A multi-study analysis of gut microbiome data from the blue mussel (Mytilus edulis) emphasises the impact of depuration on biological interpretation.

The blue mussel (Mytilus edulis) is a suspension feeder which has been used in gut-microbiome surveys. Although raw 16S sequence data are often publicly available, unifying secondary analyses are lacking. The present work analysed raw data from seven projects conducted by one group over 7 years. Although each project had different motivations, experimental designs and conclusions, all selected samples were from the guts of M. edulis collected from a single location in Long Island Sound. The goal of this analysis was to determine which independent factors (e.g., collection date, depuration status) were responsible for governing composition and diversity in the gut microbiomes. Results indicated that whether mussels had undergone depuration, defined here as voidance of faeces in a controlled, no-food period, was the primary factor that governed gut microbiome composition. Gut microbiomes from non-depurated mussels were mixtures of resident and transient communities and were influenced by temporal factors. Resident communities from depurated mussels were influenced by the final food source and length of time host mussels were held under laboratory conditions. These findings reinforce the paradigm that gut microbiota are divided into resident and transient components and suggest that depuration status should be taken into consideration when designing and interpreting future experiments.

Nylon microfibers develop a distinct plastisphere but have no apparent effects on the gut microbiome or gut tissue status in the blue mussel, Mytilus edulis.

Ingestion of microplastics (MP) by suspension-feeding bivalves has been well-documented. However, it is unclear whether exposure to MP could damage the stomach and digestive gland (gut) of these animals, causing ramifications for organism and ecosystem health. Here, we show no apparent effects of nylon microfiber (MF) ingestion on the gut microbiome or digestive tissues of the blue mussel, Mytilus edulis. We exposed mussels to two low concentrations (50 and 100 particles/L) of either nylon MF or Spartina spp. particles (dried, ground marsh grass), ca. 250-500 µm in length, or a no particle control laboratory treatment for 21 days. Results showed that nylon MF, when aged in coarsely filtered seawater, developed a different microbial community than Spartina spp. particles and seawater, however, even after exposure to this different community, mussel gut microbial communities resisted disturbance from nylon MF. The microbial communities of experimental mussels clustered together in ordination and were similar in taxonomic composition and measures of alpha diversity. Additionally, there was no evidence of damage to gut tissues after ingestion of nylon MF or Spartina spp. Post-ingestive particle processing likely mediated a short gut retention time of these relatively large particles, contributing to the negligible treatment effects.

The relationship between microplastics in eastern oysters (Crassostrea virginica) and surrounding environmental compartments in Long Island Sound.

Microplastics (MP, <5 mm) are found in coastal waters across various environmental compartments (biota, water, marine snow, sediment). The eastern oyster (Crassostrea virginica) is a commercially important species that ingests MP; however, oysters are discriminant suspension feeders that do not consume all particles to which they are exposed. This study explored the relationship between MP in oysters on a recreational oyster bed and the surrounding environmental compartments in Long Island Sound (LIS; USA). The quantity and types of MP in oysters, water, marine snow, and sediment samples were determined. Precautions were taken to minimize and monitor MP contamination in the field and laboratory to improve the quality of data collected. Microplastics were isolated from samples via chemical digestion, and any suspected particles were identified using micro-Fourier transform infrared spectroscopy. A total of 86 MP were identified out of 885 suspected particles across environmental media. The highest MP count in an individual oyster was nine, indicating low concentrations of MP in oysters and the surrounding environment. Few polymers, except polyethylene terephthalate, were shared between oysters and the surrounding environmental compartments. Sediments contained the highest number of MP across all environmental compartments (42 total). These data aid in determining the types of MP (polymer composition, shape, size) to which oysters are exposed and identified those ingested. The low numbers of MP recorded, coupled with the lack of alignment of polymers between oysters and their surrounding environment, demonstrates further that oysters are a poor bioindicator species for MP pollution.

Determining the Properties that Govern Selective Ingestion and Egestion of Microplastics by the Blue Mussel (Mytilus edulis) and Eastern Oyster (Crassostrea virginica).

