Oyster Crassostrea Virginica Gmelin Research Articles

Spatiotemporal variability of pH in coastal waters of New Brunswick (Canada) and potential consequences for oyster aquaculture

There is a void in the knowledge of the acidification status of Eastern Canada's coastal waters. This knowledge is crucial to evaluating the threats posed to marine life, particularly oyster farming, a flagship of New Brunswick seafood production. In this study, we measured the temporal variability of pH and related environmental parameters in three bays of Northeastern New Brunswick. We also evaluated the potential impact of the observed pH levels on the Eastern oyster (Crassostrea virginica Gmelin, 1791), based on the available literature on this species’ sensitivity to acidification. We investigated the presence of inherent cycles of pH with the Fourier transform and the spectral filtering technique. Our results show that pH is highly variable in the studied area, with values ranging from 7.31 to 8.90. A seasonal effect was apparent, as the pH fluctuations were set at the lowest level in winter when the cover of ice and snow on the bay was present. The spectral analysis revealed a clear semidiurnal tidal pattern of pH, this variable being inversely related to the water level in summer and directly related to it in winter. The spectral subtraction of all the tidal components allowed the detection of a circadian rhythm that was not in pace with the alternation between day and night but rather slowly drifted so that the pH troughs occurred at night during the full moon period. Short periodicities of circa 8 and 6 h also existed in two of the three bays. Based on current knowledge of C. virginica’s sensitivity to acidification, this species’ recruitment, growth, and survival are unlikely to be impacted by the present pH levels in the studied area. However, further acidification might overcome the resilience of C. virginica, especially that of the larvae that are produced during the winter in commercial hatcheries.

Habitat characteristics of artificial oyster reefs influence female oystershell mud crab Panopeus simpsoni Rathbun, 1930 (Decapoda: Brachyura: Panopeidae)

Abstract Structurally complex biogenic habitats provide foraging grounds and predation refuges for a myriad of decapod crustaceans. Many of these habitats, such as reefs formed by the eastern oyster (Crassostrea virginica Gmelin, 1791), have been lost due to natural and anthropogenic reasons, leading to the construction of artificial reefs for habitat restoration. Previous studies have investigated the impacts of oyster reef restoration efforts on abundance patterns of decapod crustaceans, such as those of panopeid crabs, but largely ignored the influence of artificial, or created, habitat on other population characteristics such as reproductive output. We sampled five artificial reefs (constructed during 2014 or 2015) in Sweetwater Lake, Galveston Bay, Texas, USA in July-August 2016 to study the population characteristics of female panopeid mud crab Panopeus simpsoniRathbun, 1930. We quantified carapace width, counted eggs and measured dry egg mass to estimate reproductive output and determine how reef-age treatment, live-oyster abundance, and live-oyster size predicted female P. simpsoni abundance and egg production. Oyster size varied significantly (P < 0.001) by age treatment, and oyster abundance and size varied significantly (P < 0.001) between the reefs within each age treatment. The abundance of female P. simpsoni varied significantly (P < 0.001) between the two age treatments and was positively correlated with oyster size and reef age. The percentage of gravid females, which varied significantly (P = 0.027) among the reefs, was positively correlated with oyster abundance, and egg production was positively correlated with body size. These results suggest that oyster development on artificial reefs may influence the abundance and reproductive output of associated crustaceans such as P. simpsoni. We demonstrate that monitoring programs should not only consider population characteristics of reef-building organisms, but also the development of complex habitat structure and its impact on associated crustacean populations when determining the success of artificial habitats.

