Filter-feeding Bivalves Research Articles

Introduction into natural environments shifts the gut microbiome of captivity-raised filter-feeding bivalves

Abstract The gut microbiome is influenced by host species and the environment, but how the environment influences the microbiome of animals introduced into a new ecosystem has rarely been investigated. Freshwater mussels are aquatic fauna, with some threatened or endangered species propagated in hatcheries and introduced into natural systems as part of conservation efforts. The effects of the environment on the freshwater mussel gut microbiome were assessed for two hatchery-propagated species (Lampsilis ovata, Lampsilis ornata) introduced into rivers within their natural range. Mussels were placed in rivers for eight weeks, after which one subset was collected, another subset remained in that river, and a third subset was reciprocally transplanted to another river in the same river basin for a further eight weeks. Gut microbiome composition and diversity were characterized for all mussels. After the initial eight weeks, mussels showed increased gut bacterial species richness and distinct community composition compared to hatchery mussels, but gut microbiome diversity then decreased for mussels that remained in the same river for all 16 weeks. The gut bacterial community of mussels transplanted between rivers shifted to resemble that of mussels placed initially into the recipient river and that remained there for the whole study. All mussels showed high proportions of Firmicutes in their gut microbiome after eight weeks, suggesting an essential role of this phylum in the gut of Lampsilis species. These findings show that the mussel gut microbiome shifts in response to new environments and provide insights into conservation strategies that involve species reintroductions.

Effects of filter-feeding bivalves in benthic and pelagic habitats on plankton community and water quality in shallow systems: implications for lake rehabilitation

Effects of filter-feeding bivalves in benthic and pelagic habitats on plankton community and water quality in shallow systems: implications for lake rehabilitation

The contribution of fish to the diet of loggerhead sea turtles in the western Mediterranean revisited

Early juvenile loggerhead turtles (Caretta caretta) rely on gelatinous zooplankton, whereas individuals larger than 40 cm curved carapace length are adapted to crush hard-shelled invertebrates. Nevertheless, fish were reported to be the staple food of loggerhead turtles in the western Mediterranean 30 years ago. Here, the temporal consistency of such a fish-based diet of loggerhead turtles is assessed through gut content analysis and stable isotope analysis of samples from the Mediterranean coast of Spain spanning three decades. The gut contents of 134 juvenile loggerhead turtles (curved carapace length range: 27–71 cm) from three different periods (1991, 1999–2008 and 2010–2017) were analyzed, as well as a subsample of the same turtles (n = 10 in each period) for both bulk and compound-specific stable isotope ratios (CSIA-AA). Gut content analysis revealed a decline in the frequency of occurrence and numerical abundance of fish and an increasing contribution of gastropods and bivalves throughout time, although pelagic tunicates were always the most frequently observed prey. The δ15Nbulk of turtle bone also dropped throughout the study period, but the values of the stable isotope ratio of N in phenylalanine (δ15NPhe) indicated that 52.5% of that variability was due to a baseline shift over time. Accordingly, the trophic position estimated from CSIA-AA did not follow the decreasing pattern of δ15Nbulk, but fluctuated throughout time. The overall evidence indicates that fish consumption by loggerhead turtles in the study region declined through time, but the trophic position of loggerhead turtles did not change simultaneously. This is probably because low trophic prey such pelagic tunicates and filter-feeding bivalves and suspension-feeding gastropods were the bulk of the diet during the whole study period and fish played a minor role, even when their frequency of occurrence peaked. Past levels of high fish consumption might be due to high levels of fishery discards, currently declining because of the recent reduction of the fishing fleet.

