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A morphological study revealed that the NW American soft-bottom bivalve Nutricola tantilla is dioecious, not a protandric hermaphrodite as previously reported. We base this conclusion on the smaller males having highly specialized, glandular sperm ducts, while the larger females have simple oviducts and no transition between the two ducts occurs. The females are brooders and retain their ova in a marsupium within the dorsal part of the inner demibranchs. Sperm cells were present and associated with a mesh-like tissue among the ova or early-stage embryos. How this tissue originates is still unknown. We suggest that sperm cells dissociate from this pool and fertilize the oocytes as soon as they are ovulated. The mode of sperm storage in N. tantilla represents a unique case in that the sperm are presumably nurtured within a nonepithelial tissue. We describe the ultrastructure of the sperm cells in N. tantilla. Large, spherical cytophores become associated with a multitude of acrosomes of spermatozoa and probably represent the precursors of the spermatozeugmata that have been described previously.

The genus Macridiscus Dall, 1902 contains a few species present in warm temperate to tropical faunas of the western Pacific. Macridiscus was widely accepted as a subgenus of Gomphina until a recent suggestion that it should be separated from Gomphina and elevated to an independent genus, based on morphology and molecular data. The taxonomy of the genus Macridiscus has in the past been based solely on shell characters and there has been no agreement about the number of valid species. In this study, we explore the taxonomy and phylogeny of Macridiscus species in order to resolve the systematics of the genus, based not only on shell characters but also on mitochondrial and nuclear DNA sequences. The morphological characters, the sequences of the cytochrome c oxidase subunit I of mitochondrial DNA and the first internal transcribed spacer region (ITS1) between the 18S and 5.8S ribosomal DNA were highly concordant and clearly suggested that three species should be recognized in the genus Macridiscus: M. multifarius new species, M. semicancellata (Koch, in Philippi

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 mm long and 1 mm 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 mm 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.

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.

Using a nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry, localization and morphology of putative nitric oxide synthase-containing elements were studied in the intestine of the following bivalves: Ruditapes philippinarum, Callithaca adamsi, Mercenaria stimpsoni (Veneridae), Corbicula japonica (Corbiculidae), Nodularia vladivostokensis and Cristaria tuberculata (Unionidae). NADPH-d-positive nerve cells and plexuses were found in the intestine of all species studied. Labelled diformazan bipolar nerve cells were present in the epithelium of the intestinal groove and typhlosole of the anterior midgut, the midgut proper, and the hindgut. Their apical process extended towards the gut lumen, whereas the basal one was connected with the basiepithelial NADPH-d-positive nerve plexus. In the typhlosole of N. vladivostokensis, these cells constituted up to 2.68% of the total number of epithelial cells. The bivalve species studied exhibit a similar distribution pattern of NADPH-d-positive cells, which lie separately or form small groups of two to three in the basal part of the epithelium. Basiepithelial NADPH-d-positive plexus was connected by separate fibres with subepithelial NADPH-d-positive nerve plexus. Both the plexuses were moderately developed in all intestinal regions in the majority of the species examined.

On a sandy beach at Shoal Bay in Princess Royal Harbour, Albany, southwestern Western Australia, lives a small muricid gastropod that feeds virtually monotonically on the overwhelmingly dominant resident bivalve Katelysia scalarina. Lepsiella paivae lives buried in the sand and attacks its prey within it. Because of its small size (,13 mm shell height), bivalve prey is also small and this study demonstrates a preference for K. scalarina of 5 mm shell length, i.e. juveniles. Laboratory experiments also suggested a possible preference for attack of the right valve. Lepsiella paivae can and does, however, attack larger prey (up to 15 mm shell length), but cannot consume them completely. A second visit to Princess Royal Harbour in the Austral winter, when there was no juvenile K. scalarina present, showed L. paivae to be attacking at the sand surface, also by drilling, the small (,4 mm) gastropod Hydrococcus brazieri (Hydrococcidae). SEM studies of experimentally determined drill holes of L. paivae show them to be of variable form, some straight sided, others bevelled (like a naticid) and ,500 mm in diameter. On this sheltered Southern Ocean beach, therefore, L. paivae has specialized to attack juvenile bivalves by burrowing after them. It can, however, attack other species opportunistically on the sand surface when seasonally favoured juvenile bivalve prey are not present.

