Abundance of microplastics at and near a shellfish aquaculture farm: An eastern oyster (Crassostrea virginica) transplant study.

Microplastics (MP) have repeatedly been found in commercially cultured species of bivalves. There are concerns regarding the amount of MP released into the environment by aquaculture activities, and questions regarding possible higher MP loads in farm-grown shellfish compared to levels in shellfish collected from recreational beds. To explore this concept, seawater, aquaculture gear, and eastern oysters (Crassostrea virginica) were sampled from an aquaculture site in Niantic Bay, CT, USA, and a 2-week transplantation experiment was performed in which oysters were transplanted between the aquaculture site and a plastic-free cage off the dock at the University of Connecticut-Avery Point campus. The digestive gland-stomach complex (gut) was dissected from the oysters and MP were extracted from the adjacent seawater and oyster gut samples using previously validated extraction methods. Extensive quality assurance and control measures were taken to reduce MP contamination. Particles in all samples were isolated, imaged under a stereomicroscope, and characterized (size, shape, polymer) using ImageJ software and micro-Fourier transform infrared spectroscopy. Water samples contained 0-0.3 MP/L and oyster gut samples contained 0-1.3 MP/g wet weight indicating very low concentrations of MP at the farm (0-2 MP/individual) or away from the farm (0-3 MP/individual). Aquaculture gear in this area is not contributing to MP ingestion in farmed oysters or elevated MP levels in the surrounding water.

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.

Differences in Specific Mass Density Between Dinoflagellate Life Stages and Relevance to Accumulation by Hydrodynamic Processes.

One previously unstudied aspect of differences between sexual and asexual life stages in large-scale transport and accumulation is density (mass per unit volume) of cells in each life stage. The specific density was determined for Scrippsiella lachrymosa cells in medium with and without nitrogen (N) enrichment through density-gradient centrifugation. Growth medium without N addition is often called "encystment medium" when used for the purpose of resting cyst formation in cyst-forming dinoflagellates; mating gametes are usually seen after 2-3 days. Significant differences in specific density were found after 2 days in encystment medium simultaneously with the observation of typical gamete swimming behavior and mating. The specific density of cells in encystment medium was 1.06 g · cm-3 ; whereas, the specific density of cells in growth medium was 1.11 g · cm-3 . Cells in encystment medium were found to have significantly increased lipid content, reduced chlorophyll content, and reduced internal complexity. The findings may explain differential transport of less dense and chemotactically aggregating gametes into surface blooms in contrast to denser vegetative cells that perform daily vertical migration and do not aggregate. Passive accumulation of non-migrating gametes into layers in stagnant water also can be explained, as well as sinking of zygotes when the storage of highly dense starch increases. Resting cysts had a density of over 1.14 g · cm-3 and would sink to become part of the silt fraction of the sediment. We suggest that differences in behavior and buoyancy between sexual and asexual life stages cause differences in cell accumulation, and therefore large-scale, environmental transport could be directly dependent upon life-cycle transitions.

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.

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.