Confirmation of the shell-boring oyster parasite Polydora websteri (Polychaeta: Spionidae) in Washington State, USA

Invasions by shell-boring polychaetes such as Polydora websteri Hartman have resulted in the collapse of oyster aquaculture industries in Australia, New Zealand, and Hawaii. These worms burrow into bivalve shells, creating unsightly mud blisters that are unappealing to consumers and, when nicked during shucking, release mud and detritus that can foul oyster meats. Recent findings of mud blisters on the shells of Pacific oysters (Crassostrea gigas Thunberg) in Washington State suggest a new spionid polychaete outbreak. To determine the identity of the polychaete causing these blisters, we obtained Pacific oysters from two locations in Puget Sound and examined them for blisters and burrows caused by polychaete worms. Specimens were also obtained from eastern oysters (Crassostrea virginica Gmelin) collected in New York for morphological and molecular comparison. We compared polychaete morphology to original descriptions, extracted DNA and sequenced mitochondrial (cytochrome c oxidase I [mtCOI]) and nuclear (small subunit 18S rRNA [18S rRNA]) genes to determine a species-level molecular identification for these worms. Our data show that Polydora websteri are present in the mud blisters from oysters grown in Puget Sound, constituting the first confirmed record of this species in Washington State. The presence of this notorious invader could threaten the sustainability of oyster aquaculture in Washington, which currently produces more farmed bivalves than any other US state.

Microbial Inhibition by Bacteria Isolated from Pallial Cavity Fluids and Associated Mucus of the Eastern Oyster Crassostrea virginica (Gmelin)

Thirteen aerobic, mesophilic marine bacterial strains were isolated from the pallial (mantle) cavity fluids and associated mucus of the eastern oyster Crassostrea virginica Gmelin. All strains were Gram-negative and identified to genus using 16S RNA gene sequence analysis. Each isolated strain was tested for its ability to inhibit growth of six Gram-negative and five Gram-positive bacterial tester strains, as well as the yeast Candida albicans, using an in vitro agar diffusion screening method to detect antimicrobial activity (Romenenko et al. 2008). All of the marine bacteria isolated from the oyster pallial cavity fluids showed some ability to inhibit tester strains. Two isolates, F (red) and M, inhibited the greatest variety of indicator strains and produced the largest zones of inhibition. Gram-negative indicator strains were more susceptible to antimicrobial activity of pallial fluid isolates than Gram-positive strains. None of the isolated bacteria was shown to inhibit C. albicans. In addition, targeted 16S metagenome libraries from the Chesapeake Bay were screened for the presence of bacteria isolated from this study. Results indicate that there are commonalities between bacteria associated with oysters from Long Island Sound and the Chesapeake Bay systems.

Genetic signature analysis of Perkinsus marinus in Mexico suggests possible translocation from the Atlantic Ocean to the Pacific coast of Mexico

BackgroundThe protozoan Perkinsus marinus (Mackin, Owen & Collier) Levine, 1978 causes perkinsosis in the American oyster Crassostrea virginica Gmelin, 1791. This pathogen is present in cultured C. virginica from the Gulf of Mexico and has been reported recently in Saccostrea palmula (Carpenter, 1857), Crassostrea corteziensis (Hertlein, 1951) and Crassostrea gigas (Thunberg, 1793) from the Mexican Pacific coast. Transportation of fresh oysters for human consumption and repopulation could be implicated in the transmission and dissemination of this parasite across the Mexican Pacific coast. The aim of this study was two-fold. First, we evaluated the P. marinus infection parameters by PCR and RFTM (Ray’s fluid thioglycollate medium) in C. virginica from four major lagoons (Términos Lagoon, Campeche; Carmen-Pajonal-Machona Lagoon complex, Tabasco; Mandinga Lagoon, Veracruz; and La Pesca Lagoon, Tamaulipas) from the Gulf of Mexico. Secondly, we used DNA sequence analyses of the ribosomal non-transcribed spacer (rNTS) region of P. marinus to determine the possible translocation of this species from the Gulf of Mexico to the Mexican Pacific coast.ResultsPerkinsus marinus prevalence by PCR was 57.7% (338 out of 586 oysters) and 38.2% (224 out of 586 oysters) by RFTM. The highest prevalence was observed in the Carmen-Pajonal-Machona Lagoon complex in the state of Tabasco (73% by PCR and 58% by RFTM) and the estimated weighted prevalence (WP) was less than 1.0 in the four lagoons. Ten unique rDNA-NTS sequences of P. marinus [termed herein the “P. marinus (Pm) haplotype”] were identified in the Gulf of Mexico sample. They shared 96–100% similarity with 18 rDNA-NTS sequences from the GenBank database which were derived from 16 Mexican Pacific coast infections and two sequences from the USA. The phylogenetic tree and the haplotype network showed that the P. marinus rDNA-NTS sequences from Mexico were distant from the rDNA-NTS sequences of P. marinus reported from the USA. The ten rDNA-NTS sequences described herein were restricted to specific locations displaying different geographical connections within the Gulf of Mexico; the Carmen-Pajonal-Machona Pm1 haplotype from the state of Tabasco shared a cluster with P. marinus isolates reported from the Mexican Pacific coast.ConclusionsThe rDNA-NTS sequences of P. marinus from the state of Tabasco shared high similarity with the reference rDNA-NTS sequences from the Mexican Pacific coast. The high similarity suggests a transfer of oysters infected with P. marinus from the Mexican part of the Gulf of Mexico into the Mexican Pacific coast.