Pentachlorophenol impairs the antimicrobic capability of blood clam via undermining humoral immunity and disrupting humoral-cellular crosstalk

Due to past massive usage and persistent nature, pentachlorophenol (PCP) residues are prevalent in environments, posing a potential threat to various organisms such as sessile filter-feeding bivalves. Although humoral immunity and its crosstalk with cellular one are crucial for the maintaining of robust antimicrobic capability, little is known about the impacts of PCP on these critical processes in bivalve mollusks. In this study, pathogenic bacterial challenge and plasma antimicrobic capability assays were carried out to assess the toxic effects of PCP on the immunity of a common bivalve species, blood clam (Tegillarca granosa). Moreover, the impacts of PCP-exposure on the capabilities of pathogen recognition, hemocyte recruitment, and pathogen degradation were analyzed as well. Furthermore, the activation status of downstream immune-related signalling pathways upon PCP exposure was also assessed. Data obtained illustrated that 28-day treatment with environmentally realistic levels of PCP resulted in evident declines in the survival rates of blood clam upon Vibrio challenge along with markedly weakened plasma antimicrobic capability. Additionally, the levels of lectin and peptidoglycan-recognition proteins (PGRPs) in plasma as well as the expression of pattern recognition receptors (PRRs) in hemocytes were found to be significantly inhibited by PCP-exposure. Moreover, along with the downregulation of immune-related signalling pathway, markedly fewer chemokines (interleukin 8 (IL-8), IL-17, and tumor necrosis factor α (TNF-α)) in plasma and significantly suppressed chemotactic activity of hemocytes were also observed in PCP-exposed blood clams. Furthermore, compared to that of the control, blood clams treated with PCP had markedly lower levels of antimicrobic active substances, lysozyme (LZM) and antimicrobial peptides (AMP), in their plasma. In general, the results of this study suggest that PCP exposure could significantly impair the antimicrobic capability of blood clam via undermining humoral immunity and disrupting humoral-cellular crosstalk.

Chemical contamination affecting filter-feeding bivalves in no-take marine protected areas from Brazil

Marine protected areas (MPAs) are zones geographically delimited under pre-defined management goals, seeking to reduce anthropogenic threats to biodiversity. Despite this, in recent years reports of MPAs affected by chemical contamination has grown. Therefore, this study addresses this critical issue assessing legacy and current chemical contamination in filter-feeder bivalves obtained in very restrictive no-take MPAs from Brazil. The detected pollutants encompass polycyclic aromatic hydrocarbons (PAHs), linear alkylbenzenes (LABs), and persistent organic pollutants (POPs) like dichlorodiphenyltrichloroethane (DDTs) and polychlorinated biphenyls (PCBs). Despite protective measures, bivalves from nine MPAs exhibited high LABs (13.2–1139.0 ng g−1) and DDTs levels (0.1–62.3 ng g−1). PAHs were present in low concentrations (3.1–29.03 ng g−1), as PCBs (0.7–6.4 ng g−1), hexachlorobenzene (0.1–0.2 ng g−1), and Mirex (0.1–0.3 ng g−1). Regardless of the sentinel species, MPAs and management categories, similar accumulation patterns were observed for LABs, DDTs, PAHs, and PCBs. Based on the limits proposed by Oslo Paris Commission, the measured levels of PAHs, PCBs and were below the environmental assessment criteria. Such findings indicate the no biological effects are expected to occur. However, they are higher considering background conditions typically measured in remote or pristine areas and potential simultaneous exposure. Such findings indicate an influence of anthropogenic sources, emphasizing the urgency for monitoring programs guiding strategic management efforts to safeguard these areas.

Microplastics in Atlantic Ribbed Mussels (Geukensia demissa) from the Delaware Inland Bays, USA

