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Siphons represent a remarkable example of evolutionary convergence in different bivalve lineages. In Protobranchia, the siphons, when present, are used exclusively to channel the water currents used for respiration, waste removal and gamete release. Their emergence is thought to be associated with the taxonomic and morphological diversification within the Nuculanida. While siphons have been extensively studied in other bivalve groups, particularly the Heterodonta, gaps remain in the knowledge of protobranch siphons, especially on their morphology, including the musculature and innervation, and their functioning. The few data on Protobranchia siphons are restricted to some Nuculanoidea, and information is still missing for other nuculanid lineages, such as the Sareptoidea. The present study provides a description of the siphonal morphology of Aequiyoldia eightsii , a sareptid bivalve. Histological data were obtained to investigate their organization and infer their functioning. All traits observed were compared with those of heterodont siphons. In A. eightsii , the siphons are fused with each other and distinguished by their reduced longitudinal musculature in bundles that do not form layers, by the presence of a blood lacunal system that fills most of the wall of the siphon and by the innervation that is likely diffuse. This suggests that the siphons are hydrostatic organs with a hydraulic mechanism whose expansion occurs when the lacunar system is filled with blood. Our results provide the first morphological and functional details of a protobranch siphon and thus contribute to knowledge on the comparative anatomy of Bivalvia. [ABSTRACT FROM AUTHOR]

Some bivalves are important in fisheries and aquaculture, and provide significant economic benefits to humans. The diversity in shell colour among bivalves has received attention due to its potential in breeding and its relevance to the study of speciation and adaptation. In this study, the relationship between whole-genome-scale gene expression and shell coloration in the clam Cyclina sinensis was analysed using high-throughput Illumina sequencing technology. A total of 15,367 annotated genes and 4,425 novel genes were detected based on transcript assembly. The results of the transcriptome analysis show that 49 genes were differentially expressed in the outer mantle fold tissue of purple- vs white-shelled specimens. These genes include three lipoprotein receptor genes, low-density lipoprotein receptor gene (ldlr), low-density lipoprotein receptor-related protein 4 (lrp4) and scavenger receptor cysteine-rich protein gene (srcr). These genes may be involved in the transport of carotenoid pigments. Moreover, as shown by the Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses, several receptor activity terms and a glycosphingolipid biosynthesis pathway were detected, and these might be involved in shell coloration. Together with the analyses of differentially expressed genes, our results suggest a possible molecular mechanism governing the selective transport of carotenoids in the mantle of purple-shelled clams and shell colour formation. The results of this study may help to elucidate the colour formation mechanism in clam shells. [ABSTRACT FROM AUTHOR]

Neolepton cobbi is a small bivalve (maximum length 3.4 mm) occurring on sandy or soft muddy bottoms in the southern Magellan Region. Its life span is 12-14 months and population turnover takes place in the late austral summer (December-January). It is a protandric hermaphrodite; sex change occurs at a nonde-fined size, although females smaller than 1.3mm length were never found. During the functional male phase, clusters of oogonia were found in the posterodorsal region of the gonad. Among 152 specimens examined histologically, only one case of functional hermaphroditism was found, in which male and female tissues occupied differentiated regions of the gonad. Neolepton cobbi is a brooding species. Secretions from a glandular region of the mantle edge contribute to the agglutination of fertilized eggs, forming elongate, paired egg capsules that are attached to the anteroventral margin of the maternal individual. Up to three pairs of egg capsules were found on each female shell, with a total of about 150 eggs/embryos. Each pair of egg capsules corresponds to one of the three different spawning events during the sole breeding season (austral spring) in which each female participates. The hatchlings are released as small (260 μm length) crawling juveniles. The shell margin at the point of attachment of egg capsules is flattened due to a disruption of growth, resulting in a secondary sexual character. The timing of egg production and the possibility of increasing the number of egg capsules per female according to the time elapsed from the start of the breeding season are viewed as a solution to the limitations on fecundity and space available for brooding, which are imposed by small adult size. [ABSTRACT FROM AUTHOR]

