Filament lumachelle on top of Middle Jurassic oolite limestones: event deposits marking the drowning of a Tethysian carbonate platform (Subbetic,southern Spain)

Bioclastic accumulations often occur on top of Tethysian carbonate platforms and crinoids are a common constituent of these bioclastic deposits on Lower and Middle Liassic carbonate platforms. In contrast, the relevant literature contains few examples in which the main constituent of the lumachels is thin-shelled bivalves (filaments). This paper presents a study of a filament lumachelle cropping out on top of a Middle Jurassic carbonate platform. The carbonate platform is represented by the Jabalcuz Formation, found in one of the northernmost Subbetic units (south of Jaén city). The lumachelle marks the demise of the carbonate platform and has special features that characterize the drowning phase. This process has been related with the syn-rift extensional tectonics associated to the opening of the Tethys westwards. Stratigraphically, the lumachelle occurs on top of shallow-water oolite limestones (Middle Jurassic) and is overlain by radiolarian-rich pelagic and resedimented deposits (Latest Callovian–Oxfordian). It occurs as a body (about 1.5 km wide and up to 8 m thick) made up entirely of densely packed thin bivalve shells. A remarkable feature of the bivalve shell beds is stromatolite-like crumpled lamination at the outcrop. The observations made at the outcrop scale, by microscope under transmitted light, and by cathodoluminiscence favor a diagenetic origin for this striking structure. Other hypotheses, such as its possible relation with seismicity, cannot be confirmed. The filaments would have filled one of the former basins that originated in relation with syn-rift fault-block tectonics leading to the demise of the carbonate platform. Tectonics was one of the main factors setting in motion a carbonate productivity crisis and the inhibition of a diverse benthic community. Once production failed in the carbonate factory, storms and probably hurricanes as well, swept shell deposits from the shallowest areas of the shallow-water carbonate platform and accumulated them in a coevally formed small half-graben basin. At least three main depositional stages can be differentiated in the fill of this half-graben basin, which was a sediment trap for the accumulation and preservation of the tiny bivalve shells against ebb surges. The massive accumulation of valves, the shortage of micrite around the filaments, outcrop morphology and facies relationships, along with regional geology, are among the arguments supporting this interpretation.     

Sperm structure and sperm transfer in Pseudopythina subsinuata (Bivalvia; Galeommatoidea)

Males and females of the commensal protandric bivalve Pseudopythina subsinuata have paired seminal receptacles, the interior of which contains many slender elongate cells. The testis produces small euspermatozoa and comparatively few and much larger paraspermatozoa. The ?16-μm-long and 3-5-μm-thick paraspermatozoa have a terminally placed irregularly cork-screw-shaped acrosome and a bundle of ca. 16 flagella emerging from behind the nucleus. The role of the paraspermatozoa is obscure. Euspermatozoa are transferred to the seminal receptacles of the females and attach with the tip of the acrosome to the elongate cells. Most females contain one to three “sperm trees”, structures consisting of a short stem and numerous branches. They are firmly implanted in the abfrontal part of the gill filament and protrude into the posterior part of the suprabranchial (brooding) chamber. Implantation of the trees causes the gill tissue to swell around the stem and some of the nearest filaments to coalesce. All branches are densely coated with euspermatozoa that are attached by means of their acrosomes. It is conjectured that the syncytial and multinucleate trees arise from seminal receptacle cells that detach from the receptacle and thereupon fuse. A similar process is known in the allied P. tsurumaru, but the resulting structure (“sperm-carrying body”) is not attached to the gills.     

A novel role for dpp in the shaping of bivalve shells revealed in a conserved molluscan developmental program

During the molluscan evolution leading to the bivalves, the single dorsal shell was doubled. To elucidate the molecular developmental basis underlying this prominent morphological transition, we described the cell cleavage and expression patterns of three genes, brachyury, engrailed, and dpp in the Japanese spiny oyster Saccostrea kegaki, and examined the function of dpp in this species. The cleavage pattern of the S. kegaki embryo was nearly the same as the previously described pattern of other bivalve species, suggesting that the pattern itself is highly important for the establishment or the maintenance of the bivalve body plan. The expression pattern of a brachyury homolog in S. kegaki (SkBra) was similar to the pattern in gastopods even at the single cell level despite the deep divergence of gastropods and bivalves. Engrailed and dpp were previously found to be expressed around the shell anlagen in gastropods. Like that of gastropods, an engrailed homolog in S. kegaki (SkEn) was found to be expressed around the shell anlagen. However, the dpp homolog in S. kegaki (SkDpp) was expressed only in the cells along the dorsal midline. ZfBMP4 treatment experiments revealed the importance of dpp in establishing the characteristic shape of the bivalve shell anlagen.     

Indicators of short-term growth in bivalves: Detecting environmental change across ecological scales

