Crassostrea ariakensis

History of Spread

Crassostrea ariakensis (Chinese River Oyster, Suminoe Oyster) is native to the coasts of Japan, China, India, and Pakistan (Carriker and Gaffney 1996), where it is harvested and cultured. This species first attracted U.S. aquaculturalists in the 1970s as an oyster that would have marketable qualities in the summer, when the dominant commercial Pacific Oyster (C. gigas) has soft flesh due to spawning.C. ariakensis was inadvertently introduced to the West Coast with stocks of C. gigas, and had been reared in several West Coast oyster hatcheries (Breese and Maloof 1977). In the 1990s, culture of C. ariakensis on the West Coast was still very localized, and wild populations were not reported (Langdon and Robinson 1996).

On the East Coast of North America, the decline of Eastern Oyster populations (C. virginica), and reduced recruitment, due in part to the diseases MSX (Haplosporidum nelsoni) and 'Dermo' (Perkinsus marinus) has led to interest in importing exotic, disease-resistant oysters. This interest has been strongest in VA, because the higher salinities of its oystering waters have led to near-eradication of native oysters by the diseases (Burreson and Mann 1994; Calvo et al. 1999; Leffler 2002). Experiments with Pacific Oysters (C. gigas) suggested that this species is poorly adapted to the high summer temperatures and low salinities of many East Coast estuaries (Burreson and Mann 1994; DeBrose and Allen 1996; Calvo et al. 1999).

In the Chesapeake Bay region, experimental plantings of 250 sterile triploid Crassostrea ariakensis began in 1998, at 'low salinity' (<15 ppt, Coan and Great Wicomico Rivers) and 'medium salinity' 15-20 ppt, (Woodas Creek, York River) sites in VA Chesapeake Bay tributaries, and at 'high salinity' (>25 ppt, Bogues Bay, Burtons Bay) locations in VA Atlantic coastal bays. At the more saline of the 'low-salinity' sites, C. ariakensis showed superior disease resistance and growth to the native Eastern Oyster C. virginica as well as to the Pacific oyster (C. gigas). Overall, C. ariakensis mortality averaged 13-16% over the salinity gradient, compared to 81-100% for C. virginica. Growth in C. ariakensis was faster, with mean shell heights were 93-137 mm, over the salinity gradient, staring from a mean of 64 mm, compared to 70-73 mm, starting from a mean of 60 mm (Calvo et al. 2001). Survival was also superior to that seen in C. gigas in similar trials, in 1997 (63, 17, and 4% mortality at low, medium, and high salinities, Calvo et al. 1999). These results have encouraged additional research on C. ariakensis. Little is known about the ecology of this oyster, or its interactions with native biota (Allen 1999; Luckenbach 1999). Thus, the question of whether fertile C. ariakensis should be allowed in aquaculture, or planted in open waters, was highly contentious.

Growth trials with sterile triploid Crassostrea ariakensis are also taking place in NC, in Chadwicks Bay. Initial results there were promising, with higher rates of growth than were seen in Chesapeake Bay (North Carolina Shellfish Growers Association 2000).

In Chesapeake Bay, field trials with triploid Crassostrea ariakensis have expanded. In 2000, the Virginia Seafood Council, with the approval of the Virginia Marine Resources Commission, conducted trials with 6,000 triploid oysters, and in 2001, 60,000 triploid C. ariakensis were planted in Virginia waters for more extensive trials (Blankenship 2001; Huslin 2001). The Virginia Seafood Council proposed to stock 1 million triploids in 2002 (Blankenship 2001). The expansion of these trials prompted concerns among different state, federal, and nongovernmental organizations. Many of these varying viewpoints were expressed in a symposium in October 2001, sponsored by several state and federal organizations, and held at the College of William and Mary, Williamsburg VA (Hallerman et al. 2001).

Industry groups urged immediate research on diploid (fertile) Crassostrea ariakensis, because of the dire state of the Chesapeake Bay industry. Industry trials confirmed this oyster’s rapid growth and consumer acceptance (Wesson, in Hallerman 2001). Virginia growers wanted to go ahead with the introduction of 1 million triploid oysters in 2002, and expand to saturate the regional demand of 270 million oysters by 2007, if necessary, using less effective chemical methods to induce triploidy. Maryland producers felt that if the Virginia project proceeded, they should be allowed to introduce diploid C. ariakensis to stay competitive (Hallerman et al. 2001).

A science discussion group outlined possible risks of triploid introduction, including the encouragement of illegal introductions of diploid, possibly disease-bearing, stock from Northwest or Asian hatcheries or markets, reversion of triploids, accidental releases from hatcheries and aquaculture systems, and the ecological consequences of a new reproducing oyster in the bay. These risks include competition with C. virginica, alteration of Bay foodwebs and habitats, and the risk that C. ariakensis could become a nuisance species (Hallerman et al. 2001). Since the stock introduced in Virginia were raised for several generations in the laboratory, risks of protozoan or bacterial diseases are virtually absent, but a larger, though still low, risk remains from viral diseases (Burreson, in Hallerman et al. 2001). Mesocosm experiments suggest that C. virginica is somewhat more competitive than C. ariakensis, in a disease-free environment, owing to its tendency to accumulate less silt, by growing vertically and forming reefs, as opposed to C. ariakensiss, which tends to set singly and lie flat (Luckenbach, in Hallerman et al. 2001; Leffler 2002).