Four species of Pandora have been reported from southern South America: P. cistula Gould, 1850, P. braziliensis G. B. Sowerby II, 1874, P. diffissa Mabille & Rochebrune, 1889 and P. patagonica (Dall, 1915). The group has received little taxonomic attention in this area, resulting in arbitrary and wrong usage of these names, for which several contradictory synonymies have been proposed. This study provides the first revision of Pandora species living in southern South America, including photographs of the type material and descriptions of shells, gross anatomy and living animals. Out of the four species previously mentioned, P. braziliensis is here regarded as valid, including P. patagonica and P. diffissa as synonyms. The fourth species, P. cistula, remains known only from its type specimen. In addition, a new species, P. brevirostris, is described from the shallow waters of Argentina. [ABSTRACT FROM AUTHOR]

Attaching objects to bivalve shells is an increasing requirement for conservation projects, mark-recapture studies and behavioural analysis. We investigated the suitability of eight different glues to attach a metallic object to a mussel shell and confirmed the utility of the best glue in a filtration behaviour experiment using Anodonta anatina. We used removal forces up to 1.35 kg cm-2 7 d after submerging in water and found an epoxy resin and a cyanoacrylate adhesive to perform best. Not all epoxy resins or cyanoacrylate adhesives performed equally. The best performing cyanoacrylate adhesive was used to glue magnets and rubber-coated Hall sensors to 26 mussels for a filtration-behaviour experiment. Nine months after attachment, all magnets and rubber-coated Hall sensors remained attached and withstood 1.35 kg cm-2. The epoxy resin was not chosen, because it contained Bisphenol A and required a more complex application procedure. Mussel filtration behaviours were monitored for 96 h in the presence of algae. The results showed that the presence of algae stimulated the filtration rate of A. anatina in the first 24 h. Over the experimental period, the mussels' mean filtration duration was 20±12 h, while the resting duration was 16±7 h. We identified a pronounced circadian rhythm, despite the long filtration duration and variation in behaviour patterns. However, at any one time some mussels were observed to be filtering. The mussels were more likely to open their shells and become active at around 20:00. Thus the majority of mussels were active at midnight and a minority at noon. We interpret the strong response to increased algal concentration and the pronounced circadian rhythm as evidence that neither the glue nor the attached sensors disturbed the mussels even in a short-term experiment. Hence, we recommend the selected cyanoacrylate adhesive for use in tagging projects and behavioural studies of freshwater mussels. [ABSTRACT FROM AUTHOR]

The present study focuses on two apparent species: the giant thyasirid Conchocele bisecta (Conrad, 1849), which is the dominant species of the benthic community in a gas hydrate area with cold-water methane-rich vents at a depth of about 800 m on the slope off Paramushir Island (Kuril Islands, Sea of Okhotsk) and small unidentified thyasirid bivalves from this same community. An examination of the shell morphology of these thyasirids showed that the small bivalves were in fact young specimens of C. bisecta, characterized by a high individual and age variability. A transmission electron microscopic study of C. bisecta revealed gills with 'Type 3' filaments, which were extended abfrontally and had a distinct bacteriocyte zone with extracellular symbionts. The symbiotic bacteria found were spherical, similar to thiotrophic symbionts of other thyasirids. The isotopic δ13С values of C. bisecta soft tissues (from −39.6 to −33.8‰) were much heavier than those of methane in the Paramushir gas-hydrate area and matched the range characteristic of symbiotrophic bivalves harbouring sulphur-oxidizing chemoautotrophic bacteria. The variations in δ13С and δ15N recorded for large and small C. bisecta can be related to ontogenetic differences in life habit: small individuals are totally buried in the sediment, while large ones are half-buried. Data from fatty acid (FA) analysis indicate that sulphur-oxidizing symbionts constitute almost the entire nutrition of C. bisecta, with no significant contribution of symbiotic or free-living methanotrophs. Furthermore, neither FA nor isotopic compositions provided evidence for photosynthetic sources as food items for C. bisecta through filter feeding. [ABSTRACT FROM AUTHOR]