The smooth clam, Callista chione (Linnaeus, 1758), is a shallowburrowing filter feeder, inhabiting sandy sediments, from just offshore to a depth of about 130m. It is widely distributed in Mediterranean and Atlantic waters, from the British Isles to the Moroccan coast. In the Mediterranean Sea, C. chione is among the most abundant bivalve species inhabiting shallow softbottomed shores and, in some areas, the most prominent suspension-feeding bivalve species in terms of biomass. The smooth clam is economically important in France, Portugal, Spain (Catalan Sea) and Italy (Adriatic Sea), and part of an extensive clam fishery carried out by artisanal dredging. Although it is not a very marketable species, C. chione is currently widely exploited as a consequence of the overexploitation of other clams, such as Ensis siliqua, Spisula solida, Chamelea gallina and Venus striatula. However, a decline of populations due to overfishing has been observed over a number of years, thus a reduction of fishing effort is now suggested. Despite the fact that C. chione is a common inhabitant of the sandy shores of the Thracian Sea, no systematic fishery exists similar to that present along the coasts of the western Mediterranean. Smooth clams are collected almost circumstantially by divers and are usually landed in small quantities together with other bivalves (Galinou-Mitsoudi, personal communication). Thus, no reliable fishery statistics are available. As C. chione is the object of an intensive uncontrolled fishery, mainly in the western Mediterranean, the study of its population dynamics is important for developing appropriate fishery management strategies. However, research on the biology and ecology of C. chione is limited. In the eastern Mediterranean, records refer only to its geographical distribution. The purpose of the current study is to contribute to the knowledge of the biology of the species in the Thracian Sea by providing information on shell banding, morphometric relationships, age and growth rates. Smooth clams were collected during December 2001 and June 2002 from two sites (St S and St K) along the western coast of Thassos Island, Greece, which are free of any source of major pollution. The two sites differ in wave exposure, anthropogenic activity and clam population density. At both sites the sediment mainly consisted of fine and medium sand, which together accounted for 78–92% of the particles. The redox-potential discontinuity (RPD) layer lay significantly closer to the surface at St K and within the depth range inhabited by C. chione. This resulted in a black biofilm on the shell periostracum, probably of anaerobic bacteria, which disappeared within 2–3 days when the clams were kept in aerobic conditions (immersed in continuously renewed seawater). Sampling of clams was carried out at depths between 1 and 3m at 0.5 m intervals, using a metal box quadrat (100 £ 100 £ 10 cm), which was pushed into the sediment and sifted by hand in situ. A random sample was also collected by hand from shallower depths at both sites in order to increase the number of captured animals. Fifteen clams from each site at each sampling season, chosen to cover the full size range of each sample, were pooled (shell length: 16–72mm) and size parameters, such as shell length (maximum distance of the anterior–posterior axis), shell height (distance of the ventral–dorsal axis, across the shell middle axis), shell width (maximum distance on the lateral axis, between both valves of the closed shell; to the nearest 0.1mm), shell weight, wet flesh weight and dry flesh weight (to the nearest 0.0001 g) were measured. Linear regressions were fitted to the log10-transformed data and the constants a (intercept 1⁄4 the initial growth coefficient) and b (slope 1⁄4 relative growth rate of size parameters) were estimated. t-Tests (H0 1⁄4 1, H0 1⁄4 3) with a confidence level of 95% (a 1⁄4 0.05) were also applied in order to determine whether the relative growth of size parameters is allometric. In order to identify spatial or seasonal fluctuations in the morphometric relationships of the clam, an analysis of variance with a covariate (ANCOVA) was carried out. Morphometric relationships between the shell-size parameters were compared only between the two sampling sites. The relationships between the wet and dry flesh weight against the shell length, and that of the wet flesh weight versus the dry flesh weight (which examines the flesh water content) were compared spatially and seasonally, as these parameters in bivalves are known to vary throughout the year. Characteristic ring-like patterns were noticed on the external surface, consisting of a wide opaque area of shell accretion followed by a transparent narrow area. It was assumed that they form annual increments. Acetate peel replicas of polished and etched shell sections were additionally prepared in order to examine the general shell structure and microgrowth patterns. The patterns of growth lines (see later) were used to identify the position of each growth ring. Distance measurements (to the nearest 0.1mm) were taken from the umbo to the end of each ring along the axis of maximum growth. Both external rings and microgrowth patterns were used for age determination as it soon became apparent that all growth lines identified on the shell surface were also clearly visible in the peel replicas of shell sections, and that measurements taken in the shell surface were in full correspondence with those taken on the peels. The von Bertalanffy growth model was applied to the size-at-age data and growth parameters (L1, K, t0) were estimated using a nonlinear regression under the FiSAT II software package. Morphometric relationships between several size parameters in C. chione are presented in Table 1. The allometric coefficient (b, slope) of these relationships is related allometrically in all cases and reflects ontogenetic changes in the shell and flesh. The shell of C. chione tends to become proportionally higher, wider and heavier, possibly providing better anchorage in the mobile sediment for larger animals. The flesh grows relatively faster than shell length, while the water content is reduced as the clams grow. The spatial comparison of the relationships between shell variables did not show any significant differences (ANCOVA, Correspondence: P. K. Leontarakis; e-mail: fri@otenet.gr

New sequence data from the D6 region of 28S rRNA and rDNA are presented for 28 species of molluscs and two annelids. These sequences were analyzed along with previously published molluscan sequences. Five of the molluscan classes were represented: Gastropoda, Cephalopoda, Bivalvia, Scaphopoda, and Polyplacophora, the last two by only one species each. Allowing for presumed long branch effects of Helicinidae, Patellidae, and the annelids, cladistic analysis supported monophyly of the following groups: Cephalopoda, Nautiloida, Coleoida, Gastropoda, Apogastropoda, Caenogastropoda, Neogastropoda, Rissooidea, Pulmonata, Stylommatophora, and a subset of Heterodonta including Sphaeriidae, Dreissenidae, and Veneridae but not Cardiidae. No resolution of relationships among molluscan classes was obtained.

A new method for measuring the pumping activity of infaunal bivalves is described. The method is based on the simultaneous recording of three temperatures: (1) the temperature of the sea water above the bivalve, (2) the temperature of the sediment and (3) the internal temperature of the bivalve. When thermic shifts between sediment and sea water are induced by the sun and/or the ground water, comparison of the three temperatures provides information on the pumping activity of the bivalve. The method was applied during the day to Tapes decussatus and Ruditapes philippinarum (Veneridae). Their pumping activity appears continuous except during emersion at ebb. Sestonic concentrations as high as 325 mg I" 1 induced no detectable interruption. Analysis of the data suggests that in situ pumping rate is not constant but fluctuates.

L'examen de ce genre et des caracteres utilises par Bernard relevent des doutes sur l'ancetre commun de ces bivalves et l'utilite de regrouper ces taxons au niveau de la sous-famille. Les ressemblances utilisees par Bernard pour regrouper ces genres peuvent resulter de la convergence associee a la petite taille des individus