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Suspension-feeding bivalve molluscs perform important ecological roles by coupling pelagic and benthic systems during their feeding activities. Particle capture, and thus feeding, is dependent on particle encounter and retention on the gill filaments, with several factors influencing this process. Over the past 30 yr, different types of synthetic microspheres have been used to examine aspects of particle capture and ingestion by bivalves. Critics of this work have posited that manufactured particles may contain surfactants, chemicals commonly used in manufacturing to reduce surface tension, that could produce spurious capture and ingestion rates. The goal of this work was to experimentally assess whether the presence of different types of surfactants on manufactured polystyrene particles can result in instantaneous effects on particle capture by the blue mussel Mytilus edulis. The effects of 3 different types of common surfactants (sodium dodecyl sulfate, benzalkonium chloride, Triton-X) on clearance rates (CR) and capture efficiencies (CE) were tested. Results indicated that none of the surfactant treatments had an effect on CR. Treatment with one of the surfactants (Triton-X) significantly lowered CE for 3 µm sized spheres compared to the control spheres (Milli-Q treated). None of the other tested surfactants significantly affected CE when compared to the control treatment. These data add to an understanding of particle handling by bivalves, and suggest that concentrations of surfactants found on commercially available microspheres used for experiments or found in the environment have little immediate effect on feeding processes.

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.

Rapid improvements in the technology used to assess microbial communities have led to an expansion of the breadth and scope of microbial ecology research over the past 20 y. The rapid increase in microbiome research, spurred by nextgeneration amplicon sequencing, has allowed researchers to characterize the microbial communities of organisms and environments which were previously poorly understood. Research on marine invertebrates is still developing, with the mechanisms that determine interactions between the host and their associated microbes largely unknown. Bivalves are of particular interest because of their crucial ecological and economic roles. With the anticipation of shifting environmental conditions, defining the natural variation and role of bivalve-associated microbes is vital to more complex research questions. The current knowledge of the spatial and temporal distributions of bivalve-associated microbes, microbial functional and genetic diversity, host-specific and tissue-specific interactions, and core microbial community composition is summarized here, as well as the beginning stages of understanding the role of microbiome in bivalve physiology and disease susceptibility.

Historically, particle capture efficiency (CE) in suspension-feeding bivalve molluscs has been shown to be strongly dependent on particle size, increasing asymptotically to a maximum of about 100% for particles ca. ≥ 4 μm in diameter. Recent advances in the analysis of the particulate matter of seston have allowed for more precise studies of bivalve feeding under natural conditions. Some studies have reported that the mechanisms associated with particle capture exhibit physiological plasticity, and under certain conditions smaller cells and particles are captured in preference to larger ones. For bivalves, however, there is no mechanistic explanation that would account for such fine-scale control of CE based on size. The current study experimentally assessed the seasonal control of CE by the blue mussel, Mytilus edulis , employing a flow-through system to examine particle capture of natural seston. The natural particle field was analyzed using two different types of particle analyzers, the LISST-100X and the Coulter Multisizer IIe. Mussels were simultaneously delivered synthetic microspheres of defined diameter (2–45 μm) to control for the effects of seasonal differences in the size and shape of natural particles. The capture of microspheres was quantified by means of flow cytometry (FCM), and results cross-checked with the Multisizer. Additionally, gene expression of a mucosal lectin (MeML) associated with the feeding organs of mussels was examined as a biomarker for physiological response to seasonal changes in the particle-food supply. Results demonstrated that for microspheres ≥ 4 μm CE of mussels was always near 100%, and did not change seasonally. In contrast, there was an apparent seasonal shift in CE of natural particles, with particles 17-to-35 μm in equivalent spherical diameter (ESD) occasionally being captured at lower efficiencies than particles 4-to-15 μm in ESD (e.g., during September and December). No relationship between MeML expression and seasonal CE was found. These findings call into question the physiological plasticity of CE in mussels and alternative hypotheses are presented. We suggest that the purported changes in CE are not a consequence of behavioral or physiological responses of mussels, but rather a result of one or more of the following confounding factors; 1) instrument artifacts that can arise as a result of the way in which laser and electronic particle counters calculate ESD to estimate particle size; 2) disaggregation of flocculent material collected from control chambers; 3) post-capture escape of highly motile phytoplankton cells from the infrabranchial camber; 4) qualitative factors of the particles that could affect capture; or 5) mathematical happenstance of calculating CE on particle-size classes that contain widely different numbers of particles.