Oyster Utilization and Host Variation of the Oyster Pea Crab (Zaops ostreum)

ABSTRACT The oyster pea crab Zaops ostreum (Say, 1817), is a parasite of the eastern oyster Crassostrea virginica Gmelin (1791). This study examined the frequency of occurrence and impact of pea crabs on adult (50–70 mm shell height [SH]) oysters from natural intertidal reefs in Hewletts Creek, Wilmington, NC and artificially created intertidal reefs from Jones Island, NC. The reefs at Hewletts Creek and Jones Island were also sampled in the fall of 2012 to quantify the presence of pea crabs in two cooccurring bivalve species, the scorched mussel Brachidontes exustus (Linneaus, 1758), and the ribbed mussel Geukensia demissa (Dillwyn, 1817). To evaluate the impact of pea crabs on juvenile oysters (mean SH 11.07 ± 0.16 mm), oysters were deployed in Hewletts Creek at three densities during July 2013 and then retrieved in October 2013. The percent occurrence of pea crabs in adult oysters was low on the natural reefs (1%–3.7%), greater on the created reefs (10%), whereas the percent occurrence in the deployed ...

Improving management support tools for reintroducing bivalve species (eastern oyster [Crassostrea virginica Gmelin]) in urban estuaries.

Successful reintroduction of "ecologically extinct" bivalve species into anthropogenically impaired urban estuaries is problematic when employing existing management tools used in estuaries where bivalves are present (GIS-based restoration models, expanding existing shellfish beds, placement of shell substrate, physical oceanographic parameters). A significant management challenge is appropriate site selection. We are proposing the inclusion of a biological parameter (evaluation of tissue histopathology) in an inexpensive and rapid site selection model to inform management decision making and identify sites with the greatest potential for reintroduction success. Use of biological biomarkers is not a new concept, but it is important that they be included in a multitiered management approach to bivalve reintroduction. This Case Study tested adult Eastern Oysters (Crassostrea virginica Gmelin) from locations that supported comparable short-term survival rates by evaluating growth and tissue health and/or disease. Biomarkers indicated oyster tissues at one site were normal, the female:male sex ratio was 50:50, and female oysters were in spawning condition. Conversely, oyster tissues at the second site exhibited multiple abnormalities, samples were 100% male, and the incidence of disease was high. Using the biomarker tool, we evaluated 4 additional sites where oysters exhibited short-term (1 year) survival. At 2 locations, we observed chronic health impacts that would preclude reintroduction, including samples from one site where a wild population was surviving. We also analyzed tissue and shell heavy metal contents. Soft tissue metal concentrations in Meadowlands samples were at the high range of scientific literature values, averaging 1.1% of total body weight, whereas tissue metal concentrations at the Keyport site were within acceptable ranges. Although initial survival and growth rates at both locations were comparable, site-specific urban stressors reduced oyster fitness at 1 of the 2 locations. We are proposing an Estuarine Reintroduction Site Selection Model, which includes a biological in situ parameter, to increase the probability of successfully managing a sustainable oyster reintroduction before commencing expensive large-scale restoration activities.