Due to the prevalence of plastic pollution in coastal ecosystems, aquatic organisms are at high risk for accumulating microplastics (MPs). Filter-feeding bivalves, such as mussels and oysters, may be exposed to, and subsequently accumulate, MPs due to the high volume of water they pass through their bodies. This study assessed the levels of MPs within Atlantic ribbed mussels (Geukensia demissa), a common filter feeder found along the United States Atlantic Coast, from 12 sites within Rehoboth Bay, Indian River Bay, and Little Assawoman Bay, collectively known as the Delaware Inland Bays. Composited mussels from each site were digested using potassium hydroxide and filtered. Microplastics were physically identified, sorted based on color, and counted using a digital microscope. Microplastics, almost entirely dominated by synthetic microfibers, were found in all mussels well above laboratory blanks. Across all sites, 40% of microfibers were black, and 27% of fibers were clear. The composite concentrations of MPs ranged from 0.25 to 2.06 particles/g wet tissue, with a mean of 0.08 ± 0.06. In general, higher concentrations were found in mussels collected at sites that were adjacent to more urbanized land use versus those from rural sites. At two sites, individual mussels, in addition to composites, were analyzed and had MP concentrations ranging from 11 to 69 particles/mussel. This study represents the first evaluation of MPs in this ecologically important coastal species and suggests its viability as a biomonitoring species for microplastic pollution.

Paralytic shellfish toxins producing dinoflagellates cause dysbacteriosis in scallop gut microbial biofilms

Filter-feeding bivalves could accumulate paralytic shellfish toxins (PSTs) produced by harmful dinoflagellates through diet. Despite that bivalves are resistant to these neurotoxins due to possessing PST-resistant sodium channel, exposure to PSTs-producing dinoflagellates impair bivalve survival. We hypothesized that ingesting PSTs-producing dinoflagellates may influence the gut microbiota, and then the health of bivalves. To test this idea, we compared the gut microbiota of the scallop Patinopecten yessoensis, after feeding with PST-producing or non-toxic dinoflagellates. Exposure to PSTs-producing dinoflagellates resulted in a decline of gut microbial diversity and a disturbance of community structure, accompanied by a significant increase in the abundance and richness of pathogenic bacteria, represented by Vibrio. Moreover, network analysis demonstrated extensive positive correlations between pathogenic bacteria abundances and PSTs concentrations in the digestive glands of the scallops. Furthermore, isolation of a dominant Vibrio strain and its genomic analysis revealed a variety of virulence factors, including the tolC outer membrane exporter, which were expressed in the gut microbiota. Finally, the infection experiment demonstrated scallop mortality caused by the isolated Vibrio strain; further, the pathogenicity of this Vibrio strain was attenuated by a mutation in the tolC gene. Together, these findings demonstrated that the PSTs may affect gut microbiota via direct and taxa-specific interactions with opportunistic pathogens, which proliferate after transition from seawater to the gut environment. The present study has revealed novel mechanisms towards deciphering the puzzles in environmental disturbances-caused death of an important aquaculture species.

Deep-sea mining: using hyperbaric conditions to study the impact of sediment plumes in the subtidal clam Spisula solida.

With the growing interest to exploit mineral resources in the deep-sea, there is the need to establish guidelines and frameworks to support hazard and risk assessment schemes. The present study used a subtidal species of filter-feeding bivalve, the clam Spisula solida, as a proxy to better understand the impacts of sediment plumes in marine organisms under hyperbaric conditions. Four concentrations of suspended sediments (0g/L, 1g/L, 2g/L, and 4g/L) were used in a mixture with different grain sizes at 4 Bar for 96h. Functional (filtration rate-FR) and biochemical endpoints (catalase-CAT, glutathione s-transferase-GST, and lipid peroxidation-LPO) were analyzed in the gonads, digestive gland, and gills of S. solida after a 96-h exposure at 4 Bar (the natural limit of the species vertical distribution). The FR showed a decreasing trend with the increasing sediment concentrations (significant effects at 2 and 4g/L). Additionally, significant changes were observed for some of the tested oxidative stress biomarkers, which were concentration and tissue-dependent, i.e., CAT activity was significantly elevated in gills (1g/L treatment), and GST was decreased in digestive gland (1g/L treatment). Overall, the results show that suspended sediments, at 2 and 4g/L, have negative functional impacts in the bivalve S. solida providing additional insights to improve hazard assessment of deep-sea mining. These findings represent a step forward to ensure the mitigation of the potential negative effects of deep-sea resource exploitation.