The Spheniopsidae are today represented by five living species of Spheniopsis and nine of Grippina, distributed in the Pacific and Atlantic Oceans. Little is known of their anatomy and the phylogenetic position of the family within the Bivalvia is debated. In order to investigate these questions, the functional morphology of Grippina coronata obtained from the continental shelf off Rio de Janeiro and Espı'rito Santo States, Brazil, has been examined. Less than 2 mm in shell length, the siphonal apparatus of G. coronata is complex, with seven sensory papillae, and the ctenidia are reduced to transverse septa pierced by four pairs of ciliated pores. There are no labial palps and the stomach is of Type II with epibenthic harpacticoid and ostracod prey identified inside it. Although there is an intestine producing faeces, the stomach also possesses a unique waste storage pouch for exoskeletal remains of digested prey. Collectively, these features suggest that the Spheniopsidae comprise carnivorous taxa belonging to the Cuspidarioidea within the Anomalodesmata. Grippina coronata is a self-fertilizing simultaneous hermaphrodite that, uniquely, broods fertilized oocytes within the ovarian follicles and, thereby, provides the first example of intrafollicular fertilization and brooding in the Bivalvia. Release of the encapsulated oocytes must be by parental death, which coincidentally releases the exoskeletal remains from the storage pouch. Such post mortem semelparity creates a life-history trait hitherto unrecognized in the Bivalvia. [ABSTRACT FROM AUTHOR]

The morphological variability of freshwater bivalve species, observed between and within river basins, may hamper their correct identification, even by experienced researchers. Classic morphometric measurements, i.e. shell length, height and thickness, or their ratios, are generally insufficient to distinguish populations and/or species. These issues may be overcome using a geometric morphometric method, which allows analysis of the overall shape of the individual, independently of its size. Thus, we aimed to test the usefulness of two geometric morphometric tools, landmarks and sliding semilandmarks, to evaluate the morphological variability of Unio delphinus Spengler, 1783 in three habitats of the Guadiana Basin (SW Iberian Peninsula, Europe): estuary, river and stream. We used 13 landmarks located on the shell interior (at the teeth, muscle scars and pallial line) and 35 sliding semilandmarks for the shell contour. These morphometric analyses showed that the shell shape of U. delphinus differs significantly among different habitats. Estuarine and stream shells are the most disparate (James index = 649.114, permutation P-value <0.001) and variability is not related to variations in shell size. The main differences in shell morphology are the following: (1) estuarine shells are more elongate, while riverine shells are more subovate; (2) the anterior curvature at the umbo is steeper in estuarine and riverine shells; (3) estuarine shells have an arched curvature at the ventral part of the shell, which is absent in specimens from the other habitats. Our data suggest that the morphology of U. delphinus shells might be influenced by the water flow characteristics of each habitat, since shells exhibited characteristics that are typically observed in freshwater mussels from lotic and lentic habitats. [ABSTRACT FROM AUTHOR]

The embryonic and larval development of Ruditapes decussatus, from fertilization to metamorphosis, is described using scanning electron microscopy (SEM). Shell formation during embryonic development is investigated using SEM and transmission electron microscopy (TEM). Released oocytes are about 67 µm in diameter. Gastrulation takes place by epiboly and starts 7 h after fertilization (T0 + 7 h). The early gastrula is characterized by the appearance of a large, open cavity posteriorly and a small, round blastopore anteriorly, which represent the shell field and the blastopore, respectively. In TEM views, the open cavity expands under and posterior to the developing prototrochal pad. The shell field comprises a few cells in which microvilli progressively regress; these cells will secrete the periostracum. After 13 h (T0 + 13 h) the late gastrula has differentiated into a typically pyriform and motile trochophore. The periostracum emerges from periostracum-secreting cells (T1) and spreads over shell-secretory cells (T3). Thus, the trochophore is laterally compressed and the periostracum inserts to the mantle edge. Typical straight-hinged D-shaped larvae develop from a trochophore by 26 h postfertilization (T0 + 26 h) and by then the valves completely enclose the soft body of the larva. At T0 + 39 h, the newly hatched veliger larva is already enclosed in the fully calcified prodissoconch I (PI) and the prototroch has transformed into the velum. Late D-larvae already have a developed digestive system and start exogenous feeding. At the same time, the prodissoconch II is newly secreted at the margin of the PI. Settlement occurs at 27 d postfertilization (T0 + 27 d), when the larvae are about 207 µm long. Once metamorphosis is completed, the mantle folds begin the secretion of the dissoconch shell and the postlarval stage is reached. [ABSTRACT FROM AUTHOR]