Marine benthic communities function within the constraints of changing environmental conditions. To better understand the dynamics of this relationship, increased knowledge of physiological mechanisms that link environmental conditions to ecological responses is imperative. Here we develop a synthesis demonstrating the potential to link biochemical/physiological and population/community level responses based on the effects of increased sedimentation, hypoxia, food availability, and habitat characteristics on bivalve physiology using two soft-sediment, suspension-feeding species, Austrovenus stutchburyi and Paphies australis. Nucleic acid ratios (RNA:DNA, RNA:protein or total RNA content) have been successfully used as indirect measures of short-term growth in a range of marine organisms. Bivalve nucleic acid ratios were analysed over several temporal and spatial scales in a combination of laboratory experiments, field experiments, transplants and surveys, and only by synthesising results from several different studies were clear patterns in bivalve RNA dynamics revealed. The magnitude of response in RNA content (RNA) varied seasonally, along environmental gradients, and with bivalve species and size class. The major factor affecting RNA was enhanced food availability, which appeared to have the potential to mask any negative effects of stressors. Over the short-term (10-14 days), elevated suspended sediment concentrations or single deposition of terrigenous clay did not affect RNA, but over the longer term (months), transplantation of bivalves to turbid sites and repeated deposition of clay had a slight negative effect on RNA. In the laboratory, hypoxia did not affect the fast increase in RNA in response to added food or the slow decrease in RNA in response to starvation. In addition, a species-specific baseline RNA content was indicated, possibly representing the basal metabolism of a species. Thus, RNA was more sensitive to factors facilitating growth than to factors inhibiting growth, indicating that RNA could be a good indicator of factors related to energy acquisition, while not being as sensitive to stressors. By assessing the variability in response across time and space scales, and considering the context and time scale of both the stressor and the response, organism-level measures may be used as part of an integrated approach in impact assessment and ecological experiments.     

Effect of varying levels of dietary essential fatty acid during early ontogeny of the sea scallop Placopecten magellanicus

This study investigated the biological effects of various dietary essential fatty acids levels to sea scallop larvae, Placopecten magellanicus. Scallop larvae were fed three diets from D-veliger to settlement. Diet A consisted of Isochrysis sp. and Pavlova lutheri, diet B was a mix of Isochrysis sp. and Chaetoceros muelleri and diet C consisted of the same two species, but C. muelleri was grown under silicate deprivation to alter the fatty acid composition. Pediveligers (28 days old) were sampled prior to settlement for fatty acid analysis, growth measurement and survival assessment. Survival and settlement success were measured at the end of the experiment (day 40). Our results show that feeding regime greatly influenced larval size, settlement and fatty acid composition. Diet A was severely deficient in arachidonic acid (20:4n-6, AA), leading to the poorest larval growth, survival and lipid content. Nevertheless, larvae fed diet A selectively accumulate AA by a factor three compared to the dietary amount. Shell size of 28-day-old larvae was positively correlated with AA content and negatively correlated with eicosapentaenoic acid (20:5n-3, EPA)-AA ratio, thus suggesting that these two variables are of major interest for the optimisation of larval growth in sea scallops. Finally, larvae fed diet C displayed 20% higher shell size at day 28 than larvae fed diet A and B, likely in relation to the dietary amount of saturated fatty acid (SFA). However, the moderate survival and settlement success of these groups of larvae might be associated with a relative deficiency in docosahexaenoic acid (22:6n-3, DHA). This study underlines that the overall balance between polyunsaturated fatty acid (PUFA) needs to be considered to adequately fed sea scallop larvae.     

Pernin: a novel, self-aggregating haemolymph protein from the New Zealand green-lipped mussel, Perna canaliculus (Bivalvia: Mytilidae)

A protein, designated pernin, found in the New Zealand green-lipped mussel, comprises almost all of the protein in cell-free haemolymph. It occurs as large, aggregate structures of several hundred units resembling small virus-like particles. Pernin is a non-pigmented, glycosylated protein, composed of 497 amino acids, which has an estimated molecular mass of 60 kDa. It is exceptionally rich in histidine (13.7%) and aspartic acid (12.3%), amino acids both known to participate in the binding of divalent metal cations. In addition, pernin has serine protease inhibitor activity, likely due to a sequence of eight N-terminal amino acid residues, separated from the remainder of the protein via a histidine–aspartate spacer. The pernin monomer comprises three regions of obvious sequence duplication. These make up approximately 95% of the pernin molecule and have sequences clearly homologous to the active-site domain of Cu–Zn SODs (superoxide dismutases). Despite several of the metal ion co-ordinating histidine residues being retained, pernin contains no Cu or Zn. It is, however, associated with Fe with an apparent stoichiometry of 1 atom of Fe to 6 molecules of pernin. Since pernin has no demonstrable SOD activity, these SOD-derived sequences presumably have been modified for another function.     

New genotypes and factors associated with Cryptosporidium detection in mussels (Mytilus spp.) along the California coast

A 3 year study was conducted to evaluate mussels as bioindicators of faecal contamination in coastal ecosystems of California. Haemolymph samples from 4680 mussels (Mytilus spp.) were tested for Cryptosporidium genotypes using PCR amplification and DNA sequence analysis. Our hypotheses were that mussels collected from sites near livestock runoff or human sewage outflow would be more likely to contain the faecal pathogen Cryptosporidium than mussels collected distant to these sites, and that the prevalence would be greatest during the wet season when runoff into the nearshore marine environment was highest. To test these hypotheses, 156 batches of sentinel mussels were collected quarterly at nearshore marine sites considered at higher risk for exposure to livestock runoff, higher risk for exposure to human sewage, or lower risk for exposure to both faecal sources. Cryptosporidium genotypes detected in Haemolymph samples from individual mussels included Cryptosporidium parvum, Cryptosporidium felis, Cryptosporidium andersoni, and two novel Cryptosporidium spp. Factors significantly associated with detection of Cryptosporidium spp. in mussel batches were exposure to freshwater outflow and mussel collection within a week following a precipitation event. Detection of Cryptosporidium spp. was not associated with higher or lower risk status for exposure to livestock faeces or human sewage sources. This study showed that mussels can be used to monitor water quality in California and suggests that humans and animals ingesting faecal-contaminated water and shellfish may be exposed to both host-specific and anthropozoonotic Cryptosporidium genotypes of public health significance.