Phytoplankton as indicators of global warming?

AbstractTerrestrial plants are sensitive indicators of global warming because their annual cycles of growth and senescence are changing as warming proceeds. Single celled algae are distinct life forms capable of population bursts in any season, so there is uncertainty about phytoplankton phenology as a comparable indicator of global warming. We analyzed 4+ decades of monthly chlorophyll a measurements at two sites in San Francisco Bay and found abrupt shifts during summer months leading to a 48‐day advance in the annual pattern of chlorophyll‐a accumulation at one site and a 36‐day delay at the other. These large phenological changes were not associated with changing temperature, but they were associated with changes in top–down control by bivalve filter feeders as biological communities were restructured by (1) introduction of a non‐native clam, and (2) a shift in atmospheric forcing of the NE Pacific. This study illustrates that changes in phytoplankton phenology are not necessarily responses to or indicators of global warming. However, they can be indicators of human disturbances and natural climate oscillations having effects large enough to mask the effect of climate warming.

Simultaneous increases of filter-feeding fish and bivalves are key for controlling cyanobacterial blooms in a shallow eutrophic lake

Eutrophication and cyanobacterial blooms have severely affected many freshwater ecosystems. We studied the effects of filter-feeding fish and bivalves on algal populations using a mesocosm experiment and long-term monitoring data from Lake Taihu (China). The mesocosm study, comprised of a two-way factorial design with the clam Corbicula fluminea and the fish Aristichthys nobilis at three biomass levels, resulted in lower chlorophyll a (Chl a) in high fish treatments, but no significant differences in the low and medium fish treatments. Chl a also decreased with an increase in clam biomass in the high fish treatments. Moreover, filter-feeding fish resulted in a decrease in algal sizes (e.g., the colony size of Microcystis aeruginosa was inversely related to fish biomass) which likely increased the filter-feeding efficiency of bivalves. Biomass of filter-feeding fish was found to be a key factor driving the synergistic effects of filter-feeding fish and bivalves in waters dominated by Microcystis colonies. Long-term monitoring revealed increasing trends in Chl a concentration, total fish catch per unit effort (TF-CPUE), and filter-feeding fish (FF-CPUE), and slightly decreasing trends in bivalve biomass and nitrogen to phosphorus ratios (N:P) from 2006 to 2016. Bivalve biomass and N:P were negatively correlated with Chl a, while FF-CPUE was not significantly related to Chl a. The current filter-feeding fish biomass in Lake Taihu is estimated to be too low to drive synergistic algal control effects together with bivalves. Furthermore, the lack of filter feeders in Lake Taihu may lead to top-down control by predators that cannot counteract the bottom-up effects of nutrients on phytoplankton. Collectively, these long-term monitoring and experimental data support the combined use filter-feeding fish and bivalves for managing cyanobacteria blooms in Lake Taihu.

Taphonomic signatures on the pearl oyster Pinctada from Arabian Gulf, Saudi Arabia

A total of 886 valves of the pearl oyster Pinctada were collected from 12 sites in Al-Uqair beach along the Saudi Arabian Gulf coast in January 2021 in order to document their taphonomic signatures. Thirteen ichnospecies of 5 ichnogenera were identified and illustrated. These traces were produced by clionid sponges (Entobia cretacea, E. ovula, E. geometrica, E. laquea, E. cateniformis, Entobia isp.), durophagous drillers (Oichnus paraboloides, O. ovalis, O. simplex, and Oichnus isp.), traces of vermetid gastropods (Renichnus isp.) polychaete annelids (Caulostrepsis isp.) and barnacle attachment scars (Anellusichnus circularis). The Pinctada shells act as hard substrate for colonization by serpulid worm, Spirorbis sp., bryozoans, barnacles, and other bivalves. Ichnogenus Oichnus was the most abundant (53.73%), followed by Entobia (44.58%), Anellusichnus (0.51%), Caulostrepsis (0.34%), and Renichnus(0.84%). The thin-shelled and smooth skeletons of Pinctada were preferable for the abundant durophagous drillers (Oichnus traces) and clionid sponges (Entobia traces) during the lifetime of the pinctadas, in contrast to endolithic bivalves (Gastrochaenolites borings) which need thicker seashells for the settlement. Occurrence of different encrusters and bioeroders on the internal surfaces of many pinctadas confirmed the postmortem origin of the signatures. Disarticulation, fragmentation, and abrasion among the collected pinctadas might be attributed to their mode of life as epifaunal byssate, filter-feeder bivalves in the shallow littoral and sublittoral zones of the continental shelf under strong currents conditions.