The microbial communities and overall health of the eastern oyster, Crassostrea virginica, have long been topics of interest due to the fundamental economic and ecological roles this species maintains. A broad scale characterization of the oyster microbiome over spatial and seasonal scales, however, has never been carried out. The primary goal of this study was to examine the factors mediating microbial communities of the gut and pallial fluid of C. virginica at three sites within the Long Island Sound estuary, with a focus on both genetic structure (T-RFLP) and physiological profiling (EcoPlates) of the microbiome. Results indicated that the genetic structure of microbial communities of oysters was minimally separated across sites, but was influenced by season. Although the microbial community structure was similar, the number of carbon sources utilized by these communities (richness) varied across site, season, and anatomical location within the host. Parameters including oyster condition index, Dermo disease, and ambient water temperature were measured to assess their influence on the oyster microbiome. Only water temperature was found to have a significant relationship with microbial community structure and richness. Results suggest that a core microbiome may exist within the eastern oyster, specifically for those populations that are not genetically distinct.

article i nfo Article history: The mechanisms by which bivalve molluscs differentiate among particles are not clearly understood. Recent evidence suggests that particle selection by bivalves can be mediated by interactions between carbohydrates attached to a particle's surface and lectins present in the mucus covering the feeding organs. The physico- chemical surface properties of particles have also been proposed as factors contributing to particle selection. In this study, we examined the effects of surface charge and wettability of 10-μm spheres on particle selection by two species of suspension-feeding bivalves: eastern oysters (Crassostrea virginica) and blue mussels (Mytilus edulis). Microspheres were delivered to bivalves in particle-selection assays, and the proportions of spheres rejected as pseudofeces and egested as feces were determined using flow cytometry. Results sug- gest that when given a choice, both mussels and oysters rejected some types of microspheres (e.g., aluminum oxide) and ingested other types (e.g., polystyrene). In some assays, microspheres with very different surface charges or wettabilities were handled similarly, indicating that neither property alone was a qualifier for selection. The differences in surface properties between pairs of microspheres were also considered and used as variables in discriminant analyses. For oysters and mussels, the generated models explained ca. 25% and 72% of the variation in the data, respectively. In both models, wettability was more important than charge in classifying data into their correct group (rejected, preferentially ingested, or not selected). Results from this study support the idea that non-specific physicochemical interactions can play a role in mediating selection in suspension-feeding bivalves.

While many coastal ecosystems previously supported dense meadows of seagrass and dense stocks of bivalves, the impacts of overfishing, eutrophication, harmful algal blooms, and habitat loss have contributed to the decline of these important resources. Anthropogenic nutrient loading and subsequent eutrophication has been identified by some researchers as a primary driver of these losses, but others have described potential positive effects of eutrophication on some estuarine resources. The Peconic Estuary, Long Island, NY, USA, offers a naturally occurring nutrient-loading gradient from eutrophic tidal creeks in its western reaches to mesotrophic bays in the eastern region. Over 2 years, we conducted an experiment across this gradient to examine the effects of eutrophication on the growth of estuarine species, including juvenile bivalves (northern quahogs (Mercenaria mercenaria), eastern oysters, (Crassostrea virginica), and bay scallops (Argopecten irradians)) and slipper limpet (Crepidula fornicata). Water quality and phytoplankton community biomass and composition were concurrently monitored at each site, and the effects of these variables on the growth of estuarine species were analyzed with multiple regression model. Eutrophication seemed to impact shellfish through changes in the quality of food and not the quantity since the growth rates of shellfish were more often correlated with densities of specific cell types or quality of seston rather than bulk measures of phytoplankton and organic seston. Northern quahogs and eastern oysters grew maximally within eutrophic locales, and their growth was positively correlated with high densities of autotrophic nanoflagellates and centric diatoms in these regions (p

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.

This study provides evidence that the suspen- sion-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 fami- lies. Additionally, the agglutination of RBC and microalgae was inhibited by several carbohydrates (fetuin, lipopolysac- charide (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 mus- sels preferentially ingested microspheres coated with the neoglycoproteins glucosamide-BSA and mannopyranosyl- phenyl-BSA but rejected in pseudofeces microspheres coated with BSA alone. The positive selection for neogly- coprotein-coated microspheres was inhibited when mussels were pre-incubated in seawater containing a solution of the same neoglycoprotein. Two surface properties of the micro- spheres, 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 selec- tion of particles involves interactions between epiparticulate carbohydrates and lectins in the mucus produced by feeding organs.

The enzyme-labeled fluorescence (ELF-97®) substrate produces an insoluble, fluorescent yellow-green product at the site of alkaline-phosphatase (AP) activity. Fifteen coastal phytoplankton species were tested to determine if AP activity could be detected in phosphate- depleted media. All species tested, except Synechococcus bacillaris, expressed AP activity. Subsequently, threshold concentrations of soluble reactive phosphate (SRP) at which AP activity could be detected by ELF-97® were deter- mined for Chaetoceros neogracile, Chlorella autotrophica, Isochrysis sp., Prorocentrum minimum ,a ndTetraselmis chui. Microalgal species differed significantly in the SRP con- centration at which AP activity was first detectable (10.1- 16.4 µM), well above concentrations normally considered limiting for phytoplankton. P. minimum began to express AP activity at a higher SRP concentration than the other algal species; this may be attributable to a relatively high DNA/cell ratio in P. minimum, compared to the other phytoplankton. Thus, phytoplankton species may respond to phosphorus deficiency at high SRP concentrations.

