

The relationship between three genera considered basal in the Chromodorididae (Cadlina, Tyrinna, Cadlinella) has not yet been resolved by traditional morphological means. Here we examined the sperm ultrastructure of Tyrinna nobilis, Tyrinna evelinae, Cadlina flavomaculata and Cadlina cf. nigrobranchiata, with the expectation of finding phylogenetically informative characters. No Tyrinna or Cadlina species showed sperm similarities to Cadlinella. Both Cadlina species and Tyrinna nobilis (but not T. evelinae) exhibited coarse striations in the acrosomal pedestal. The putative fibers that occurred between the coarse striations of the pedestal are condensed into a layer in Cadlina and Tyrinna, but not in other species that also have coarse striations (Gymnodoris), and may constitute evidence for a close relationship. Tyrinna evelinae possessed fine acrosomal striations, which was shared with other Chromodorididae, Actinocyclidae and the cryptobranchs Rostanga and Aphelodoris. We also examined the sperm ultrastructure of ‘Chromodoris’ ambiguus, an animal which has shown molecular affinities to species of Cadlina, and not Chromodoris. The sperm of ‘C.’ ambiguus did not exhibit the typical Cadlina characteristics, but also showed important differences to other investigated Chromodoris species.     

Macrogeographic genetic variability in the gastropod mollusk Littorina sitkana from the northwest Pacific

Variation at four highly polymorphic allozyme loci (inorganic pyrophosphatase, peptidase, and two esterase loci) was examined in 25 settlements of the marine snail Littorina sitkana (Mollusca, Gastropoda). The sampling localities covered a wide part of the species range: from the Peter the Great Bay (the Sea of Japan) at the southwest to the Mednyi Island (Commander Islands) at the northeast. Like other littorines lacking the pelagic stage, L. sitkana was characterized by significant genetic differentiation (G(ST) for the pooled sample was 0.310). Cluster analysis and nonmetric multidimensional scaling conducted on a matrix of pairwise genetic distances between all of the settlements studied revealed four genetically different groups: southern Primorye, northern Prymorye, Sakhalin, and Kuril-Commanders. The population-genetic structure of the L. sitkana settlements is close to that described by the isolation-by-distance and stepping-stone models: the geographic and the genetic distances between the most settlements examined are distinctly correlated.     

An Ultrastructural Study of Basommatophoran Spermatozoa (Mollusca, Gastropoda)

Spermatozoa of the basommatophoran pulmonate families Ellobiidae ( Ophicardelus ornatus Ferussac), Amphibolidae ( Salinator fragilis Lamarck, Salinator solida von Martens), Siphonariidae ( Siphonaria funiculata Reeve) and Lymnaeidae ( Lymnaea lessoni (Deshayes)) were studied using transmission electron microscopy. Spermatozoa of all species, like most euthyneuran spermatozoa, possess ( 1 ) an acrosome composed of an apical vesicle and acrosomal pedestal (many differences between families), (2) a helically keeled, posteriorly invaginated nucleus, ( 3 ) a midpiece composed of paracrystalline and matrix materials and a variable number of incorporated glycogen-filled helices (one in Salinator and Siphonaria , two or three in Lymnaea , three in Ophicardelus ) and (4) an axoneme associated with coarse fibres (periodically banded in neck region) and rows of intra-axonemal granules. The wide diversity of spermatozoon structure in the species studied, in particular the midpiece structure of Siphonaria (which resembles closely that of certain opisthobranchs) indicates that the Basommatophora may not represent a valid taxonomic unit. A comparison of basommatophoran sperm with other euthyneurans and with euspermatozoa of prosobranchs is given.     

The Vetigastropoda and the systematics of streptoneurous Gastropoda (Mollusca)

The status of the Vetigastropoda, created 1980 to unite zeugobranch and trochoid archaeo-gastropods, is reconsidered and confirmed in the light of recent investigations. The most important synapomorphic characters of the Vetigastropoda are the ctenidial sense organs (so-called bursicles), the epipodial sense organs, and the special structure of the oesophagus. The conditions of the archaeogastropod oesophagus are comparatively described.
An examination of the relationships of the Vetigastropoda establishes the archaeogastropod nature of the Architaenioglossa. It also demonstrates the monophyletic origin of all higher gastropods, including the Caenogastropoda (s.str.) as well as the Pentaganglionata (= Euthy-neura). The Campanilidae, Valvatidae, Rissoellidae, Omalogyridae, and the Allogastropoda (= Architectonicoidea and Pyramidelloidea) share characteristics with both of the above-mentioned groups, but cannot be classified within any of them. These groups are regarded as subsequent offshoots between the caenogastropod and the pentaganglionate level of organization. The level of Archaeogastropoda is redefined by the hypoathroid/dystenoid nervous system and simple tentacle nerves; the remaining Streptoneura are classified as Apogastropoda with epiathroid nervous system, parapedal commissure, and bifurcated tentacle nerves.     

