Asian Oyster Research Articles

Eleven novel microsatellite markers for the Asian oyster Crassostrea ariakensis

Clones from four partial genomic libraries of the Asian oyster Crassostrea ariakensis were screened to reveal 64 microsatellite-containing sequences. Polymerase chain reaction primers were designed for 34 of these loci. Reactions were optimized for 11 primer pairs and markers were evaluated in eight family crosses and one wild population from northern China. Zero to three null alleles per locus were identified in family crosses, and two of the 11 loci showed allele frequency deviations from Hardy-Weinberg equilibrium in the wild population. These markers have proven useful for genetic studies of C. ariakensis.

Differential Production of Feces and Pseudofeces by the Oyster Crassostrea ariakensis When Exposed to Diets Containing Harmful Dinoflagellate and Raphidophyte Species

The Asian oyster, Crassostrea ariakensis, is being evaluated for its potential success in the restoration of oyster populations in Chesapeake Bay. Critical to an understanding of its potential success is knowledge of the impacts of common harmful algae in its diet; blooms of such algae are common in Chesapeake Bay. In these experiments, C. ariakensis were exposed to a standard algal diet, Isochrysis sp., alone, and in combination with the harmful dinoflagallates Prorocentrum minimum and Karlodinium veneficum and the raphidophytes Heterosigma akashiwo and Chattonella subsalsa. Two experiments were run, with varying proportions of Isochrysis to the test algal species. Feces and pseudofeces were examined microscopically and by high- performance liquid chromatography for changes in diagnostic pigments relative to the initial diet and for production of degradation pigments of chlorophyll. All species were cleared from suspension to varying degrees by the oysters. In the Isochrysis control and in the Isochrysis + P. minimum treatment, intact, solid feces were produced, and the highest proportion of chlorophyll degradation pigments were found in the Isochrysis control diet suggesting algal digestion. Thin, ''ropey'' feces and pseudo-feces were observed in the K. veneficum + Isochrysis treatment. Virtually no degradation pigments were found for oysters fed a diet containing K. veneficum, suggesting lack of digestion and assimilation of algal food. With the raphidophytes, reduced production of feces and pseudofeces was evident, and these contained a lower proportion of recognizable harmful algal cells, and higher proportion of degradation pigments than found with the other test species. Amorphous and cellular material that appears to be partly derived from the oyster digestive system was evident in the feces of oysters exposed to K. veneficum, H. akashiwo, and C. subsalsa; this was particularly pronounced in oysters exposed to H. akashiwo and suggests damage to the oyster gut. The impact of the presence of harmful algae in the diet of the oysters thus varied by algal species, but in all cases oyster digestion was altered relative to the control diet.

Estimation of Annual Mortality Rates for Eastern Oysters (Crassostrea virginica) in Chesapeake Bay Based on Box Counts and Application of Those Rates to Project Population Growth of C. virginica and C. ariakensis

In an effort to restore the ecological role of oysters in Chesapeake Bay and the economic benefits of a commercial fishery, the states of Maryland and Virginia are considering introducing the nonnative Asian oyster (Crassostrea ariakensis) into the Bay. As part of an ecological risk assessment (ERA) to evaluate the proposed action and alternatives, demographic modeling is being used to project the change in populations of both the Asian and the native eastern oyster (C. virginica) in the Bay across space and time. Annual mortality rates are vital input to the demographic model. We present two approaches for parameterizing mortality rates for C. virginica by salinity ranges and disease-intensity categories and discuss how these rates could be applied to project population growth for the Asian oyster. We estimated mortality rates from empirical data collected during annual dredge surveys of oyster beds in Maryland. We compared counts of recent boxes (dead oysters without fouling or sedimentation on the inner valve surfaces, including “gapers” of one or two weeks old with tissue remaining in the shell), old boxes (dead oysters without tissue remnants but with fouling, sedimentation or both on the inner valve surfaces), and live oysters in market-size and small classes. Our mortality estimates based on counts of recent boxes consistently differentiated between years with high disease intensity and those with low disease intensity, between wet and dry years, and between salinity zones. In contrast, traditional estimates of yearly mortality based on total box counts often were out of phase with measured levels of disease intensity and weather (dry or wet). To model populations of C. ariakensis, we propose to adjust the mortality rates for C. virginica based on research results that provide estimates of differences between the two species' resistance to MSX and dermo and to other mortality factors, such as predation.