Oysters accumulate relatively low levels of domoic acid (DA) compared to other bivalves. Mafra et al. (2009, in this Theme Section) identified feeding mechanisms of oysters that may lead to low DA accumulation during monospecific blooms of Pseudo-nitzschia multiseries. However, several differ- ent species of Pseudo-nitzschia, as well as other diatoms and flagellates, may co-occur during a bloom. Therefore, the present study investigates pre-ingestive feeding processes that operate when oysters Crassostrea virginica are exposed to mixed phytoplankton assemblages containing P. multiseries of varying cell length. Guided by video-endoscopy, material transported along the ventral and dorsal gill tracts was sampled and analyzed to determine the site for sorting of microalgae on the pallial organs. There was no preferential rejection of P. multiseries in pseudofeces when oysters were exposed to the alga in a mixed suspension with other diatom species (Thalassiosira weissflogii or Chaetoceros muelleri). In contrast, P. multiseries was preferentially rejected when mixed with the flagellates Iso- chrysis galbana or Rhodomonas lens, suggesting a qualitative mechanism for particle sorting. This occurred on the gills, followed by further selection on the palps. Oysters also preferentially rejected larger P. multiseries cells (82 to 90 µm) relative to smaller ones (24 to 28 µm) on the gills, while no fur- ther selection based on size occurred on the palps. This effect is attributed to the fact that P. multiseries cells with a length that exceeds the width of the principal filament aperture (ca. 68 µm) are more likely directed to the ventral tract and rejected in pseudofeces. These findings offer an additional explanation for the relatively low DA levels found in oysters during natural Pseudo-nitzschia spp. blooms.

Many studies on the biology of bivalves have focused on identifying the importance of various environmental factors in regulating feeding behaviour in order to predict ingestion and growth responses in a variety of habitats. When taking into consideration the diversity of studies undertaken and the variety of methodologies employed, it is perhaps not surprising that there have been inconsistencies in both interspecific and intraspecific comparisons. In order to compare and contrast the way in which feeding activities of a mussel species ( Mytilus edulis ) and a scallop species ( Placopecten magellanicus ) respond to environmental factors and seston characteristics, we exposed both species simultaneously to a fluctuating assemblage of natural particles found in their local environment. Similar sizes of mussels (40–73 mm) and scallops (40–88 mm) were held in flow through chambers at two different sites in Atlantic Canada on four separate occasions, during which environmental conditions, including seston characteristics, were monitored and individual clearance rates were measured. Because the same individuals were measured over time, repeated-measures statistical procedures were used to analyze the data. Results indicate that weight standardized clearance rates of mussels were significantly higher than those of scallops only at the highest concentration tested (12.8 mg l − 1 ), which translates into almost 3 times more material being filtered. Scallops appeared to respond to fluctuations in environment parameters and suspended food particles more consistently than mussels. These data suggest that the feeding behaviour of these two species is mediated differently by the same environmental parameters. Our study also demonstrates the importance of selecting appropriate statistical tests (e.g., repeated measures) for the analyses of data obtained from repeated sampling of the same individuals over time, and the need to consider species-specific feeding responses when developing models that predict the influence of bivalve feeding on ecosystem function.

Transparent exopolymer particles (TEP) are known to form from dissolved precursors released by phytoplankton and bacteria. Many benthic suspension feeders also have been shown to produce significant amounts of TEP under field and laboratory conditions. Although TEP from phyto- plankton have been implicated in enhancing aggregation and sedimentation of suspended material, the role of TEP from suspension feeders in aggregate formation is still unknown. In order to deter- mine if TEP produced by suspension feeders enhance the formation of marine aggregates, organic matter from actively feeding blue mussels Mytilus edulis and sea vases Ciona intestinalis held in recirculating chambers was collected. Control water was collected from chambers without animals. Animal-conditioned and control water were then mixed with 1 of 2 non-sticky particles (silica-amino beads or diatomaceous earth). The suspensions were transferred to 250 ml bottles, which were placed on a roller table to generate aggregates. Particle aggregation was quantified by examining the size distribution of particles over time by means of an electronic particle counter (particles between 6 and 90 μm), and by counting large (greater than ca. 100 μm) aggregates under a microscope at the end of the experiment. We found that significantly more aggregates formed in the animal-conditioned water than in control water after 48 h, for both species and both kinds of added particles. The number of large aggregates that formed was related to TEP concentration, and not related to total or dissolved organic carbon. Our results suggest that suspension feeders produce significant quantities of TEP that can enhance particle aggregation.