The Effects of a Simulated Suspended Sediment Plume on Eastern Oyster (Crassostrea virginica) Survival, Growth, and Condition

Bottom sediments are resuspended into the water column during dredging operations. These resuspended sediments are an often cited concern used to justify restrictions applied to dredging schedules in many areas of the USA. One example of a temporal restriction, commonly referred to as an environmental window, involves dredging schedules in the James River, Virginia, because of potential impacts on the eastern oyster (Crassostrea virginica Gmelin). Yet, effects’ data are lacking to understand the effects of suspended sediments to C. virginica. To address this data gap, we performed a laboratory study mimicking sediment resuspension during annual dredging operations in the James River. Field-collected oysters were exposed for 7 days under flow-through conditions to 0, 100, 250, and 500-mg/L total suspended solids (TSS) in a unique exposure system where oyster movements could be electronically monitored. Endpoints analyzed were survival, percent of time open, total number of shell movements, weight change, and condition index. Data indicated no significant effects of suspended sediment on these endpoints after 7 days of exposure. Weight change in oysters attached vertically to monitor their movements was significantly less than in oysters not monitored in every treatment. No significant differences in condition index, an indicator of oyster growth sensitive to environmental pollutants, were observed among treatments measured 30 days postexposure. Correlations performed for each treatment among monitored oyster endpoints found significant negative associations between number of movements and percent open in the 100, 250, and 500-mg/L TSS treatments and in all treatments combined. These data will help reduce the uncertainty surrounding the effects of suspended sediments on C. virginica.

Spatial genetic features of eastern oysters (Crassostrea virginica Gmelin) in the Gulf of Mexico: northward movement of a secondary contact zone.

The eastern oyster (Crassostrea virginica Gmelin) is an economically and ecologically valuable marine bivalve occurring in the Gulf of Mexico. This study builds upon previous research that identified two divergent populations of eastern oysters in the western Gulf of Mexico. Allelic and genotypic patterns from 11 microsatellite markers were used to assess genetic structure and migration between the previously described oyster populations in Texas. The main findings are as follows: (1) there are two distinct populations (FST = 0.392, P < 0.001) of oysters that overlap in the Corpus Christi/Aransas Bay estuarine complex in Texas, (2) the distribution of genotypes among individuals in the contact zone suggests limited hybridization between populations, (3) the variables of salinity, temperature, dissolved oxygen, turbidity and depth are not correlated with allele frequencies on reefs in the contact zone or when analyzed across Texas, and (4) there is little evidence of directional selection acting on the loci assayed here, although patterns at four markers suggested the influence of balancing selection based on outlier analyses. These results are consistent with long-term historical isolation between populations, followed by secondary contact. Recent hydrological changes in the area of secondary contact may be promoting migration in areas that were previously inhospitable to eastern oysters, and observed differences in the timing of spawning may limit hybridization between populations. Comparison of these findings with the results of an earlier study of oysters in Texas suggests that the secondary contact zone has shifted approximately 27 km north, in as little as a 23-year span.

Effects of Substrate Type on Demographic Rates of Eastern Oyster (Crassostrea virginica)

ABSTRACT Several restored oyster (Crassostrea virginica Gmelin) reefs in Pamlico Sound, North Carolina, have experienced recent population crashes, potentially caused in part by clionid boring sponge infestation of the marl rock (a calcium carbonatemud composite material) commonly used as a reef substrate and of the shells of oysters that colonize the marl reef foundation. The composition and porosity of marl may make it vulnerable to infestation by carbonate bioeroders, particularly clionid sponges. The objective of this study was to quantify the effects of carbonate (CaCO3; oyster shell, marl) versus noncarbonate (non-CaCO3; granite, concrete) reef-building substrates on C. virginica demographic rates, including oyster density and oyster growth, on experimental reefs constructed along the salinity gradients of two estuaries in coastal North Carolina. There were no differences in oyster density among substrate types through the first 6 mo of reef sampling, although at 12 mo after reef construction, diffe...