The presence of thin organic layers embedded in the nacreous layer in the shell of several species of Unionoida has been widely considered to be largely a reaction to prevent further shell dissolution once part of the shell has been corroded. Their microstructural characteristics have also been used as taxonomic characters to separate the families Margaritiferidae and Unionidae. This paper focuses on the description and interpretation of these organic layers in samples of four extant species of European Unionidae coming from acidic and alkaline waters, six extant species of Margaritiferidae, one Cretaceous Unionidae and two Cretaceous Margaritiferidae species. Our results demonstrate that organic layers have characterized species of Unionoidea at least since the Cretaceous and occur in specimens coming from both acidic and alkaline waters. Moreover, the presence of these layers in noncorroded shells can be interpreted as a preventative or ‘prophylactic’ measure against corrosion. Their thickness is the only useful character verified to separate the two families. In the Margaritiferidae, organic layers were found to be present in Margaritifera margaritifera, M. valdensis, M. idubedae, M. laosensis, M. laevis and M. monodonta, and absent in M. auricularia and M. marocana. In Unionidae, they appear in Protopleurobema numantina, Potomida littoralis and in some individuals of Unio mancus, U. delphinus and U. tumidiformis. We argue that organic layers played a fundamental role in shell protection during the invasion of freshwater by the earliest Unionoida, while today they function to improve fitness in acidic water in some species. [ABSTRACT FROM AUTHOR]

Ensis macha has a wide distribution along the Pacific and Atlantic southern coasts of South America. The pattern of morphological shell variation of this razor clam was analysed in 12 populations sampled along its whole Atlantic distribution, covering a latitudinal range of 3,700 km. The southernmost extant population of E. macha was recorded in the Beagle Channel. Variation in shell outline was assessed using elliptic Fourier analysis of adult individuals followed by different multivariate analyses. Significant shell-shape variation was found along the latitudinal range. Elliptical Fourier analysis revealed that trends in shell slenderness and curvature were associated mainly with salinity and water depth of the localities. Each locality presented a distinct shell shape, different from all other localities. Nevertheless, the populations could be grouped into four large assemblages between which a latitudinal gradient of shell shape was observed. Phenotypic divergence in shell shape is suggestive of population differentiation, but assessing the underlying processes is not straightforward because of the complex interaction between genetic and environmental factors. The possible evolutionary processes that mediate the effects of environmental variables and geographical distances on the shell-shape variation are discussed. [ABSTRACT FROM PUBLISHER]

A scanning electron microscope study of the periostracum of 50 species of venerid bivalves revealed that periostracal calcification in the form of aragonitic needles and shorter pins is widespread within the family. Together with organic and sediment coatings that are found in some species, these needles form an integral part of the functional shell. Visible as a white ‘crust’ on the outside of shells, long slender needles (up to 400 µm long and 1 µm wide) without adherent material are seen in species of Tivela and Lioconcha and in Gomphina undulosa. Other venerids including Pitar species, Mysia undata and Compsomyax subdiaphana have short pins, capped with a fibrous organic matrix and significant coatings of sediment. Callocardia hungerfordi and Clementia papyracea have very thick sediment coatings underlain by short pins, while Gafrarium and Circe species have short pins with a thin, robust, organic coating and little particulate material. Finally, there are species, including Venus verrucosa, Chione elevata and Mercenaria mercenaria, where minute, <1 µm long pins also underpin a thin organic coating. Details of formation were studied in Tivela lamyi and Lioconcha ornata, where the needles are elongate hexagonal crystals of aragonite enveloped by an organic sheath, which grow at their proximal ends from within the periostracum, connected to the outer mantle epithelium via narrow channels. Growth of needles ceases following the onset of shell calcification. The distribution of the periostracal structures was examined in relation to a published molecular phylogeny that recognized two major clades within the family. Larger needles and pins are confined to the clade that includes subfamilies Pitarinae, Gouldinae, Meretricinae and Petricolinae, while submicron-sized pins are found only in the Venerinae and Chioninae of the second clade. Calcified periostracal structures appear to be absent in Tapetinae and Dosiniinae. [ABSTRACT FROM PUBLISHER]