Suspension-feeding molluscs are important members of coastal communities and a large body of literature focuses on their feeding processes, including the efficiency of particle capture. Some molluscs, such as bivalves, capture individual picoplankton cells (0.2–2.0 μm) with a retention efficiency of less than 50%, leading to the assumption that such particles are not an important food resource. Picoplankton, however, are often concentrated in particle aggregates of much larger size. This study investigates the ability of suspension feeders to ingest picoplankton-size particles (0.2–2.0 μm) bound in marine aggregates. We fed clams (Mercenaria mercenaria), mussels (Mytilus edulis), oysters (Crassostrea virginica), scallops (Argopecten irradians) and slipper snails (Crepidula fornicata) 1.0- and 0.5-μm fluorescent particles (either polystyrene beads or bacteria) that were (1) dispersed in seawater, or (2) embedded within laboratory-made aggregates. Dispersed 10-μm beads were also delivered so that feeding activity could be determined. Ingested fluorescent particles were recovered in feces or isolated digestive glands and quantified. Results indicate that aggregates significantly enhance the ingestion of 1.0- and 0.5-μm beads by all species of bivalves, and enhance the ingestion of bacteria (greatest cell dimension ca. 0.6 μm) by all suspension feeders examined. Differences among species in their ability to ingest aggregates and picoplankton-size particles, however, were evident. Compared to mussels and clams, scallops and oysters ingested fewer aggregates with 1.0-μm beads or bacteria, and slipper snails ingested the most dispersed beads and bacteria. These differences may be a consequence of variations in gill structure and mechanisms of particle processing. Our data demonstrate that suspension feeders can ingest picoplankton-size particles that are embedded within aggregates, and suggest that such constituent particles may be an important food resource.

Marine aggregates were evaluated for their potential role in the ecology of aquatic pathogens using underwater video surveys coupled with direct collection of aggregates in modified settling cones. Six locations, two each in New York, Connecticut, and Massachusetts, were surveyed over 8 months to explore differences in the characteristics of aggregates found in habitats populated by clams (Mercenaria mercenaria) and oysters (Cras- sostrea virginica). Microaggregate ( 500 lm) concentrations, but peak concentrations of macroaggregates and microaggregates, mean size of particles, and volume fraction of aggregated material varied among the six shallow-water habitats. Concentrations (colony- forming units per ml) of total heterotrophic bacteria (THB) and total mesophilic pathogenic bacteria (MPB) from samples of aggregates were significantly different among the four locations bordering Long Island Sound (LIS). The highest concentrations and enrichment factors in aggregates were observed in August for THB and in June for MPB. Significant correlations were detected for salinity and the concentrations and enrichment factors of THB in aggregates and for the concentrations and percentages of MPB in seawater samples. Significant correlations were also detected for temperature and the concentrations of MPB in aggregates and the enrichment factors for THB and MPB (marginal significance). Bacterial species identified in association with aggregates included: Vibrio cholerea, V. parahaemolyticus, V. vulnificus, V. alginolyticus, Aeromonas hydrophila, Pseudomonas aeruginosa, Escherichia coli, and Mycobacteria sp. These results have important implications for the way in which aquatic pathogens are collected, quantified, and monitored for risk-based surveillance in shallow-water ecosystems.

The economically important marine bivalve mollusc, Mercenaria mercenaria, (commonly called a northern quahog or hard clam), has endured considerable mortalities caused by a thraustochytrid pathogen called Quahog Parasite X (QPX). Data on the percent prevalence of QPX infections were compiled from published reports along with our data to describe the epizootiology of QPX disease. QPX infections occurred in clams collected from both cultured beds and wild populations, but a higher percentage of QPX cases (76.5%) were from cultured clam beds. In addition, samples from cultured beds had a significantly higher prevalence (29.2 ± 27.2%) of QPX infections compared with samples from wild populations (9.6 ± 9.6%). The highest prevalence of QPX infections occurred in clams from samples with an intermediate size range (shell lengths 20-55 mm). QPX infections occurred in both male and female clams, but infection prevalence does not appear to be correlated with sex or sex ratios. The geographical range of QPX-related clam mortalities was Atlantic Canada to the Eastern Shore of Virginia, USA. Only marginally significant differences were detected between the prevalence of QPX at different locations. There were no latitudinal gradients in QPX prevalence or frequencies, suggesting local factors were important in determining its distribution. Although QPX infections occurred throughout the year, no seasonal trends in the prevalence or frequencies of QPX were discernable. This summary of information available on QPX disease highlights the need for more thorough data collection regarding factors believed to be associated with its presence and severity in hard clams.

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.

We evaluated marine aggregates as environmental reservoirs for a thraustochytrid pathogen, Quahog Parasite Unknown (QPX), of the northern quahog or hard clam, Mercenaria mercenaria. Positive results from in situ hybridization and denaturing gradient gel electrophoresis confirm the presence of QPX in marine aggregates collected from coastal embayments in Cape Cod, Massachusetts, where QPX outbreaks have occurred. In laboratory experiments, aggregates were observed and recorded by entering a quahogÂs pallial cavity, thereby delivering embedded particles from the water column to its benthic bivalve host. The occurrence of pathogen-laden aggregates in coastal areas experiencing repeated disease outbreaks suggests a means for the spread and survival of pathogens between epidemics and provides a specific target for environmental monitoring of those pathogens.