Effects of Salinity on Bonamia sp. Survival in the Asian Oyster Crassostrea ariakensis

Abstract A novel Bonamia sp. discovered in Bogue Sound, NC, has recently emerged as a parasitic threat to the Asian oyster Crassostrea ariakensis. Because this oyster is being considered for introduction to the mid-Atlantic region, more data are needed to better evaluate the risks associated with this parasite. Field observations collected from North Carolina and information on other Bonamia spp. suggest an affinity for higher salinities, and direct transmissibility; in the absence of explicit experimental tests, however, this is largely hypothetical. Consequently, we used laboratory trials to test the direct transmissibility and the persistence of Bonamia sp. in infected triploid C. ariakensis transferred to and maintained at three salinities, 10, 20, and 30 psu for at least 15 wk. Under these experimental conditions, there was no evidence of direct Bonamia sp. transmission. Average parasite intensity in infected oysters transferred to and maintained at 10 and 20 psu decreased compared with oysters place...

Characteristics of Crassostrea ariakensis (Fujita 1913) and Crassostrea virginica (Gmelin 1791) in the Discharge Area of a Nuclear Power Plant in Central Chesapeake Bay

Since 1978, the Maryland Department of Natural Resources has investigated the uptake and depuration by C. virginica of radionuclides released by the Calvert Cliffs Nuclear Power Plant (CCNPP), which is located on the western shore of Chesapeake Bay in southern Maryland. This ongoing program presented a unique opportunity to compare radionuclide transfer dynamics and other parameters such as growth, mortality, disease status, and meat condition for Crassostrea virginica and C. ariakensis in a side-by-side, in-situ environmental setting. Introducing the Asian oyster, C. ariakensis, into Chesapeake Bay has been proposed to help restore an oyster population to historical levels, to enhance the Bay ecosystem, and to contribute to the revival of the commercial oyster fishery. The population of the indigenous C. virginica has declined dramatically throughout the bay in part as a result of infection by two non-native protozoan parasites that cause MSX (Haplosporidium nelsoni) and Dermo (Perkinsus marinus) diseases. Other factors including habitat loss, reduced water quality, and overfishing have also reduced population size. Crassostrea ariakensis is known to be more tolerant to these diseases and thus may have greater potential for survival and growth in Chesapeake Bay than the native oyster. Triploid C. ariakensis and diploid C. virginica were immersed in cages in the vicinity of the CCNPP discharge for two exposure periods between July and December 2004. After retrieval, oysters were analyzed for radionuclide concentrations, growth rate, mortality, disease status, and meat condition. Fouling of oysters and cages was also observed. Quantities of radionuclides released by the CCNPP for both exposure periods were insufficient to produce detectable concentrations in either C. virginica or C. ariakensis, precluding a comparison of uptake of Co58, Co60, or Ag-110 m, radionuclides that have been detectable historically. Shell lengths were not significantly different between the two species for the two sampling periods, despite the fact that there were considerable age differences between them, but C. ariakensis did show a greater rate of growth. Meat weights and condition indices for C. ariakensis were significantly greater than for C. virginica (P < 0.01). Mortality and Dermo prevalence and intensity were also significantly greater for C. virginica than for C. ariakensis (P < 0.01). Although quantities of radionuclides were insufficient to produce detectable concentrations in either species, this study provided a unique opportunity to examine C. ariakensis in an area of the Maryland Chesapeake Bay where C. ariakensis has not previously been observed.