Inducible defenses in the eastern oyster Crassostrea virginica Gmelin in response to the presence of the predatory oyster drill Urosalpinx cinerea Say in Long Island Sound

The response of the eastern oyster C. virginica to the presence of the oyster drill Urosalpinx cinerea was examined from July to September 2011. Several aspects of oyster growth were measured, including wet weight, shell weight, and dorsal shell area for oysters collected near Groton, Connecticut (41.32036 N, −72.06330 W). Wet weight and shell weight growth were significantly higher in the presence of the predator U. cinerea, while tissue weight showed no difference from the control. The control group showed more shell area growth and a much lower ratio of shell weight growth to shell area growth. Differences in shell weight to area ratio indicated that C. virginica dramatically shifted from lateral shell growth to shell thickening in the presence of U. cinerea. This inducible defense has not been previously shown for C. virginica and could play an important role in the predator–prey interaction between these two species.

Oyster Shell Dissolution Rates in Estuarine Waters: Effects of pH and Shell Legacy

ABSTRACT Oyster shell is a crucial component of healthy oyster reefs. Shell planting has been a main component of oyster restoration efforts in many habitats and has been carried out on scales from individual and grassroots efforts to multiagency efforts across entire estuaries. However, the cycling and lifetime of the shell that makes up the bulk of an oyster reef has only recently received attention, and most of the work to date has focused on the role of epi- and endobionts on shell degradation. Here we report findings from a laboratory study in which we manipulated pH in a flow-through control system using water from the mesohaline mouth of the Patuxent River to measure dissolution rates of intact oyster shell. Shells from the Eastern oyster (Crassostrea virginica Gmelin 1791) with three different legacies were exposed to 4 levels of pH that encompass a range typical of the mesohaline waters of the Chesapeake Bay (∼7.2–7.9 on the NBS scale). Mass loss over a 2-wk period was used to measure dissolution...

Toxin profile change in vegetative cells and pellicle cysts of Alexandrium fundyense after gut passage in the eastern oyster Crassostrea virginica

Vegetative cells and pellicle cysts of the toxic dinoflagellate Alexandrium fundyense Balech were fed to the eastern oyster Crassostrea virginica Gmelin under controlled conditions. Para - lytic shellfish toxins (PSTs) were measured in vegetative cells and pellicle cysts prior to feeding and directly after passage through the oyster alimentary canal and defecation as intact cells. Oysters fed with vegetative cells and those fed with pellicle cysts accumulated toxins. One experimental treat- ment tested for direct uptake of toxins from the water (oysters and A. fundyense cells were separated by a screen); PSTs were not accumulated from the water by the oysters. There were no significant changes in total, per-cell toxicity after passage through the oyster alimentary canal, suggesting lim- ited transfer of toxins from intact cells to the oysters. However, there were statistically significant changes in the toxin composition of cells following gut passage. Vegetative cells and pellicle cysts from feces had increased amounts of saxitoxin (STX) and decreased amounts of gonyautoxin 4 (GTX4) per cell, compared to amounts prior to gut passage. Following gut passage, pellicle cysts showed better survival in the feces than vegetative cells, which is consistent with the view of pellicle- cyst formation as a successful survival strategy against adverse conditions.

Interactive effects of cadmium and hypoxia on metabolic responses and bacterial loads of eastern oysters Crassostrea virginica Gmelin