Transparent exopolymer particles (TEP) play an important role in aggregation, phyto- plankton bloom dynamics and the fate of organic carbon. Various types of plankton are known to release exopolymers that contribute to the TEP pool. Little is known, however, about other potential sources of TEP and TEP precursors in the oceans, especially in nearshore waters. Suspension-feeding bivalves can form dense assemblages in coastal waters and can process large volumes of water dur- ing feeding activities (e.g. 5 l h -1 g -1 dry wt). They secrete mucus to maintain clean body surfaces and to aid in capturing and transporting food material. We hypothesized that mucopolysaccharides, which are released from bivalves in dissolved or particulate form, contribute to the TEP pool. To test this hypothesis, laboratory experiments were carried out to measure the concentration of TEP and DOC in chambers with actively feeding oysters Crassostrea virginica over a 9 h period. Field experi- ments were conducted to measure TEP and DOC production by oysters in situ, using clear Plexiglas chambers on an intertidal sand-flat in New Brunswick, Canada. TEP concentrations were determined with an Alcian blue staining technique and spectrophotometric detection. DOC concentrations were determined using high-temperature catalytic oxidation (HTCO) with a platinum catalyst. Laboratory results indicated that oysters significantly enhanced TEP and DOC concentrations compared to con- trol levels over 9 h. Likewise, field experiments demonstrated elevated TEP concentrations in exper- imental chambers compared to control treatments. Our results support the hypothesis that oysters contribute to the production of TEP in coastal marine ecosystems.

Particle feeding on suspended and deposited material is a common mode of food collection among many groups of the Metazoa. In particular, many members of the Mollusca, most notably the bivalves, have specialized in particle feeding. Because suspension- and deposit-feeding activities often play significant roles in ecosystem processes, particular attention has been given to the mechanisms of particle feeding by these molluscs, and to the impacts of environmental factors upon these mechanisms. Early studies of the feeding organs of bivalve molluscs were descriptive and emphasis soon was placed upon the actual uptake of particulate material from the environment and identification of this material. Introduction of electronic particle counters led to a better understanding of the rates and efficiencies at which particle removal occurred, and the interface between food availability in the ocean and responses of these animals to temporal and spatial variability. While some early workers inferred the capabilities of these animals to select particles, it was the introduction of more advanced technologies (e.g., flow cytometry, video endoscopy, confocal microscopy) that allowed more detailed studies of the mechanisms associated with particle uptake and selection by these animals. These techniques have provided a more comprehensive analysis which clearly demonstrate that the mechanisms associated with particle feeding and selection are complex, with species-specific processes based upon both physical and chemical characteristics of the particles. In this review, we summarize prior research on particle selection in suspension- and deposit-feeding bivalve molluscs, discuss current theory and controversy regarding these processes, and propose areas for further study. In particular, more research is needed to elucidate the mechanisms responsible for particle selection, particulate characteristics that impact selection, and the intrinsic and extrinsic factors that mediate these processes.

Attempts have been made, especially in Asia, to displace harmful algal blooms (HABs) by spraying ‘china clay’ and ‘loess’ over affected coastal waters. The impact of this technique on benthic communities and processes is not known. We examined the effects of yellow loess on clearance rates of the benthic, filter-feeding invertebrates, Crassostrea virginica (Gmelin 1791), C. gigas (Thunberg 1793), Mytilus edulis (Linnaeus 1758), M. trossulus (Gould 1850), Argopecten irradians (Lamarck 1819), and Crepidula fornicata (Linnaeus 1758). An Obelia species of hydroid was also studied and the percent time open was analysed. Depletion rates were measured using a range of loess concentrations suspended in culture with unicellular algae (Rhodomonas lens) in 0.45 μm filtered seawater. The effects of loess on clearance rates and behaviour were species-specific. C. virginica was not impacted until clay concentrations reached 1.0 g L−1, while A. irradians showed a significant decrease at 0.01 g L−1. M. edulis showed a significant decrease in clearance rates at the 1 and 10 g L−1 concentrations. For hydroids, the percent time open was significantly lower than the control at 0.01, 0.1, and 10 g L−1. We clearly demonstrate that loess has a significant negative impact on filter-feeding invertebrates. The use of clay as a strategy for mitigation of HABs should be approached with extreme caution. While the control of active blooms may eventually be possible, it may not necessarily be an environmentally advisable or responsible approach to dealing with HABs.