Molecular phylogenetics of Caenogastropoda (Gastropoda: Mollusca)

Caenogastropoda is the dominant group of marine gastropods in terms of species numbers, diversity of habit and habitat and ecological importance. This paper reports the first comprehensive multi-gene phylogenetic study of the group. Data were collected from up to six genes comprising parts of 18S rRNA, 28S rRNA (five segments), 12S rRNA, cytochrome c oxidase subunit I, histone H3 and elongation factor 1alpha. The alignment has a combined length of 3995 base positions for 36 taxa, comprising 29 Caenogastropoda representing all of its major lineages and seven outgroups. Maximum parsimony, maximum likelihood and Bayesian analyses were conducted. The results generally support monophyly of Caenogastropoda and Hypsogastropoda (Caenogastropoda excepting Architaenioglossa, Cerithioidea and Campanilioidea). Within Hypsogastropoda, maximum likelihood and Bayesian analyses identified a near basal clade of nine or 10 families lacking an anterior inhalant siphon, and Cerithiopsidae s.l. (representing Triphoroidea), where the siphon is probably derived independently from other Hypsogastropoda. The asiphonate family Eatoniellidae was usually included in the clade but was removed in one Bayesian analysis. Of the two other studied families lacking a siphon, the limpet-shaped Calyptraeidae was associated with this group in some analyses, but the tent-shaped Xenophoridae was generally associated with the siphonate Strombidae. The other studied hypsogastropods with an anterior inhalant siphon include nine families, six of which are Neogastropoda, the only traditional caenogastropod group above the superfamily-level with strong morphological support. The hypotheses that Neogastropoda are monophyletic and that the group occupies a derived position within Hypsogastropoda are both contradicted, but weakly, by the molecular analyses. Despite the addition of large amounts of new molecular data, many caenogastropod lineages remain poorly resolved or unresolved in the present analyses, possibly due to a rapid radiation of the Hypsogastropoda following the Permian-Triassic extinction during the early Mesozoic.     

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.     

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     

Rhythmic egg-release in Littorina littorea (Mollusca: Gastropoda)

Batches of female Littorina littorea from four estuarine sites were kept in aquaria throughout their breeding season (December to July), some permanently submerged in circulating sea-water and others in a regime of simulated semi-diurnal tides of constant small amplitude. Females without the tidal experience released few eggs with little relation to the lunar month. Those in the tidal regime released more than four times the total number of eggs and released far more in periods of new and full moon than in periods of half-moon. Over the whole season, release was 1–5–3–25 times faster (at different sites) during full and new moon but the difference was more accentuated at the height of the season in February to early March and in early May. While individual females differed considerably and there were minor variations between the four sites, it is concluded that a two-weekly rhythm relates egg release to the spring-neap tide cycle and that this rhythm requires a tidal cycle of immersion and emersion for its maintenance. The special significance of such a rhythm for a species with pelagic eggs and larvae in an estuary is discussed.     

Ultrastructure and Phylogenetic Significance of Notaspidean Spermatozoa (Mollusca, Gastropoda, Opisthobranchia)

Spermatozoa of five notaspidean opisthobranchs [Berthellina citrina, Berthella ornata, Pleuro-branchus peroni, Pleurobranchaea maculata, Umbruculum sinicum] were examined using TEM. In all five species, the acrosome (sensu lato) consists of an apical vesicle (the acrosomal vesicle) and acrosomal pedestal. The acrosomal pedestal overlaps the nuclear apex, and in P. peroni (and possibly B. ornata) is periodically banded—-the first reported incidence of this type of substructure in any euthyneuran acrosome. Although sperm nuclei of P. peroni, B. ornata and B. citrina differ in length and also the number of keels present (nucleus 7 μm long with four/five keels present in Pleurobranchus; 17 μm long with one keel in Berthella; 15 μm long with a very weak keel in Berthellina), the basal invagination to which the centriolar derivative, axoneme and coarse fibres are attached is always poorly developed, and very little overlap between nucleus and midpiece occurs. In P. maculata and U. sinicum, the nucleus forms a helical cord around the axoneme and mitochondrial derivative such that it is not possible to recognize exclusively ‘nuclear’ and ‘midpiece’ regions of the spermatozoon. In all notaspideans investigated, (1) the axoneme, coarse fibres and glycogen helix are enclosed by the paracrystalline and matrix components of the mitochondrial derivative and (2) a dense ring structure (attached to the plasma membrane) and glycogen piece are observed. While the glycogen piece is very short (0.85–1.43 μm) with a very degenerate axoneme in B. citrina, B. ornata and P. peroni, this region of the spermatozoan is well developed (30–35 μm long) in U. sinicum and exhibits a fully intact 9 + 2 axoneme. The ‘glycogen piece’(or its presumed homologue) in P. maculata spermatozoa is very short (0.65 μm), devoid of any axonemal remnant and constructed of a hollow, internal cylinder attached to an outer (incomplete) shell, and contains scattered (glycogen) granules. Spermatozoal structure supports a close relationship between the genera Berthellina, Berthella and Pleurobranchus. These three genera have more distant links with Pleurobranchaea, while Umbraculum maintains an isolated, specialized position within the Notaspidea.     

On the taxonomy of Littorina africana (Mollusca: Gastropoda)

L. africana and L. knysnaensis are regarded as two morphs of a single species which exhibits a genetic cline along the south-eastern coast of southern Africa. The dark brown morph knysnaensis dominates the western, cooler end of the cline and is replaced by the pale blue morph africana at the warmer end of the cline. These conclusions are based on evidence from the latitudinal distributions, the complete range of intermediate forms regarding shell colour and shell morphology and the lack of differences in redular morphology, penial morphology or habitat.     

Genetic and morphological variability in a population of Diphyllobothrium dendriticum (Nitzsch, 1824)

Plerocercoids of Diphyllobothrium dendriticum were isolated from whitefish (Coregonus clupeaformis) from McGregor Lake, Alberta. Adult D. dendriticum were obtained by infection of golden hamsters (Mesocricetus auratus). Analysis of morphological variables indicated high variability and some overlap with D. latum. Correlations in morphology were due to the covariance of shape variables. Isozyme analysis also showed high variability, with two thirds of the enzymes studied being polymorphic. Correlations with morphology indicate a relationship between rare patterns for malate dehydrogenase and esterase and increased size of adults. Southern hybridisation with ribosomal gene probes produced two common patterns and a rare intermediate form, indicating that the species is variable even within a local population. Correlations between rDNA pattern type and morphological variables supported the hypothesis of a genetic base for the morphological variability.