A new genus Abyssogena is established for A. phaseoliformis (Métivier, Okutani & Ohta, 1986) and A. kaikoi (Okutani & Métivier, 1986), which were previously assigned to the genus Calyptogena Dall, 1891, and also for two new species, A. southwardae and A. novacula. The most characteristic features of Abyssogena are an elongate shell up to about 280 mm in length; a pallial line starting from the ventral margin of the anterior adductor scar; secondary pallial attachment scars developed dorsal to the pallial line; radially arranged hinge teeth with a reduced anterior cardinal tooth in the right valve; and presence of an inner ctenidial demibranch only. Abyssogena occurs in deep water from 2,985 to 6,400 m and is distributed in the Pacific and Atlantic Oceans at cold seeps along continental margins and hydrothermal vents at mid-oceanic ridges. Some species have a remarkably wide geographic distribution; A. southwardae is present throughout the Atlantic and A. phaseoliformis is present in Japan, Kuril-Kamchatka, as well as Aleutian Trenches. No fossils of Abyssogena are known. [ABSTRACT FROM AUTHOR]

The taxonomy of two sympatric surf clams Donax marincovichi Coan, 1983 and Donax obesulus Reeve, 1854, inhabiting the coastal Humboldt Current Upwelling System is revisited. Because both species are exploited by artisanal fisheries, it is essential to verify that they are indeed distinct species that have to be managed separately. In this study, both taxa were sampled across their shared distributional area and specimens were indentified according to their respective morphological characteristics. Although width/height and height/length ratios revealed significant differences within sampling areas, the two morphotypes were frequently incongruent for taxonomically important morphometric parameters. In addition, they showed no significant mitochondrial genetic differentiation within or among populations and exhibited indistinguishable sperm ultrastructure. We conclude that the two morphotypes do not represent distinct species and should be included together under D. obesulus. [ABSTRACT FROM PUBLISHER]

The genus Calyptogena (Bivalvia: Vesicomyidae) comprises highly specialized bivalves living in symbiosis with sulphur-oxidizing bacteria in reducing habitats. In this study, the genus is revised using shell and anatomical features. The work is based on type material, as well as on the extensive collection of vesicomyids obtained during twelve expeditions to the Pacific and Indian Oceans. Nine Recent species are ascribed to the genus Calyptogena, four of which are new: C. pacifica Dall, 1891, C. fausta Okutani, Fujikura & Hashimoto, 1993, C. rectimargo Scarlato, 1981, C. valdiviae (Thiele & Jaeckel, 1931), C. gallardoi Sellanes & Krylova, 2005, C. goffrediae n. sp., C. starobogatovi n. sp., C. makranensis n. sp. and C. costaricana n. sp. The characteristic features of Calyptogena are: shell up to 90 mm in length, elongate-elliptical or elongate; presence of escutcheon; presence of broad posterior ramus (3b) of right subumbonal cardinal tooth as well as right posterior nymphal ridge; absence of pallial sinus as a result of attachment of intersiphonal septal retractor immediately adjacent to ventral surface of posterior adductor; absence of processes on inner vulva of inhalant siphon; presence of inner demibranch only, with descending and ascending lamellae with interlamellar septa not divided into separate tubes. The most closely related taxa to Calyptogena are probably the genus Isorropodon Sturany, 1896, and the group of species represented by ‘Calyptogena’ phaseoliformis Métivier, Okutani & Ohta, 1986. These groups have several characters in common, namely absence of pallial sinus, presence of single inner pair of demibranchs and absence of processes on inner vulva of inhalant siphon. The worldwide distribution of the genus Calyptogena suggests that methane seeps at continental margins are the major dispersal routes and that speciation was promoted by geographical isolation. Recent species diversity and fossil records indicate that the genus originated in the Pacific Ocean. Sufficient data to discuss the distribution at species level exist only for C. pacifica, which has a remarkably narrow bathymetric range. Published studies on the physiology of C. pacifica suggest that adaptation to a specific geochemical environment has led to coexisting vesicomyid genera. The bacteria-containing gill of C. pacifica and other Calyptogena species is one of the most specialized in the family Vesicomyidae and may reflect these ecological adaptations. [ABSTRACT FROM PUBLISHER]