Suspension-feeding bivalve molluscs can assume a large ecological role by linking benthic and pelagic ecosystems. Therefore, a knowledge of the factors that influence feeding rates and processes at the level of the individual is important in understanding bivalve-dominated environments. We examined the roles of diet quality and concentration on particle processing by the ctenidia of four species of bivalves: the mussels Mytilus edulis L. and Mytilus trossulus Gould, and oysters Crassostrea virginica (Gmelin) and Crassostrea gigas (Thunberg). Bivalves were delivered diets of three different qualities at three different concentrations (1–2×103, 104, 105 particles ml−1). The high-quality diet consisted of the cryptophyte Rhodomonas lens Pascher et Ruttner; the low-quality diet consisted of particles prepared from ground Spartina sp. detritus; the medium-quality diet consisted of a 50:50 mixture (by particle number) of both particle types. Particle transport velocities on the ventral groove and dorsal tracts were then measured by means of video endoscopy. Ventral-groove transport velocities of M. edulis were the most responsive, demonstrating a significant increase with increasing diet quality, and a significant decrease with increasing diet concentration. Transport velocities in the ventral groove and dorsal tracts of C. virginica were not significantly affected by changes in diet quality, but significantly increased with increasing diet concentration. Transport velocities of M. trossulus and C. gigas demonstrated little change with diet quality or concentration, indicating that responses are species specific. Our data suggest that differential control of particle transport on the bivalve ctenidium is one of the underlying mechanisms that contribute to compensatory feeding responses exhibited by the entire organism.

The technique of endoscopic video observation was used to study feeding processes of Placopecten magellanicus (Gmelin), collected from Bull Arm, Newfoundland in August 1991 and 1992, under near-natural feeding conditions. The fate of captured particles depended on the extent of ingestive or handling capacity saturation. Under low (1 to 10 particles μl-1) to medium (10 to 20 particles μl-1) particle concentrations, most particles were incorporated in continuous anteriorly directed slurries in the dorsal ciliated tracts of the gill arch and dorsal bends. As particle concentration or exposure time to the lower particle concentrations increased, four endogenous mechanisms of ingestion volume control were increasingly observed: (1) rejection of dense mucus-particle masses from the principal filament troughs onto the ventrally beating cilia and associated currents of the ordinary filament plicae, counter to and below the incoming pallial current maintained by the principal filament cilia; (2) intermittent stopping of the anteriorward flow in the dorsal ciliated tracts; (3) reduction or cessation of input from the principal filaments to the dorsal ciliated tracts; (4) detachment of the dorsal bends from the mantle to establish a shunt from the infrabranchial to the suprabranchial cavity. Chemical and histochemical tests of purified fluid withdrawn from the dorsal ciliated tracts indicate that mucus is present at all particle concentrations. Mucus therefore participates both in normal feeding and in ingestion volume control on the bivalve gill, although different mechanisms, and types of mucus, effect transport of material in the dorsal (feeding) and ventral (cleaning) ciliated tracts.

As the application of nanomaterials to science and technology grows, the need to understand any ecotoxicological effects becomes increasingly important. Recent studies on a few species of fishes and invertebrates have provided data which suggest that harmful effects are possible. The way in which nanoparticles are taken up by aquatic organisms, however, has been little studied. We examined uptake of nanoparticles by two species of suspension-feeding bivalves (mussels, Mytilus edulis; oysters, Crassostrea virginica), which capture individual particles1 microm with a retention efficiency of15%. Given this limitation, it would appear that nanoparticles could not be ingested in large numbers. During certain times of the year, however,70% of suspended particles are incorporated within aggregates that are100 microm in size. Therefore, we delivered bivalves fluorescently labeled, 100-nm polystyrene beads that were either (1) dispersed or (2) embedded within aggregates generated in the laboratory. Results indicate that aggregates significantly enhance the uptake of 100-nm particles. Nanoparticles had a longer gut retention time than 10-microm polystyrene beads suggesting that nanoparticles were transported to the digestive gland. Our data suggest a mechanism for significant nanoparticle ingestion, and have implications for toxicological effects and transfer of nanomaterials to higher trophic levels.

The benthic suspension feeding ascidian, Halocynthia pyriformis (Rathke, 1806), is often exposed to high concentrations of resuspended sediment in the Bay of Fundy. Resuspended sediment can change diet quantity and quality that may alter the ascidian's ability to feed and gain energy. The feeding activity of H. pyriformis exposed to bottom sediment was examined using standard physiological techniques and video endoscopy. Ascidians were exposed to natural seston plus additions of bottom sediment ranging in concentration from 0 to 46 mg l(-1). For each sediment concentration, clearance rate, ingestion rate, and retention efficiency of the ascidians was estimated using flow-through feeding chambers. Samples of suspended particles and feces were collected to estimate absorption efficiency and absorption rate. Results indicate that with increasing sediment concentration, ingestion rate increased to a constant level, absorption rate increased linearly despite a logarithmic decrease in absorption efficiency, and the retention of small particles (2-5mgr;m) increased while retention of larger particles (5-15mgr;m) decreased. As sediment concentration increased, squirting frequency increased and diameter of the siphon was reduced. Endoscopic observation of feeding structures and processes and the measurement of particle velocity was performed on ascidians exposed to 0 and 10 mg l(-1) of bottom sediment. An increase in squirting frequency at the high concentration facilitated the rejection of unwanted material and altered the structure and transport velocity of mucus. Mucus velocity was five times slower at 10 mg l(-1) than at 0 mg l(-1), however, the overall distance of mucus travel and the probability of clogging was reduced at 10 mg l(-1). H. pyriformis appears to compensate for episodic changes in the quantity and quality of available food particles by altering siphon-opening diameter, squirting frequency, structure and transport of mucus, and retention efficiency to maintain constant clearance rates.