Bioaccumulation of estuarine pollutants in leaf oysters (Isognomon ephippium) on the mid-north coast, New South Wales, Australia

Filter feeding bivalves are useful bioindicators for the detection of biologically available pollutants. We investigated trace metals, metalloids, and pesticides in leaf oyster (Isognomon ephippium) soft tissue and shells and compared them to sediment in five estuaries in northern New South Wales, Australia. Concentrations of Pb, Cr, Mn, Ni, Fe and Al were higher in sediments, whereas Zn, Cd, Ag, Hg, Se and As bioaccumulated in the soft tissue. The amount of Cu, Hg and Ni in the sediment from Tweed and Richmond River estuaries exceeded the Australian national sediment quality guideline values. Only one pesticide, atrazine, was detected in leaf oyster soft tissue. Combinations of six elements in the soft tissue were the best predictors of oyster condition index and shell size, whereas sediment contaminants showed weak relationships. Overall, the bioaccumulation of metals and metalloids increases with leaf oyster size and reduces leaf oyster condition, suggesting these large bivalves are useful bioindicators for pollution in estuarine environments.

Impacts of divalent cations (Mg2+ and Ca2+) on PFAS bioaccumulation in freshwater macroinvertebrates representing different foraging modes

Per- and polyfluoroalkyl substances (PFAS) have extensively contaminated freshwater aquatic ecosystems where they can be transported in water and partition to sediment and biota. In this paper, three freshwater benthic macroinvertebrates with different foraging modes were exposed to environmentally relevant concentrations of eight perfluoroalkyl carboxylates (PFCA), three perfluoroalkyl sulfonates (PFSA), and three fluorotelomer sulfonates (FTS) at varying divalent cation concentrations of magnesium (Mg2+) and calcium (Ca2+). Divalent cations can impact PFAS partitioning to solids, especially to sediments, at higher concentrations. Sediment dwelling worms (Lumbriculus variegatus), epibenthic grazing snails (Physella acuta), and sediment-dwelling filter-feeding bivalves (Elliptio complanata) were selected due to their unique foraging modes. Microcosms were composed of synthetic sediment, culture water, macroinvertebrates, and PFAS and consisted of a 28-day exposure period. L. variegatus had significantly higher PFAS bioaccumulation than P. acuta and E. complanata, likely due to higher levels of interactions with and ingestion of the contaminated sediment. “High Mg2+” (7.5 mM Mg2+) and “High Ca2+” (7.5 mM Ca2+) conditions generally had statistically higher bioaccumulation factors (BAF) than the “Reference Condition” (0.2 mM Ca2+ and 0.2 mM Mg2+) for PFAS with perfluorinated chain lengths greater than eight carbons. Long-chain PFAS dominated the PFAS profiles of the macroinvertebrates for all groups of compounds studied (PFCA, PFSA, and FTS). These results indicate that the study organism has the greatest impact on bioaccumulation, although divalent cation concentration had observable impacts between organisms depending on the environmental conditions. Elevated cation concentrations in the microcosms led to significantly greater bioaccumulation in the test organisms compared to the experimental reference conditions for long-chain PFAS.