Suspension feeding bivalve molluscs interact with different types of microplastics (MP) suspended in the water column. Most bivalves are selective suspension feeders and, thus, do not consume all particles to which they are exposed. Selection depends upon the physicochemical properties and size of the particle. Recent work has provided evidence that blue mussels, Mytilus edulis, and eastern oysters, Crassostrea virginica, ingest and egest microspheres (polystyrene) and microfibers (nylon) differently, but whether other factors, such as polymer type and shape, mediate selection have not been explored. To investigate these factors, mussels and oysters were offered similar sized nylon (Ny) and polyester (PES) microfibers or polyethylene (PE) and polystyrene (PS) microspheres, or different sized PES microfibers during a 2 h exposure. Feces and pseudofeces were collected separately and analyzed for MPs, and the data were used to develop a linear regression model for selection. Results demonstrated clear species-specific differences in the efficiency of particle selection. Both mussels and oysters, however, exhibited size-based rejection of PES microfibers, ingesting a higher proportion of shorter fibers than longer fibers. Polymer type did not impact selection of fibers or spheres. The relative size of particles (area and perimeter) was found to be the most important factor in predicting whether a MP will be rejected or ingested.

Direct Comparison of Fecal and Gut Microbiota in the Blue Mussel (Mytilus edulis) Discourages Fecal Sampling as a Proxy for Resident Gut Community.

Bivalves have ecological and economic importance but information regarding their associated microbiomes is lacking. As suspension feeders, bivalves capture and ingest a myriad of particles, and their digestive organs have a high throughput of particle-associated microbiota. To better understand the complement of transient and resident microbial communities, standard methods need to be developed. For example, fecal sampling could represent a convenient proxy for the gut microbiome and is simple, nondestructive, and allows for sampling of individuals through time. The goal of this study was to evaluate fecal sampling as a reliable proxy for gut microbiome assessment in the blue mussel (Mytilus edulis). Mussels were collected from the natural environment and placed into individual sterilized microcosms for 6 h to allow for fecal egestion. Feces and gut homogenates from the same individuals were sampled and subjected to 16S rRNA gene amplicon sequencing. Fecal communities of different mussels resembled each other but did not resemble gut communities. Fecal communities were significantly more diverse, in terms of amplicon sequence variant (ASV) richness and evenness, than gut communities. Results suggested a mostly transient nature for fecal microbiota. Nonetheless, mussels retained a distinct resident microbial community in their gut after fecal egestion that was dominated by ASVs belonging to Mycoplasma. The use of fecal sampling as a nondestructive substitute for direct sampling of the gut is strongly discouraged. Experiments that aim to study solely resident bivalve gut microbiota should employ an egestion period prior to gut sampling to allow time for voidance of transient microbes.

The interactive effects of titanium dioxide nanoparticles and light on heterotrophic bacteria and microalgae associated with marine aggregates in nearshore waters.

Titanium dioxide nanoparticles (n-TiO2) are emerging contaminants and the ecological impact of these materials to the nearshore environment is largely unknown. The reactivity of n-TiO2 increases with light exposure, and the photocatalytic effects have been shown on cultures of bacteria and microalgae in the laboratory. The purpose of this study was to assess the response of natural bacterial and microalgal communities associated with marine aggregates to n-TiO2 under conditions similar to those found in the photic zone of nearshore waters. Nano and bulk TiO2 particles were incorporated into marine aggregates over 4 days under two light conditions: 6:18 and 0:24 (hours light:dark). The abundance and metabolic response of heterotrophic bacteria and viability of microalgae associated with aggregates were assessed. Although the proportion of living microalgae was unchanged, the abundance, total metabolic activity and functional diversity of heterotrophic bacteria were significantly altered by irradiated n-TiO2.

Gut Microbiomes of the Eastern Oyster (Crassostrea virginica) and the Blue Mussel (Mytilus edulis): Temporal Variation and the Influence of Marine Aggregate-Associated Microbial Communities.