Many species of microalgae release considerable amounts of their photoassimilated carbon into the water. This article reviews the impact of these released metabolites on the behavior and physiology of phytophagous suspension-feeding invertebrates (metazoans). The discussion is divided between planktonic (larvae and adult) and benthic suspension-feeders. Emphasis is placed on marine interactions, factors influencing microalgal metabolite production and methods of assessing their influence on suspension-feeders.

Bivalve molluscs form dense populations that exert profound effects on the particle loads and phytoplankton composition of coastal waters1. It has long been known that bivalves can select among different particle types, including selecting against those of poor nutritional value2,3,4,5, but because of difficulties in observing particle transport processes in the pallial cavity in vivo, the mechanism of selection was not known. We now use a combination of video endoscopy6 and flow cytometry7 to show that oysters can select living particles from non-living detritus on the gills. Our methods could aid the study of suspension feeding in many animal groups.

Previous models of particle feeding have focused on optimal solutions for particle acquisition or absorption. We propose two conceptual approaches to treat particle feeders as an integrated system of compartments, in hopes of understanding critical limiting factors that might be overlooked by focusing on only one part. The compartment model treats a particle feeder as a series of structures that process particles, with characteristic residence times within compartments and transfer points between them. These might change with overall particle food value and proportion of poor particles. As a non-exclusive alternative, the pathway model considers particle transfer as being analogous to enzyme control systems, with feedback loops that may involve interactions such as negative feedback between compartments that engage in no direct transfer. We examine these models in the light of some studies of particle handling by the deposit-feeding bivalves Yoldia limatula, Macoma secta, and M. nasuta, and the suspension-feeding oyster Crassostrea gigas. In Y. limatula, palp overloading results in feedback that shuts down the particle-collecting palp proboscis. In Macoma, nearly all particles are rejected, suggesting that rejection is necessary because digestion and gut residence time are limiting factors. We suggest that a whole-system approach is important in understanding particle processing by deposit feeders and suspension feeders. Additional key words: feeding, deposit feeding, suspension feeding, symmorphosis The purpose of this paper is to provide a conceptual framework for analyzing the processing of food particles by bivalve molluscs. We model the component organ systems as a series of compartments with transfer points between them, but with potential feedback loops among all potential compartments. Because the various compartments-ctenidium, palp, gut, and their components-have differing structures and functions, mechanisms are necessary for transfers of particles between them, e.g., the mucus string that transports particles between the ctenidium and palp in suspension feeders. Owing to disparate evolutionary origins and inherent structural difficulties, the supply of particles from one compartment to the next might cause overloading or rates of particle supply from one compartment that are below the capacity of another. In other words, the particle-processing system found in bivalve molluscs and other organisms might not function optimally with fixed transfer rates. Alternatively, the system might consist of a series of components whose individual processing rates are fixed and perfectly adapted to each other, much as has been suggested in arguments for other complex systems (Taylor & Weibel 1981; Weibel et al. 1991). We will first present a general compartment model for examining particle feeders such as bivalve molluscs. We describe compartments specifically for a selected group of bivalve species and show how problems ranging from phylogeny to the study of particle transfer between compartment structures can be studied. We will also use another conceptual model, derived from enzyme pathway analysis, to point out how interactions among the compartments can be considered. Finally, we will present some data on two species of deposit-feeding bivalves and some preliminary data on a suspension feeder to demonstrate how this approach allows a sharpening of our understanding of particle feeding and processing. This content downloaded from 207.46.13.52 on Mon, 24 Oct 2016 04:13:08 UTC All use subject to http://about.jstor.org/terms Models of particle processing Bivalve particle feeding Benthic particle feeders are typically divided into deposit feeders and suspension feeders. These represent a continuum rather than two discrete modes, although specific mechanisms of collection and processing may differ. Within the superfamily Tellinacea, for example, we encounter a range of morphologies, from those best suited for suspension feeding to those more efficient at deposit feeding (Yonge 1949; Pohlo 1966, 1967). Many particle-feeding species, including bivalves, switch from deposit to suspension feeding, depending upon substratum and hydrodynamic conditions (Taghon et al. 1980; Dauer 1983; Miller 1984; Olafsson 1986; Miller et al. 1992). The food source for both feeding types, however, is often quite similar, as it may consist of material resupplied by storm and other resuspension events (Miller et al. 1984; Frechette et al. 1989; Judge et al. 1993). In both bivalve feeding types, particles are collected actively, followed by one or more particle-processing stages, which usually involve particle rejection by the ctenidium and palp before particles enter the gut. Some food particles then pass directly through a tubular gut while others may be shunted to a diverticular structure where complete intracellular digestion occurs. Benthic particle feeders can be examined with regard to two principal problems. How do they deal with (1) variation of particle load (volume of particles supplied per unit time), and (2) varying proportions of particles of differing quality? Both deposit feeders and suspension feeders are confronted with widely varying particle loads and ranges of quality. Deposit feeders deal with high loads of particles that are mostly indigestible and that may require rejection before appropriate particles enter the gut (Lopez & Levinton 1987). They deal with this problem partly by non-random search and particle-collection strategies (Taghon et al. 1978; Levinton 1980; Whitlatch & Weinberg 1982), by variation of ingestion rate when local patches of high quality particles are encountered (Taghon 1981; Taghon & Jumars 1984), and by rejection of low quality particles (Hylleberg & Gallucci 1975; Taghon 1982). The bivalve Macoma nasuta typically rejects over 95% of the material that enters its incurrent siphon (Hylleberg & Gallucci 1975). By contrast, some sea cucumbers feed nearly indiscriminately, and digestive processing must be a major consideration in maximizing energy uptake (Powell 1977; Sloan & von Bodungen 1980). Suspension feeders also deal with a wide variety of particle loads and with particles of varying quality. At extreme loading, suspension feeders may adjust pumping rates, but bivalve molluscs also collect particles on the ctenidium and then reject those that are non-nutritive in the form of pseudofeces (Ki0rboe et al. 1980). There is growing evidence for qualitative selection (Newell & Jordan 1983; Ward & Targett 1989; MacDonald & Ward 1994) even of similar-sized organic particles (Shumway et al. 1985). Ingestion rates and other feeding processes appear to respond to changing content of non-nutritive particles (Bayne et al. 1989; Iglesias et al. 1992). Depending upon the degree of non-nutritive particle loading, cockles may respond by altering rates of particle uptake and the degree of rejection of particles as pseudofeces (Iglesias et al. 1992). In sum, particle feeders appear to respond in manifold and complex ways to the two cardinal problems outlined above. Previous conceptual approaches Three distinct but certainly not contradictory approaches have been applied to the modeling of particle processing, as well as other functional problems. The first approach involves models that place constraints upon feeding. Certain features of bivalve feeding have been argued to have characteristic hydrodynamic properties, and several authors have devised models that limit bivalves to certain physical mechanisms of particle processing, such as mechanical transport (Whitlatch & Weinberg 1982; J0rgensen 1989). More general models of particle feeding predict that certain particle sizes are collected with less efficiency than others, owing to hydrodynamic constraints (Rubenstein & Koehl 1977; Shimeta 1993). Processing rates also may be fixed, which could lead to characteristic transfer rates and loadings in a particle-processing chain. The second approach involves development of compensation models of feeding. Compensation is a change in behavior (e.g., rate of water transport through the siphon) following a change in feeding conditions (e.g., change in particle concentration in the water) that increases the rate of food uptake by the animal over the rate that would be found if no compensation occurred. For example, some studies report changes in rate of inflow that compensate for changes in phytoplankton concentration (Bayne et al. 1988). These models implicitly assume an array of optimal solutions corresponding to changing conditions and that bivalves can adjust at least in the direction of these optimal states. Finally, optimal models are more explicit renditions of compensation models. Such models take a series of boundary conditions and predict an optimal solution to foraging or digestion. For example, Penry & Jumars (1987) argued that during deposit feeding, particles 233 This content downloaded from 207.46.13.52 on Mon, 24 Oct 2016 04:13:08 UTC All use subject to http://about.jstor.org/terms Levinton, Ward, & Thompson Fig. 1. Conceptualization of a bivalve mollusc as a system of compartments with transfer between