Organophosphate esters (OPEs) in the marine environment: Spatial distribution and profiles in French coastal bivalves

Organophosphate esters (OPEs), chemicals widely used in industrial production, electronics and domestic products, have become ubiquitous environmental contaminants. In this study, the levels and spatial distribution of 11 OPEs (aryl, alkyl and halogenated) were investigated in over 100 samples of filter-feeding bivalves collected yearly between 2014 and 2021 at sites of contrasted pressure along the French coasts. OPEs were found in virtually all samples, indicating their widespread spatial and temporal occurrence in coastal bivalves and the relevance of their biomonitoring. The median concentrations were between 0.4 (TMPP) and 4.9 ng g−1 dry weight (TCIPP), with TCIPP, TNBP and EHDPP found at the highest median values. TCEP and TBOEP were not frequently detected overall, but each year, the same sites showed repeatedly high concentrations. Structurally-related OPEs generally correlated, but the geographical distributions were not predictable from known anthropogenic pressures (population in the catchment area, industry), with little comparability with other hydrophobic contaminants. If the relation between sources of OPEs and bioaccumulated levels remains uncertain, local hotspots, rather than riverine/atmospheric transportation, could account for their geographical distribution. A systematic review of the levels of OPEs found in filter-feeding bivalves worldwide revealed comparable levels in our study with those reported elsewhere; however, the levels across and within (when available) studies generally spanned several orders of magnitude, indicating high spatial and temporal heterogeneity. In view of the growing concerns regarding OPEs, this study provides essential reference data for future studies of their occurrence on European coasts and supports the need for a more systematic (bio)monitoring of this class of contaminant.

Mussel antiviral transcriptome response and elimination of viral haemorrhagic septicaemia virus (VHSV)

As filter-feeding bivalves, mussels have been traditionally studied as possible vectors of different bacterial or viral pathogens. The absence of a known viral pathogen in these bivalves makes it particularly interesting to study the interaction of the mussel innate immune system with a virus of interest. In the present work, mussels were challenged with viral haemorrhagic septicaemia virus (VHSV), which is a pathogen in several fish species. The viral load was eliminated after 24 h and mussels evidenced antiviral activity towards VHSV, demonstrating that the virus was recognized and eliminated by the immune system of the host and confirming that mussels are not VHSV vectors in the marine environment. The transcriptome activating the antiviral response was studied, revealing the involvement of cytoplasmic viral sensors with the subsequent activation of the JAK-STAT pathway and several downstream antiviral effectors. The inflammatory response was inhibited with the profound downregulation of MyD88, shifting the immune balance towards antiviral functions. High modulation of retrotransposon activity was observed, revealing a mechanism that facilitates the antiviral response and that had not been previously observed in these species. The expression of several inhibitors of apoptosis and apoptosis-promoting genes was modulated, although clear inhibition of apoptosis in bivalves after severe viral infection and subsequent disease was not observed in this study. Finally, the modulated expression of several long noncoding RNAs that were correlated with genes involved in the immune response was detected.

Three decades of change in the Skagerrak coastal ecosystem, shaped by eutrophication and coastal darkening

Global coastal ecosystems are under accelerating pressure from human activities and climate change. In this study we explore a long-term time series (mostly 1990–2016) from major Norwegian rivers, together with coastal time series from the Norwegian Skagerrak coast. The aims are to: 1) test long-term trends in riverine loadings to Skagerrak, changes in coastal water quality and pelagic and benthic species composition, and 2) to describe the relationships between human drivers (eutrophication and coastal darkening) and community structure of benthic communities. Riverine discharge and transport increased by 23–85% over the time period, corresponding to a 40–78% increase in concentrations of suspended particulate material in coastal waters and reduced surface salinity, connected to the reported coastal darkening of coastal waters. There was a worsening in ecological status for hard-bottom benthic communities (0–30 m) and a reduction in the lower growth depth limit of several macroalgae species. A structural shift in the community composition from macroalgae to increased abundance of animals was found to be related to increased coastal darkening and reduced eutrophication. The concentration of coastal inorganic nutrients (DIN, PO4) declined by 27–36%, in response to management efforts to reduce eutrophication in European regional seas. Results indicate that reduced eutrophication was related to increased ecological status of the deep soft-bottom community (350 m), with a reduction in opportunistic polychaetes and an increase in filter feeding bivalves. This work highlights how climate change and other human-induced changes in a boreal ecosystem intensifies land-ocean interactions, and calls for more adaptive monitoring, where traditional water quality programs and policies evolve iteratively as new information emerges and the major drivers of the systems change.