Gut microbial community structure was evaluated for two species of bivalve molluscs, the eastern oyster (Crassostrea virginica) and the blue mussel (Mytilus edulis) collected from Long Island Sound, Connecticut, over the course of a year. These bivalves utilize a shared feeding mechanism, which may result in similar gut microbial communities. Their particle diet, marine aggregates, and surrounding environment, aggregate-free seawater (AFSW), were also collected for comparison. Due to the suspension-feeding activities of bivalves, the potential for aggregate- and AFSW-associated microbiota to influence their microbial communities may be significant. Both taxonomic and functional diversity of the samples were assessed. 16S rRNA gene amplicon sequencing indicated that oysters and mussels maintained similar, but not identical, gut microbiomes, with some temporal variation. Throughout the year, bivalve species had gut microbial community compositions that were more similar to one another than to aggregates. Within a month, bivalves shared on average a quarter of their total operational taxonomic units (OTUs) with each other and a 10th of their total OTUs with aggregates. During months with warm water temperatures, individuals within each of the four sample types had similar alpha diversity, but again, temporal variation was observed. On a functional level, bivalve gut microbial communities exhibited variation attributed to host species and season. Unlike oysters, mussel gut bacterial communities maintained high richness and evenness values throughout the year, even when values for the particle diet and AFSW were reduced. Overall, a core gut bivalve microbiome was present, and it was partially influenced by the marine aggregate microbial community.IMPORTANCE This work investigates the influence that extrinsic factors, diet, and the environment can have on the microbiomes of shellfish. Over the course of a year, the gut microbial communities of two species of bivalves, oysters and mussels, held under identical conditions in coastal marine waters were compared. While the mussels and oysters harbored gut microbial communities with similar composition, on a functional level, they exhibited species and temporal variation. These results indicate that intrinsic factors influence the bivalve microbiome, resulting in species variability, even when environmental conditions, feeding mechanism, and particle diet are constant. Seasonal and multispecies comparisons for bivalve-associated microbial communities are rare, and we believe this research represents an important contribution. The results presented here advance our understanding of the symbiotic interactions between marine invertebrates, the microbial communities they harbor, and the environment.

Selective Ingestion and Egestion of Plastic Particles by the Blue Mussel (Mytilus edulis) and Eastern Oyster (Crassostrea virginica): Implications for Using Bivalves as Bioindicators of Microplastic Pollution.

Microplastics (MP; 1 µm to 1 mm) of various shapes and compositions are ingested by numerous marine animals. Recently, proposals have been made to adopt bivalve molluscs as bioindicators of MP pollution. To serve as indicators of MP pollution, however, the proposed organisms should ingest, without bias, the majority of plastic particles to which they are exposed. To test this premise, eastern oysters, Crassostrea virginica, and blue mussels, Mytilus edulis, were offered variously sized polystyrene microspheres (diameters 19-1000 µm) and nylon microfibers (lengths 75-1075 × diameter 30 µm), and the proportion of each rejected in pseudofeces and egested in feces was determined. For both species, the proportion of microspheres rejected increased from ca. 10-30% for the smallest spheres to 98% for the largest spheres. A higher proportion of the largest microsphere was rejected compared with the longest microfiber, but similar proportions of microfibers were ingested regardless of length. Differential egestion of MP also occurred. As a result of particle selection, the number and types of MP found in the bivalve gut will depend upon the physical characteristics of the particles. Thus, bivalves will be poor bioindicators of MP pollution in the environment, and it is advised that other marine species be explored.

Field-Based Evidence for Microplastic in Marine Aggregates and Mussels: Implications for Trophic Transfer.