Pollution by toxic metals including cadmium (Cd) and hypoxia are important stressors in estuaries and coastal waters which may interactively affect sessile benthic organisms, such as oysters. We studied metabolic responses to prolonged hypoxic acclimation (2 weeks at 5% O 2) in control and Cd-exposed (30 d at 50 μg L −1 Cd) oysters Crassostrea virginica, and analyzed the effects of these stressors on abundance of Vibrio spp. in oysters. Hypoxia-acclimated oysters retained normal standard metabolic rates (SMR) at 5% O 2, in contrast to a decline of SMR observed during acute hypoxia. However, oysters spent more time actively ventilating in hypoxia than normoxia resulting in enhanced Cd uptake and 2.7-fold higher tissue Cd burdens in hypoxia. Cd exposure led to a significant decrease in tissue glycogen stores, increase in free glucose levels and elevated activity of glycolytic enzymes (hexokinase and aldolase) indicating a greater dependence on carbohydrate catabolism. A compensatory increase in activities of two key mitochondrial enzymes (citrate synthase and cytochrome c oxidase) was found during prolonged hypoxia in control oysters but suppressed in Cd-exposed ones. Cd exposure also resulted in a significant increase in abundance of Vibrio parahaemolyticus and Vibrio vulnificus levels during normoxia and hypoxia, respectively. Overall, Cd- and hypoxia-induced changes in metabolic profile, Cd accumulation and bacterial flora of oysters indicate that these stressors can synergistically impact energy homeostasis, performance and survival of oysters in polluted estuaries and have significant consequences for transfer of Cd and bacterial pathogens to the higher levels of the food chain.

Non-lethal determination of sex and reproductive condition of Eastern oysters Crassostrea virginica Gmelin using protein profiles of hemolymph by Proteinchip® and SELDI-TOF-MS technology

As the demand for hatchery-produced oyster seed increases for both aquaculture and restoration purposes, techniques to improve seed production, such as accurate determination of broodstock oyster sex and gonad development, have become more important. In this study, we developed a novel method of determining oyster sex and developmental stage through protein profiling of hemolymph using a relatively new proteomic tool, Proteinchip® and SELDI-TOF-MS. Over 139 peptides/proteins were detected from oyster hemolymph, 62 of which appeared to be involved in reproductive activities. Using the protein-profile information, individual broodstock oysters were categorized successfully into one of five groups: undifferentiated, female developing, female ripe, male developing and male ripe. The accuracy of categorization, confirmed by subsequent, traditional histological methods, was 98.8% ( p < 0.05). Wide application of this method is still limited by cost; however, results of this research open doors for further study to develop more-affordable and portable methods based upon detection of specific hemolymph peptides and proteins.

Development and validation of single nucleotide polymorphism markers in the eastern oyster Crassostrea virginica Gmelin by mining ESTs and resequencing

Single nucleotide polymorphisms (SNPs) are useful markers for genetic and genomic studies. They are needed in the eastern oyster ( Crassostrea virginica), a major aquaculture species, for the mapping of genes affecting economically important quantitative traits such as disease resistance. Here we report the discovery and characterization of 46 SNPs in the eastern oyster by mining expressed sequence tags (ESTs) and resequencing. Sixty-three putative SNPs were identified from existing ESTs, of which 27 were validated. Three hundred and thirty-six putative SNPs were discovered by resequencing 6.8 Kb from 19 genes, corresponding to a remarkably high density of one SNP per 20 bp. Twenty-one of the candidate SNPs identified by resequencing were tested, and 19 were validated. Thirty-three of the 46 validated SNPs segregated in two mapping families, with 28 (85%) in accordance to expected Mendelian ratios. These markers are derived from expressed genes and should be useful for genetic analyses including the mapping and characterization of the genes that they represent.

Change in lipid composition in eastern oyster ( Crassostrea virginica Gmelin) exposed to constant or fluctuating temperature regimes

A temperature decrease usually induces an ordering effect in membrane phospholipids that can lead to membrane dysfunction. Ectotherms typically counteract this temperature effect by remodeling membrane lipids as stipulated in the homeoviscous adaptation theory (HVA). Previous studies mostly focused on the remodeling of membrane lipids during long-term acclimatization or acclimation at constant temperature regimes, whereas in nature, many organisms experience variations in temperature on a daily basis and must react to this changing thermal environment. The objective of this study was to examine the composition of membrane lipids in oysters subjected to long-term acclimation at constant temperatures (12 or 25 °C) or to environmentally realistic daily fluctuations in temperature between 12 and 25 °C for 7 d. The lipid composition of gill in oysters subjected to long-term acclimation at a constant temperature or to daily temperature fluctuations varied in a way consistent with HVA: oysters adjusted their phospholipid to sterol ratio in response to long-term acclimation to a constant temperature but not to daily temperature fluctuations. In contrast, the unsaturation index of polar lipids in oysters varied in response to both long-term acclimation to a constant temperature and to daily temperature fluctuations, mainly due to changes in 22:6n-3 and 20:5n-3. The 20:4n-6 levels in oyster gills increased as temperature rose, suggesting an increasing availability of this fatty acid for eicosanoid biosynthesis during stress responses.