Azaspiracid accumulation in Japanese coastal bivalves and ascidians fed with Azadinium poporum producing azaspiracid-2 as the dominant toxin component.

The filter-feeding bivalves often accumulate marine toxins by feeding on toxic dinoflagellates that produce marine toxins. Azaspiracids (AZAs) are a group of lipophilic polyether toxins which have been detected in a variety of organisms in many countries. In our present study, accumulation kinetics and toxin distributions in the tissues of seven bivalve species and ascidians relevant to Japanese coastal waters were investigated by experimentally feeding a toxic dinoflagellate Azadinium poporum, which produces azaspiracid-2 (AZA2) as the dominant toxin component. All bivalve species and ascidians investigated in this study had the capability to accumulate AZA2 and no metabolites of AZA2 were detected in the bivalves and the ascidians. Japanese short-neck clams, Japanese oysters, Pacific oysters and ascidians accumulated AZA2 with the highest concentrations on the hepatopancreas, whereas the highest concentrations of AZA2 were found on the gills in surf clams and horse clams. Hard clams and cockles accumulated high levels of AZA2 in both the hepatopancreas and the gills. As far as we know, this is the first report describing detailed tissue distribution of AZAs in several bivalve species other than mussels (M. edulis) and scallops (P. maximus). Variation of accumulation rates of AZA2 in Japanese short-neck clams on different cell densities or temperatures were observed.

Changes in biochemical metabolites in manila clam after a temporary culture with high-quality microalgal feed mixed with the dinoflagellate species Karlodinium veneficum and K. zhouanum

Phytoplankton composition is an important factor affecting the growth and physiological biochemical characteristics of filter-feeding bivalves. With the increasing trend in dinoflagellate biomass and blooms in mariculture areas, how the physio-biochemical traits and seafood quality of the mariculture organism are affected by the dinoflagellates, especially those at nonfatal levels, is not well understood. Different densities of two Karlodinium species, namely K. veneficum (KV) and K. zhouanum (KZ), mixed with high quality microalgal food Isochrysis galbana was applied in feeding manila clam Ruditapes philippinarum in a 14-day temporary culture, to comparatively study how the critical biochemical metabolites such as glycogen, free amino acids (FAAs), fatty acids (FAs), volatile organic compounds (VOCs) in the clam were affected. The survival rate of the clam showed dinoflagellate density and species specificity. The high-density KV group inhibited survival to 32% lower than that of the pure I. galbana control, respectively, while KZ at low concentrations did not significantly affect the survival compared with the control. In the high-density KV group, the glycogen and FAA contents decreased (p < 0.05), indicating that energy and protein metabolism were significantly affected. Amount of carnosine (49.91 ± 14.64 to 84.74 ± 8.59 μg/g of muscle wet weight) was detected in all the dinoflagellate-mixed groups, while it was not present in the field samples or in the pure I. galbana control, showing that carnosine participated in the anti-stress activities when the clam was exposed to the dinoflagellates. The global composition of FAs did not significantly vary among the groups. However, contents of the endogenous C18 PUFA precursors linoleic acid and α-linolenic acid significantly decreased in the high-density KV group compared to all the other groups, indicating that high density of KV affected the metabolisms of fatty acids. From the results of the changed VOC composition, oxidation of fatty acids and degradation of free amino acids might occur in the clams exposed to dinoflagellates. The increased VOCs, such as aldehydes, and decreased 1-octen-3-ol probably produced a more fishy taste and reduced food flavor quality when the clam was exposed to the dinoflagellates. This present study demonstrated that the biochemical metabolism and seafood qulity of the clam were affected. However, KZ with moderate density in the feed seemed to be beneficial in aquaculture for increasing the content of carnosine, a high-valued substance with multiple bioactivities.