Marine aggregates incorporate particles from the environment, including microplastic (MP). The characteristics of MP in aggregates and the role of aggregates in linking MP with marine organisms, however, are poorly understood. To address these issues, we collected aggregates and blue mussels, Mytulis edulis, at Avery Point, CT, and analyzed samples with microspectrometers. Results indicate that over 70% of aggregates sampled harbored MP (1290 ± 1510 particles/m3). Fifteen polymer types were identified, with polypropylene, polyester and synthetic-cellulose accounting for 44.7%, 21.2% and 10.6%, respectively, of the total MP count. Over 90% of MP in aggregates were ≤1000 µm, suggesting that aggregations are a sink for this size fraction. Although size, shape, and chemical type of MP captured by mussels were representative of those found in aggregates, differences in the sizes of MP in pseudofeces, feces and digestive gland/gut were found, suggesting size-dependent particle ingestion. Over 40% of the MP particles were either rejected in pseudofeces or egested in feces. Our results are the first to identify a connection between field-collected marine aggregates and bivalves, and indicate that aggregates may play an important role in removing MP from the ocean surface and facilitating their transfer to marine food webs.

Comparative toxicity of Corexit® 9500, oil, and a Corexit®/oil mixture on the eastern oyster, Crassostrea virginica (Gmelin).

Given their particle feeding behavior, sessile nature, and abundance in coastal zones, bivalves are at significant risk for exposure to oil and oil dispersant following environmental disasters like the Deepwater Horizon oil spill. However, the effects of oil combined with oil dispersants on the health of oysters are not well studied. Therefore, eastern oysters (Crassostrea virginica) were exposed in vivo to Corexit® 9500, crude oil (high-energy water accommodated fraction; HEWAF), and a Corexit®/oil mixture (chemically-enhanced water accommodated fraction; CEWAF) to evaluate potential toxic effects on immunological (phagocytosis and respiratory burst), physiological (feeding rate), and histological endpoints. Phagocytosis was significantly increased following CEWAF exposure only. Respiratory burst was significantly decreased following Corexit® exposure, but significantly increased following exposure to the highest concentration of CEWAF. Oyster feeding rates were significantly decreased following exposure to Corexit®, HEWAF, and CEWAF, and were most sensitive to CEWAF exposure. These modulations of important immunological and physiological functions could result in serious health outcomes for oysters, such as increased parasitism and decreased growth. Our experiments showed that subtle, sub-lethal effects occurred following acute in vivo exposure to Corexit®, HEWAF, and CEWAF, though oysters were not equally sensitive to the three components. Data from this study can be used for more accurate risk assessment concerning the impact of oil and Corexit® on the health of oysters.

Acute exposure to TiO2 nanoparticles produces minimal apparent effects on oyster, Crassostrea virginica (Gmelin), hemocytes.

The response of oyster (Crassostrea virginica) hemocytes was studied following exposure to anatase nanoparticles (ca. 7.4nm), surface-coated rutile nanocomposites (UV-Titan M212, ca. 86nm) and bulk titanium dioxide (TiO2) particles (anatase and rutile crystalline forms; 0.4-0.5µm). Hemocytes were collected from oysters and exposed to one of the four particle types at concentrations of 0.1, 0.5, and 1.0mg/L under dark and environmentally-relevant light conditions for periods of two and four hours. Hemocyte mortality, phagocytosis, and reactive oxygen species (ROS) production were then evaluated using flow-cytometric assays. Bulk and nanoparticulate TiO2 had little effect on viability of oyster hemocytes or on production of ROS. Significant changes in phagocytosis occurred after exposure to anatase nanoparticles for 4h under dark conditions, and UV-Titan for 2h under light conditions. Results demonstrate that TiO2 particles (bulk or nanoscale) produce minimal effects on hemocyte biomarkers examined following acute, in vitro exposures.

Photocatalytic effects of titanium dioxide nanoparticles on aquatic organisms-Current knowledge and suggestions for future research.

Nanoparticles are entering natural systems through product usage, industrial waste and post-consumer material degradation. As the production of nanoparticles is expected to increase in the next decade, so too are predicted environmental loads. Engineered metal-oxide nanomaterials, such as titanium dioxide, are known for their photocatalytic capabilities. When these nanoparticles are exposed to ultraviolet radiation in the environment, however, they can produce radicals that are harmful to aquatic organisms. There have been a number of studies that have reported the toxicity of titanium dioxide nanoparticles in the absence of light. An increasing number of studies are assessing the interactive effects of nanoparticles and ultraviolet light. However, most of these studies neglect environmentally-relevant experimental conditions. For example, researchers are using nanoparticle concentrations and light intensities that are too high for natural systems, and are ignoring water constituents that can alter the light field. The purpose of this review is to summarize the current knowledge of the photocatalytic effects of TiO2 nanoparticles on aquatic organisms, discuss the limitations of these studies, and outline environmentally-relevant factors that need to be considered in future experiments.