Fertilization Interference Between Crassostrea ariakensis and Crassostrea virginica: A Gamete Sink

Published data indicate that spawning seasons for the Asian oyster Crassostrea ariakensis and the eastern oyster C. virginica overlap. Hybrids can form, but the larvae are not viable. If C. ariakensis is introduced into Chesapeake Bay and synchronous spawning occurs with native C. virginica, hybridization could reduce the production of viable larvae (=gamete sink). We examined the effects of gamete age, sperm concentration, and ratios of heterospecific gametes on fertilization rates and hybridization between the two species. Interspecific fertilization rates were consistently lower than intraspecific rates. Fertilization rates decayed linearly with gamete age, though intraspecific fertilization rates remained above 50% for 4–6 h, indicating that long dispersal of viable gametes is possible. Fertilization rates decayed in a log-linear manner with decreasing sperm density for intra and interspecific crosses. Fertilization rates declined significantly when sperm were (1) given a choice of eggs from each species to fertilize or (2) required to compete to fertilize eggs from a single species. Hence, a gamete sink will likely occur if these two species spawn synchronously. The magnitude of the gamete sink will depend on both gamete concentrations and the relative proportion of interspecific gametes in the water column. Furthermore, genetic analysis of individual 2-day old larvae indicated that C. virginica sperm was more likely to fertilize C. ariakensis eggs than any other interspecific cross. All else being equal, the removal of C. ariakensis eggs through this mechanism may provide C. virginica with a competitive edge.

Interacting effects of temperature and salinity on Bonamia sp. parasitism in the Asian oyster Crassostrea ariakensis

The proposition to introduce the Asian oyster Crassostrea ariakensis to the mid-Atlantic region of the USA is being considered with caution, particularly after the discovery of a novel microcell haplosporidian parasite, Bonamia sp., in North Carolina. Although this parasite was found to be pathogenic in C. ariakensis under warm euhaline conditions, its persistence in C. ariakensis exposed to various temperature and salinity combinations remained unresolved. In this laboratory experiment, we tested the influence of temperature in combination with a wide range of salinities (10, 20 and 30 psu) on Bonamia sp. Temperature was either changed from warm (>20 °C) to cold (6 °C for 6 weeks) and back to warm or maintained constant and warm. Warm temperature was associated with higher host mortality than cold temperature, suggesting that temperature influenced Bonamia sp. pathogenicity. The effect of salinity was revealed under warm temperature with highest mortality levels observed in infected C. ariakensis exposed to 30 psu. When temperature was increased following low-temperature exposure, Bonamia sp. was not detected; however sub-optimal experimental conditions may have contributed to this result, making it difficult to draw conclusions regarding the reemergence of the parasite after low-temperature exposure. Although the overwintering of Bonamia sp. in C. ariakensis will need to be further investigated, the results presented here suggest that Bonamia sp. may be able to persist in C. ariakensis under a combination of low temperature and meso- to euhaline salinities.

Strong seasonality of Bonamia sp. infection and induced Crassostrea ariakensis mortality in Bogue and Masonboro Sounds, North Carolina, USA

Asian oyster Crassostrea ariakensis is being considered for introduction to Atlantic coastal waters of the USA. Successful aquaculture of this species will depend partly on mitigating impacts by Bonamia sp., a parasite that has caused high C. ariakensis mortality south of Virginia. To better understand the biology of this parasite and identify strategies for management, we evaluated its seasonal pattern of infection in C. ariakensis at two North Carolina, USA, locations in 2005. Small (<50 mm) triploid C. ariakensis were deployed to upwellers on Bogue Sound in late spring (May), summer (July), early fall (September), late fall (November), and early winter (December) 2005; and two field sites on Masonboro Sound in September 2005. Oyster growth and mortality were evaluated biweekly at Bogue Sound, and weekly at Masonboro, with Bonamia sp. prevalence evaluated using parasite-specific PCR. We used histology to confirm infections in PCR-positive oysters. Bonamia sp. appeared in the late spring Bogue Sound deployment when temperatures approached 25 °C, six weeks post-deployment. Summer- and early fall-deployed oysters displayed Bonamia sp. infections after 3–4 weeks. Bonamia sp. prevalences were ⩾75% in Bogue Sound, and 60% in Masonboro. While oyster mortality reached 100% in late spring and summer deployments, early fall deployments showed reduced (17–82%) mortality. Late fall and early winter deployments, made at temperatures <20 °C, developed no Bonamia sp. infections at all. Seasonal Bonamia sp. cycling, therefore, is influenced greatly by temperature. Avoiding peak seasonal Bonamia sp. activity will be essential for culturing C. ariakensis in Bonamia sp.-enzootic waters.