Molluscs from Tidal Channels of the Gulf of Gabès (Tunisia): Quantitative Data and Comparison with Other Lagoons and Coastal Waters of the Mediterranean Sea

The present study analyses the spatio-temporal structuration of the molluscan fauna from four tidal channels of the Gulf of Gabès. A total of 26 stations were sampled at four seasons from March 2016 to January 2017, leading to the identification of 2695 individuals and 57 species. The species richness and abundances are higher in autumn than in other seasons. The fauna is dominated by seven species, three gastropods [Cerithium scabridum Philippi, 1848, Bittium reticulatum (da Costa, 1778) and Tricolia speciosa (Megerle von Mühfleld, 1824)] and four bivalves [Abra alba (W. Wood, 1802), Loripes orbiculatus Poli, 1791, Varicorbula gibba (Olivi, 1792) and Peronaea planata (Linnaeus, 1758)], which are characteristic of habitats with detritus accumulation and seagrass meadows. These dominant species are commonly recorded in lagoons and coastal shallow waters of the Mediterranean Sea. The structure of the molluscan fauna is linked to the location of tidal channels in the Gulf of Gabès. Abundances are lower in the Mimoun channel than in the other channels, especially the Maltine channel which shows a great accumulation of organic matter and high abundances of molluscs. Low abundances are found in high-energy hydrodynamic zones with gravel sediment; conversely, the presence of macrophytes (mainly in seagrass meadows) increases molluscan diversity. Comparisons with other sites in the shallow waters of the Tunisian coast and lagoons show that the taxonomic diversity of molluscs of the tidal channels of the Gulf of Gabès is equivalent to that reported elsewhere, but the abundance per m2 is among the lowest levels recorded here. Moreover, most of the dominant species found in the Gulf of Gabès tidal channel are reported as dominant in other studies covering the Mediterranean Sea. A distance-based redundancy analysis shows that depth, sediment type and the presence of marine phanerogams and filter-feeder bivalves on fine sands and gravels account for the structure of mollusc assemblages associated with each channel.

The combined effects of filter-feeding bivalves (Cristaria plicata) and submerged macrophytes (Hydrilla verticillate) on phytoplankton assemblages in nutrient-enriched freshwater mesocosms.

Freshwater ecosystems are threatened by eutrophication, which causes persistent and harmful algal blooms. Filter-feeding bivalve mollusks and submerged macrophytes (SMs) alleviate the eutrophication effects by inhibiting phytoplankton biomass blooms. However, very little is known about whether and how the combined manipulation of filter-feeding bivalves and SMs control eutrophication and influence phytoplankton assemblages. Here, we performed a nutrient-enriched freshwater mesocosm experiment to assess the combined effects of the filter-feeding bivalve Cristaria plicata, a cockscomb pearl mussel, and the macrophyte Hydrilla verticillate on the biomass and composition of phytoplankton assemblages. We found that addition of C. plicata and H. verticillate decreased the water nutrient concentrations and suppressed overall phytoplankton biomass. Further, distinct differences in taxa between restoration and control treatments were observed and noticeably competitive exclusion of cyanobacteria in the restoration treatments occurred. An antagonistic interaction between filter-feeding bivalves and SMs was only detected for total cyanobacteria biomass demonstrating that a larger magnitude of SM restoration may override the effect of filter-feeding bivalves. Our results suggest that manipulation, through the addition of bivalves as grazers, associated with the restoration of SMs, is an efficient approach for reducing cyanobacterial blooms and alleviating eutrophication.