The contribution of marine aggregate-associated bacteria to the accumulation of pathogenic bacteria in oysters: an agent-based model.

Bivalves process large volumes of water, leading to their accumulation of bacteria, including potential human pathogens (e.g., vibrios). These bacteria are captured at low efficiencies when freely suspended in the water column, but they also attach to marine aggregates, which are captured with near 100% efficiency. For this reason, and because they are often enriched with heterotrophic bacteria, marine aggregates have been hypothesized to function as important transporters of bacteria into bivalves. The relative contribution of aggregates and unattached bacteria to the accumulation of these cells, however, is unknown. We developed an agent-based model to simulate accumulation of vibrio-type bacteria in oysters. Simulations were conducted over a realistic range of concentrations of bacteria and aggregates and incorporated the dependence of pseudofeces production on particulate matter. The model shows that the contribution of aggregate-attached bacteria depends strongly on the unattached bacteria, which form the colonization pool for aggregates and are directly captured by the simulated oysters. The concentration of aggregates is also important, but its effect depends on the concentration of unattached bacteria. At high bacterial concentrations, aggregates contribute the majority of bacteria in the oysters. At low concentrations of unattached bacteria, aggregates have a neutral or even a slightly negative effect on bacterial accumulation. These results provide the first evidence suggesting that the concentration of aggregates could influence uptake of pathogenic bacteria in bivalves and show that the tendency of a bacterial species to remain attached to aggregates is a key factor for understanding species-specific accumulation.

An examination of the factors influencing mercury and methylmercury particulate distributions, methylation and demethylation rates in laboratory-generated marine snow.

In the marine environment, settling particulates have been widely studied for their role as effective vertical transporters of nutrients and metals scavenged from the euphotic zone to the benthos. These particulates are composed of transparent exopolymers, plankton and bacterial cells, detritus and organic matter, and form various size fractions from colloids (<0.2µm) to aggregates, and finally marine snow (>300 µm). As marine snow forms in the water column, anoxic layers form around and within the aggregation potentially creating a prime environment for the methylation of mercury (Hg), which occurs primarily in low oxygen environments. To examine this process, marine aggregates were produced from sieved estuarine seawater (100 µm) in 1-L glass bottles spiked with stable isotope enriched methylmercury (CH3199Hg) and inorganic mercury (200Hg(II)) at 18° C using a roller-table. After the rolling period, different particle-size fractions were collected and analyzed, including: visible marine snow (>300µm), particulates 8 to 300 µm, and particulates 0.2 to 8µm. Particulate analysis indicated higher incorporation of both forms of Hg into marine snow compared to unrolled treatments, with greater incorporation of 200Hg(II) than CH3199Hg. In addition, inorganic Hg was methylated and CH3Hg was demethylated in the larger particulate fractions (>8µm). Methylation and demethylation rates were assessed based on changes in isotopic composition of Hg(II) and CH3Hg, and found to be comparable to methylation rates found in sediments. These results indicate that net Hg methylation can occur in marine snow and smaller aggregates in oxic coastal waters, and that this net formation of CH3Hg may be an important source of CH3Hg in both coastal and open ocean surface environments.

An examination of the ingestion, bioaccumulation, and depuration of titanium dioxide nanoparticles by the blue mussel (Mytilus edulis) and the eastern oyster (Crassostrea virginica).