Boonea (= Odostomia ) impressa (Say) is an ectoparasitic gastropod commonly found on the American oyster Crassostrea virginica (Gmelin). Field and laboratory studies were conducted to quantify effects of the ectoparasite on the physiology of oysters. After 2 wk of parasitism, three and six Boonea impressa per oyster significantly reduced both organic and inorganic growth and survival of juvenile Crassostrea virginica under laboratory conditions. Furthermore, six Boonea impressa per oyster affected feeding behavior of Crassostrea virginica by significantly reducing filtration rates of juvenile oysters, and significantly increasing the number of valve adductions of adult oysters. Filtration rates returned to normal within 2 h after Boonea impressa were removed. Previous workers have considered adverse effects of parasitism by B. impressa only in terms of removal of nutrients from the host oyster. In this study reduction in growth of parasitized oysters could not be wholly explained by the metabolic energy requirements of the ectoparasites; we suggest that interference with the feeding behavior of oysters has a significant effect on growth and survival.

The brown tide bloom, which occurred in coastal embayments of Long Island (NY), Rhode Island, and New Jersey, during the summers of 1985 and 1986 (Carpenter and Cosper, 1986; Olsen, 1986; Sieburth et al., 1986), caused starvation and recruitment failure of bivalve populations (Bricelj and Siddall, 1986; Cosper et al., 1987; Tracey, 1988). Several mechanisms for these deleterious effects were postulated including harmful effects due to released metabolites from the brown tide alga, Aureococcus anophagefferens Hargraves et Sieburth (Cosper et al., 1987; Sieburth et al., 1988; Tracey et al., 1988).