A systematic revision of the genus Neolepton in southern South America has been carried out. Neolepton concentricum, N. cobbi, N. hupei and N. falklandicum are redescribed and figured, and lectotypes for N. concentricum and N. bennetti are designated. Three new species are described, two from the Magellanic Province (N. amatoi new species and N. yagan new species) and one from the Argentine Province (N. bonaerense new species). The presence of N. umbonatum in Magellanic waters is considered doubtful. Some of the reproductive characteristics and peculiarities of the brooding behaviour of N. concentricum are described. [ABSTRACT FROM AUTHOR]

The Noah's Ark shell Arca noae is a widely distributed and locally abundant bivalve in the coastal waters of the eastern Adriatic Sea where it is commercially exploited. Information regarding its basic biology is lacking, and little is known about its population structure, recruitment, reproduction, growth and longevity. We estimated the age of three populations of A. noae collected from the Croatian Adriatic using external surface growth rings, pallial line scars on the inner shell surface, and growth lines in acetate peel replicas of sections of the umbo and outer prismatic shell layer. All the age determination methods used were successful in estimating the age of A. noae, however counts of the prismatic growth lines provided a less ambiguous estimate of the age of the shells. Arca noae is a slow-growing species reaching a size of 70 mm and an age of up to 16 years. Croatian populations might be vulnerable to over-exploitation. [ABSTRACT FROM PUBLISHER]

The article describes the larval and early post-larval shells of Camptochlamys species (Pectinidae) and Atreta (Dimyidae) from Middle Jurassic subsurface sediments in northwestern Poland. Two shells of Camptochlamys and four specimens plus some shell fragments of Atreta were examined using scanning electron microscope (SEM). The physical characteristics of each shell are described, along with their microstructure, size and dimensions.

This article discusses a study of larval shells of Oxytoma, a Jurassic bivalve found in Poland. The study examined the physical characteristics of the shell, including their length and height. It evaluated changes in larval shell size and changes in paleotemperature and showed that the shells are in juvenile or early adult stages. By examining the position of the adult resilifer, the study noted that the larval and adult ligaments originated in the center of the larval provinculum.

Callocardia hungerfordi (Veneridae: Pitarinae) lives in subtidal muds (220 to 240m C.D.) and is covered by a dense mat of mud that, effectively, camouflages the shell. [ABSTRACT FROM PUBLISHER]

A revision of the Recent species of the genus Goodallia (Astartidae) is presented. Adult shell and prodissoconch morphology of all the species are described and figured. In total, five species are recognized: G. triangularis (Montagu, 1803), G. pusilla (Forbes, 1844), G. macandrewi (Smith, 1881), G. micalii New species, and G. gofasi new species. G. triangularis and G. pusilla are widespread distributed in the Atlantic coasts of Europe and North Africa, and in the Mediterranean. G. macandrewi is restricted to the type locality in the Canary Islands, while what has been called G. macandrewi from the Mediterranean corresponds to an undescribed species here named G. micalii, distributed in the Central and Eastern Mediterranean. A second undescribed species from off the Atlantic Moroccan coast is named G. gofasi, and is known only from the type material of station B33 of the expedition CINECA CHARCOTT III. The named forms, variability and distribution of G. triangularis are discussed. [ABSTRACT FROM PUBLISHER]