Effects of Salinity on Bonamia sp. Survival in the Asian Oyster Crassostrea ariakensis

Effects of Salinity on Bonamia sp. Survival in the Asian Oyster Crassostrea ariakensis

HARMFUL ALGAE POSE ADDITIONAL CHALLENGES FOR OYSTER RESTORATION: IMPACTS OF THE HARMFUL ALGAE KARLODINIUM VENEFICUM AND PROROCENTRUM MINIMUM ON EARLY LIFE STAGES OF THE OYSTERS CRASSOSTREA VIRGINICA AND CRASSOSTREA ARIAKENSIS

The eastern oyster, Crassostrea virginica (Gmelin, 1791) has been in decline along the eastern seaboard, and especially in Chesapeake Bay, for decades because of over-harvesting, disease and declines in water quality and suitable habitat. Eutrophication has also been increasing over the past half century, leading to increases in hypoxia and harmful algal blooms (HABs). The effects of two common Chesapeake Bay HAB dinoflagellates, Karlodinium veneficum, and Prorocetnrum minimum were tested on larvae of C. virginica and the Asian oyster being considered for introduction to Chesapeake Bay, C. ariakensis. When embryos from freshly spawned C. virginica and C. ariakensis were exposed immediately to K. veneficum at 104 cells mL−1, virtually all of the developed larvae were deformed within 48 h in one experimental trial, but not in a second trial in which algae were at a different growth stage. No deformities, and mortalities of <45%, were observed in controls to which a standard diet of the haptophyte Isochrysis was added. When 2-wk-old larvae of both species were exposed to the same HAB species, the effect was a severe reduction in motility with K. veneficum, but with P. minimum only C. ariakensis was affected and not C. virginica. Comparisons were made of the frequency of these HABs in Chesapeake Bay from long-term data analysis and the temporal period of spawning. Whereas both blooms are more common during the summer months, the frequency of blooms of K. veneficum and the period of oyster spawning, June to September, coincide more strongly. To compare spatial similarity, results of a larval transport model were compared with observational data for K. veneficum. This comparison demonstrated a significant overlap in July, particularly in the northern reaches of the Bay. These eutrophication-related HABs thus have the potential to reduce survival of early life history stages of oysters and hence to reduce oyster recruitment. Any reduction in recruitment either spatially or temporally, combined with an overall reduction in sheer numbers of larvae that survive, will make restoration or establishment of significant, self-sustaining populations of natural or introduced oyster species much more difficult.

Molecular phylogenies help resolve taxonomic confusion with Asian Crassostrea oyster species

Identification of oyster species is still largely based on phenotypic characters that are highly plastic. Prompted by the proposed introduction of the Asian oyster species Crassostrea ariakensis into the Chesapeake Bay region of the U.S.A., this study uses molecular genetic information to understand the taxonomic framework surrounding C. ariakensis and to confidently distinguish among various sympatric oyster species. Putative samples of C. ariakensis and other species of cupped oysters from across Asia were collected and phylogenetic analyses were conducted on DNA sequences of both nuclear (ITS-1) and mitochondrial (COI) regions. Trees generated based on the two independent molecular datasets were highly congruent, and indicate that many oysters collected for this study as C. ariakensis were originally misidentified. Results also indicate that C. ariakensis, C. hongkongensis and C. nippona are distinct, but closely related species. There is strong support in both analyses for a close relationship between C. gigas and C. sikamea, as well as between C. belcheri and C. gryphoides, and between C. iredalei and C. madrasensis. The parsimony analyses based on these DNA markers, however, did not provide evidence to support C. angulata as a distinct species from C. gigas. Overall, the results emphasize the need for rigorous species identification, and additional extensive and intensive sampling to more accurately determine relationships among Crassostrea species, define their geographic distributions, and establish existing sympatry patterns.