The production rates of titanium dioxide (TiO2) nanoparticles for consumer products far exceed the pace at which research can determine the effects of these particles in the natural environment. Sedentary organisms such as suspension-feeding bivalves are particularly vulnerable to anthropogenic contaminants, such as nanoparticles, that enter coastal environments. The purpose of this work was to examine the ingestion, bioaccumulation, and depuration rates of TiO2 nanoparticles by two species of suspension-feeding bivalves, the blue mussel (Mytilus edulis) and the eastern oyster (Crassostrea virginica). Two representative TiO2 nanoparticles, UV-Titan M212 (Titan) and Aeroxide P25 (P25), were delivered to the animals either incorporated into marine snow or added directly to seawater at a concentration of 1.0 mg/L for exposure periods of 2 and 6 h. After feeding, the animals were transferred to filtered-seawater and allowed to depurate. Feces and tissues were collected at 0, 12, 24, 72, and 120 h, post-exposure, and analyzed for concentrations of titanium by inductively coupled plasma-mass spectrometry. Results indicated that the capture and ingestion (i.e., transfer to the gut) of TiO2 nanoparticles by both mussels and oysters was not dependent on the presence of marine snow, and weight-standardized clearance rates of bivalves exposed to TiO2 nanoparticles were not significantly different than those of unexposed control animals. Both species ingested about half of the nanoparticles to which they were exposed, and >90% of the nanoparticles were egested in feces within 12 h, post-exposure. The findings of this study demonstrate that mussels and oysters can readily ingest both Titan and P25 nanoparticles regardless of the form in which they are encountered, but depurate these materials over a short period of time. Importantly, bioaccumulation of Titan and P25 nanoparticles does not occur in mussels and oysters following exposures of up to 6 h.

Modulation of pumping rate by two species of marine bivalve molluscs in response to neurotransmitters: Comparison of in vitro and in vivo results.

Most studies regarding the neuroanatomy and neurophysiology of molluscan ctenidia have focused on isolated ctenidial tissue preparations. This study investigated how bivalve molluscs modulate their feeding rates by examining the effects of a variety of neurotransmitters, including serotonin, dopamine, and the dopamine agonist apomorphine on both isolated ctenidial tissue and in intact members of two commercially important bivalve species: the blue mussel, Mytilus edulis; and the bay scallop Argopecten irradians. In particular, we examined the effect of changes in: 1) beat of the lateral cilia (in vitro), 2) distance between ctenidial filaments and/or plicae (in vivo), and 3) diameter of the siphonal openings (in vivo) on alteration of bulk water flow through the mantle cavity. Important differences were found between isolated tissue and whole animals, and between species. Drugs that stimulated ciliary beat in vitro did not increase water processing rate in vivo. None of the treatments increased water flow through the mantle cavity of intact animals. Results suggest that A. irradians was primarily modulating lateral ciliary activity, while M. edulis appeared to have a number of ways to control water processing activity, signifying that the two species may have different compensatory and regulatory mechanisms controlling feeding activity.

False positives in Biolog EcoPlates™ and MT2 MicroPlates™ caused by calcium.

Biolog MicroPlates(TM) (e.g. EcoPlate(TM), MT2 MicroPlate(TM), GN MicroPlate(TM)) are useful tools for characterizing microbial communities, providing community-level physiological profiles to terrestrial and aquatic ecologists. The more recently designed Biolog EcoPlates have been used frequently in aquatic ecology with success. This study, however, reveals one major problem when using EcoPlates to evaluate samples within an estuarine or seawater matrix. At concentrations greater than 100 parts per million, the cation calcium begins to interfere with the microplate chemistry, causing false positive readings. Experiments, in which multiple treatments of natural and artificial seawater were tested, as well as calcium-addition experiments, demonstrate that calcium inhibits complete dissolution of the minimal growth medium in wells. Future studies involving Biolog EcoPlates and MicroPlates should take this effect into account, and the dilution of samples is strongly recommended to diminish the "calcium effect."

Early host-pathogen interactions in marine bivalves: evidence that the alveolate parasite Perkinsus marinus infects through the oyster mantle during rejection of pseudofeces.