Assessment of genetic diversity of the American oyster (Crassostrea virginica) in Veracruz, México using microsatellite markers

Abstract Genetic variation was evaluated in populations of the eastern oyster Crassostrea virginica (Gmelin) from the coast of Veracruz, eastern Gulf of Mexico. We sampled six lagoons and analyzed variation at five microsatellite loci. Significant Hardy-Weinberg deviations occurred at all loci and were attributed to the presence of null alleles. We found no isolation by distance among the populations in these lagoons, but significant heterogeneity was observed among some adjacent lagoons, possibly reflections of geographical factors and local reductions in population size. Certain extreme north and south localities were not genetically different in terms of nonsignificant pairwise FST values. Gene flow is attributed to seasonal shifts in coastal currents, larval production throughout the year, human interference with the natural lagoon processes, and restocking efforts. The observed pattern of variation could be another example of chaotic genetic patchiness in marine organisms.

Identification and mapping of disease-resistance QTLs in the eastern oyster, Crassostrea virginica Gmelin

Identification and mapping of disease-resistance QTLs (quantitative trait loci) is important for our understanding of genetic mechanisms of disease-resistance and for our ability to genetically improve cultured stocks. Disease-resistance is the most important trait for farmers of the eastern oyster ( Crassostrea virginica Gmelin), which is affected by two major diseases: MSX and Dermo. In this study, the genome of the eastern oyster was scanned with a large number of amplified fragment length polymorphism (AFLP) markers before and after Dermo-inflicted mortalities (53% and 67%) in two reference families. Significant post-mortality shifts in genotype frequency were detected at a large number of loci. Linkage analysis revealed that most markers showing frequency shifts are closely linked to each other on the genetic map, and all markers within a cluster had frequency shifts in the same direction according to their linkage phase. This finding suggests that post-mortality shifts in genotype frequency were not random, but linked to Dermo/summer mortality-resistance QTLs. Twelve putative Dermo/summer mortality-resistance QTLs were identified on female and male maps from two families, providing candidate genome regions for further analysis.

Tissue-specific accumulation of cadmium in subcellular compartments of eastern oysters Crassostrea virginica Gmelin (Bivalvia: Ostreidae)

Cadmium distribution was studied in different subcellular fractions of gill and hepatopancreas tissues of eastern oysters Crassostrea virginica. Oysters were exposed for up to 21 days to low sublethal Cd concentrations (25 μg L −1). Gill and hepatopancreas tissues were sampled and divided into organelle fractions and cytosol by differential centrifugation. Organelle content of different fractions was verified by activities of marker enzymes, citrate synthase and acid phosphatase for mitochondria and lysosomes, respectively. In both tissue types, there was a significant accumulation of cadmium in cytosol reaching 230–350 ng mg −1 protein. Among organelles, mitochondria were the main target for Cd bioaccumulation in gills (250–300 ng mg −1 protein), whereas in hepatopancreas tissues, the highest cadmium accumulation occurred in lysosomes (90–94 ng mg −1 protein). Although 75–83% of total cadmium burden was associated with the cytosol reflecting high volume fraction of this compartment, Cd concentrations in organelle fractions reached levels that could cause dysfunction of mitochondria and lysosomes. Organ- and organelle-specific patterns of cadmium bioaccumulation support our previous in vivo studies, which showed adverse effects of cadmium exposures on mitochondrial oxidation in gills and on the lysosomal system of hepatopancreas. This may have important implications for the development of biomarkers of effect for heavy metals and for understanding the mechanisms of toxic effects of metals.