A study of hinge transposition in the bivalve families Carditidae, Condylocardiidae, Crassatellidae, Astar-tidae, Trapezidae, Cardiidae, Tellinidae, and Veneri-dae, all in the subclass Heterodonta showed that all these families except Condylocardiidae, Cardudae, Tellinidae and Venendae rarely have transposed hinges.Of the 3767 valves studied, any valve with transposed anterior lateral teeth had transposed cardinal teeth on the same valve, and any valve with normal anterior lateral teeth always had normal cardinal teeth Previous work on the hinge structure of various fossil and living bivalves, other than Sphaemdae, supports this observation. The anterior lateral teeth and the cardinal teeth may be a single genetically linked unit in these families, and the posterior lateral teeth another unit. Each unit is represented by two pheno-types, namely normal and transposed dentition. With a combination of the two units by two phenotypes, therefore, four types of dentition are expected in heterodont bivalves, other than the Sphaemdae They are Phenotype I—normal anterior lateral teeth & cardinal teeth and normal posterior lateral teeth Phenotype II—normal anterior lateral teeth & cardinal teeth and transposed posterior lateral teeth, Phenotype III—transposed anterior lateral teeth & cardinal teeth and normal posterior lateral teeth, and Phenotype IV—transposed anterior lateral teeth & cardinal teeth and transposed posterior lateral teeth.In contrast to a whole transposition of gastropods with anatomical and shell asymmetry, Phenotypes II and III show that a partial transposition is fairly common within the bivalves. The partial transposition may be caused by their symmetrical organization and may be selectively nearly neutral.At present, there is no evidence to show that a transposed hinge is genetically controlled. Some evidence at the evolutionary level suggests that it is not a result of developmental noise or accident, but it may be a developmentally controlled directional asymmetry, which may be an example of a genetically controlled discontinuous character. [ABSTRACT FROM AUTHOR]

The article discusses a study on shell tubules in Condylocardiinae, which belongs to the microbivalve family Condylocardiidae. The shells of three Condylocardia species compared to Cardita calcyculata, Adacnarca limopsoides and Striarca lactea are examined. The study observed that shell tubules in Condylocardia reflect mantle cell extensions and that the similarity of shell tubules in Condylocardiidae and Carditidae supports their phylogenetic relationship.

The article focuses on the posterodorsal ridge in larval shells (P II) of Jurassic, Cretaceous, Eocene, and Recent arcoid bivalves. It cites that it is mostly likely that the ridge is the growth track of a notch in the P II shells of arcoids with planktotrophic larval development and not from injury. It adds that the posterodorsal ridge in the shells fills in the same position as the notch and its growth track in oysters however, they differ.

The article discusses research which investigated the correlation between the nacrein structure and morphology of bivavle and gastropod shells. It references previous studies on the detection of carbonic anhydrolase (CA) activity in the purified nacrein fraction. The researchers performed molecular cloning of the nacrein cDNA from the gastropod Turbo marmoratus. Described in detail is nacrein formation in the bivalve Pinctada maxima. Other highlights of the research findings are presented.

Adults of the bivalve Macoma balthica live burrowed in mud or sand, which makes their shells invisible so that a vision-related selective advantage of colour morphs is hard to imagine. Nevertheless, this species harbours a conspicuous colour polymorphism of white, yellow, orange and red. Here we present data from laboratory-reared crosses that demonstrate a genetic basis for this colour polymorphism. The results suggest a simple model of inheritance of colour class: four alleles at a single locus displaying linear hierarchy of dominance, in the following order of increasing dominance: white M. balthica. Furthermore, bicolouration may to a degree be a heritable trait, but the sample size is too small to draw any conclusions about possible inheritance mechanisms. Heritable colour variation can now, with the proposed model, be used as a genetic marker. Furthermore, the proposed model will provide the basis for further analysis of the evolutionary forces maintaining this colour polymorphism that is hidden from view for most of the life of this bivalve. [ABSTRACT FROM AUTHOR]

The reproductive cycle and shell microgrowth patterns of the venerid bivalve Ruditapes philippinarum from Matsukawa-ura, a small inlet facing the Pacific Ocean, northeastern Japan were examined. Histological examination of the gonads revealed that spawning in this species occurred twice in 2005, once between late June and early August and the other between late September and early October. Comparison of the shell microgrowth patterns with the developmental stages of the gonad in each individual revealed that in spawning individuals (mature and spawning stages), the mean lunar-day growth rates were significantly smaller than those in individuals which were not in spawning condition (spent, recovery and growing stages). In non-spawning individuals, the mean lunar-day growth rates were positively correlated with seawater temperature up to 20°C. However, in spawning individuals, no correlation was observed between shell growth and seawater temperatures >16°C. These facts suggest that physiological stress during reproduction has a negative influence on shell growth and results in spawning breaks. The present study indicate that spawning breaks can be used to identify the timing of sexual maturity and to count the number of spawning events occurring per year in extant and fossil bivalves. [ABSTRACT FROM AUTHOR]