Parasites have developed myriad strategies to reach and infect their specific hosts. One of the most common mechanisms for non-vector transmitted parasites to reach the internal host environment is by ingestion during feeding. In this study, we investigated the mechanisms of oyster host colonization by the alveolate Perkinsus marinus and focused on how oysters process infective waterborne P. marinus cells during feeding in order to determine the portal(s) of entry of this parasite to its host. We also compared the infectivity of freely-suspended cells of P. marinus with that of cells incorporated into marine aggregates to link changes in particle processing by the feeding organs with infection success and route. Finally, we evaluated the effect of oyster secretions (mucus) covering the feeding organs on P. marinus physiology because these host factors are involved in the processing of waterborne particles. The ensemble of results shows a unique mechanism for infection by which the parasite is mostly acquired during the feeding process, but not via ingestion. Rather, infection commonly occurs during the rejection of material as pseudofeces before reaching the mouth. The pseudofeces discharge area, a specialized area of the mantle where unwanted particles are accumulated for rejection as pseudofeces, showed significantly higher parasite loads than other host tissues including other parts of the mantle. Aggregated P. marinus cells caused significantly higher disease prevalence and infection intensities when compared to freely-suspended parasite cells. Mucus covering the mantle caused a quick and significant increase in parasite replication rates suggesting rapid impact on P. marinus physiology. A new model for P. marinus acquisition in oysters is proposed.

Role of epicellular molecules in the selection of particles by the blue mussel, Mytilus edulis.

This study provides evidence that the suspension-feeding blue mussel, Mytilus edulis, uses biochemical cues to recognize its food. We identified lectins in mucus from the gills and labial palps, two pallial organs involved in the feeding process. These compounds were able to agglutinate rabbit and horse erythrocytes (RBC) and several species of marine microalgae representing different families. Additionally, the agglutination of RBC and microalgae was inhibited by several carbohydrates (fetuin, lipopolysaccharide (LPS), and mannose-related residues), suggesting that a suite of lectins may be present in mucus from the gills and labial palps. Results from feeding experiments, using microspheres with tailored surfaces, demonstrated that mussels preferentially ingested microspheres coated with the neoglycoproteins glucosamide-BSA and mannopyranosylphenyl-BSA but rejected in pseudofeces microspheres coated with BSA alone. The positive selection for neoglycoprotein-coated microspheres was inhibited when mussels were pre-incubated in seawater containing a solution of the same neoglycoprotein. Two surface properties of the microspheres, charge and wettability, had little effect on the observed selection process. Our results, along with our previous findings for oysters, suggest a new concept for the mechanism of particle selection in bivalves and perhaps other suspension-feeding organisms. Specifically, the selection of particles involves interactions between epiparticulate carbohydrates and lectins in the mucus produced by feeding organs.

In vivo effects of brown tide on the feeding function of the gill of the northern quahog Mercenaria mercenaria (Bivalvia: Veneridae).

The in vivo response of adult northern quahogs, Mercenaria mercenaria, to Aureococcus anophagefferens (brown tide) at the level of the gill was determined using video-endoscopy. Feeding activity, particle-approach velocities, and ventral-groove-transport velocities were documented after the quahogs were exposed to Isochryis galbana (baseline observations) supplemented with either toxic or nontoxic A. anophagefferens at two bloom concentrations (8 x 10(5) or 2 x 10(6) cells ml(-1)). Externally, there was no evidence of adverse effects of brown tide on feeding, as siphons remained extended and dilated. Toxic brown tide at both concentrations elicited gill muscular contractions, intermittent cessation of water flow, and decreased particle loading within the pallial cavity. The 8 x 10(5) cell ml(-1) toxic treatment had no significant effect on approach velocities or ventral-groove-transport velocities after 2 h, although time-averaging showed significant reduction of the latter during the last 30 min of exposure. The higher concentration of toxic brown tide caused a significant decrease in these velocities after only 1 h. Nontoxic brown tide produced none of these effects. Thus, A. anophagefferens compromised quahog feeding by stimulating contractions of the branchial musculature and interfering with lateral and ventral groove ciliary beating. These effects were both time- and concentration-dependent and could be caused by either a dopaminergic or a serotonergic toxic factor.