Pathogens in Crassostrea ariakensis and other Asian oyster species: implications for non-native oyster introduction to Chesapeake Bay

With the drastic decline of eastern oyster Crassostrea virginica populations in the Chesapeake Bay due to over-fishing, diseases and habitat destruction, there is interest in Maryland and Virginia in utilizing the non-native oyster species Crassostrea ariakensis for aquaculture, fishery resource enhancement, and ecological restoration. The International Council for the Exploration of the Sea (ICES) recommends that non-native species be examined for ecological, genetic and disease relationships in the native range prior to a deliberate introduction to a new region. Therefore, a pathogen survey of C. ariakensis and other sympatric oyster species was conducted on samples collected in the PR China, Japan and Korea using molecular diagnostics and histopathology. Molecular assays focused on 2 types of pathogens: protistan parasites in the genus Perkinsus and herpesviruses, both with known impacts on commercially important molluscan species around the world, including Asia. PCR amplification and DNA sequence data from the internal transcribed spacer region of the rRNA gene complex revealed the presence of 2 Perkinsus species not currently found in USA waters: P. olseni and an undescribed species. In addition, 3 genetic strains of molluscan herpesviruses were detected in oysters from several potential C. ariakensis broodstock acquisition sites in Asia. Viral gametocytic hypertrophy, Chlamydia-like organisms, a Steinhausia-like microsporidian, Perkinsus sp., Nematopsis sp., ciliates, and cestodes were also detected by histopathology.

Whenr-Selection May Not Predict Introduced-Species Proliferation: Predation Of A Nonnative Oyster

Predicting outcomes of species introductions may be enhanced by integrating life-history theory with results of contained experiments that compare ecological responses of exotic and analogue native species to dominant features of the recipient environment. An Asian oyster under consideration for introduction to the Chesapeake Bay, USA, the rapidly growing Suminoe oyster (Crassostrea ariakensis), may not be as successful an invader as its r-selected life history suggests if the trade-off for rapid growth and maturation is lower investment in defenses against blue crab (Callinectes sapidus) predation than the native Eastern oyster (Crassostrea virginica). In laboratory trials, blue crabs simultaneously offered equal numbers of Suminoe and Eastern oysters consumed more nonnatives, irrespective of whether the crabs had previous experience with Suminoe oysters as prey. Satiated blue crabs consumed nearly three times as many Suminoe oysters as Eastern oysters of 25-mm shell height, and eight times as many of 35-mm shell height. Despite blue crabs consuming small (30 mm) Suminoe oysters at twice the rate of large (40 mm) Suminoe oysters, when 40-mm Suminoe were paired with 30-mm Eastern oysters, seven times as many of the larger (Suminoe) oysters were consumed. The greater susceptibility of C. ariakensis than C. virginica to blue crab predation appears to be based upon the biomechanics of shell strength rather than active selection of a more attractive food. Much less force was required to crush shells of Suminoe than Eastern oysters of similar shell height. Tissue transplant experiments demonstrated greater predation on oyster tissues in weaker C. ariakensis shells independent of tissue identity, and duration of handling time before rejection of C. virginica exceeded the time to crush C. ariakensis. These results, coupled with the present importance of blue crab predation in limiting recovery of native Eastern oysters, imply a role for blue crabs in inhibiting Suminoe oysters, if introduced, from attaining high adult densities required to restore a fishery, provide appreciable reef habitat, and reduce turbidity through filtration. Thus, in high-predation environments, allocation of resources to rapid growth and development rather than to predation defenses reflects a life-history trade-off that may promote early stages of invasion, yet prevent attainment of dense adult populations.

When the World Is Not Your Oyster

The decline of the eastern oyster Crassostrea virginica in Chesapeake Bay from overfishing, disease, and habitat degradation is well documented (1–3). Oysters provided important ecological and economic benefits to the Chesapeake Bay states of Maryland and Virginia, which have poured millions of dollars into their restoration (4). Recently, these states have effectively conceded that restoration has been ineffective (5), and they are developing an Environmental Impact Statement (EIS) to evaluate alternatives for increasing oyster populations (6). Under the EIS, the preferred (proposed) action is to introduce diploid populations of an Asian oyster, Crassostrea ariakensis, into the Bay. Such an introduction could occur in 2006 (7).