A regulatory and policy discussion group listed the types of information needed for sound decision making on Crassostrea ariakensis introductions: (1) More basic species biology; (2) Simulation modeling of risks and benefits, including economics; (3) A better knowledge of “user need” conflicts on other fisheries and boating in the Chesapeake, and implications for the broader Atlantic-Gulf region. (4) A better sense of ecological consequences of diploid introductions. The group discussed ways of insuring biosecurity, including requiring bonding of the parties involved in C. ariakensis aquaculture. At this time, the representatives of Maryland agencies opposed the introduction of C. ariakensis, or any other non-native oyster (Hallerman et al. 2001).

Federal agencies, involved in the Chesapeake Bay Program, were also opposed to Crassostrea ariakensis introductions, until adequate risk assessment was completed. They were concerned that this introduction would imperil the Chesapeake 2000 agreement among federal and state agencies, which called for a 10-fold increase in the abundance of native oysters in Chesapeake Bay They also suggested that environmental impact studies would be required, in order to obtain permits from the Army Corps of Engineers (Federal Agencies Committee, in Hallerman et al. 2001). The Chesapeake Bay’s major research institutions, the Virginia Institute of Marine Science, and the University of Maryland’s Center for Environmental Science issued position papers emphasizing the need for research and careful risk assessment (Hallerman et al. 2001). A non-governmental organization, the Chesapeake Bay Foundation, also stressed risk assessment, and the need for review by the National Academy of Sciences, but emphasized restoration of native oysters as the highest priority (Hallerman et al. 2001).

In March 2002, the National Academy of Sciences agreed to review the proposed introduction of Crassostrea ariakensis to Chesapeake Bay. The goal of this report was to determine whether available information on C. ariakensis and its ecological and economic impacts is sufficient for sound decision-making, and to predict impacts of aquaculture, and/or direct release of diploid oysters into the Bay. The published version was scheduled for release by August 2003 (Leffler 2002). In December of 2002, the Virginia Seafood council again submitted the proposal to introduce 1 million triploid C. ariakensis to Virginia waters, and in February 2003, the proposal was approved by the Virginia Marine Resources Commission. Maryland's position on C. ariakensis altered dramatically, in June 2003, when Governor Robert Ehrlich announced that the state would seek a permit from the Army Corps of Engineers to plant diploid C. ariakensis directly into the Chesapeake Bay. According to the Corps, after a plan for introduction is submitted, an environmental impact statement would be required for this introduction. Maryland wdeveloped its plan jointly with VA and NC, since all three states were interested in C. ariakensis introduction. At the time, it was believed that the impact statement would probably take about 2 years to prepare (Blankenship 2003).

The report of the National Academy of Sciences, published in August 2003, indicated that considerable research was needed before an informed decision can be made on the introduction of C. ariakensis (National Research Council 2003).

Interest in the introduction of Crassostrea ariakensis as a means of restoring Chesapeake Bay’s oyster fishery and its ecosystem was clearly increasing. As the number of stocked triploids increases, the probability of reversion to diploid status increases, as does the chance of accidental or illegal release of diploid stocks. As the local oyster industry increasingly utilizes C. ariakensis , political pressure for official diploid introductions was also likely to increase (Leffler 2003).

Issues complicating the proposed introduction of C. ariakensis include an outbreak of a Bonamia-like parasite in Bogue Sound, NC (Blankenship 2004; Burreson et al. 2004; Carnegie 2007; Schott et al. 2008) and the low genetic diversity of cultured stocks (Breitburg, personal communication; Xiao et al. 2008). The Bonamia parasite has been found in low prevalence in triploid C. ariakensis in Chesapeake Bay waters, and prefers high salinities and temperatures (above 20 ppt, above 20 C). It has caused heavy mortality of C. ariakensis in Bogue Sound, but not yet in Chesapeake waters (Schott et al. 2008). Genetic comparisons of Asian populations identified as C. ariakensis indicated that they consisted of two cryptic species (Zhang et al. 2008), and that US hatchery stocks, proposed for introduction in Chesapeake Bay, had greatly reduced genetic variaton, with the possibility of genetic bottlenecks (Xaio et al. 2008). An additional complication was the discovery of gametic interference between the two species. Sperm and eggs of the two species fuse, but fail to develop, potentially reducing recruitment of both species, or possibly favoring one species over the other (Bushek et al. 2008).

The accumulation of possible risks and uncertainties led to consideration of alternatives to mass introduction of diploids, including aquaculture of triploid C. ariakensis in the Bay, or confinement of C. ariakensis to closed systems. A modelling study conducted by the evironmental research firm Versar Inc. indicated that the probability of the establishment of diploid oysters from large-scale triploid aquaculture were substantial, and larger than previously expected (Richkus et al. 2009).

The environmental impact statement study was concluded and the decsision on the proposed introduction was announced on April 6 2009, after 5 years, 3 years longer than intially expected. The decision, made by the Army Corps of Engineers,and the states of Maryland and Virginia, was to prohibit introduction of diploid C. ariakensis, end cultivation of triploids in open waters, and instead transfer resources to restoration of the native Eastern Oyster (Fahrenthold 2009; Wheeler 2009). Virginia officials had reservations, suggesting that the decision could be reconsidered, if the risks of introduction were clarified by future research (Fahrenthold 2009). The decision does permit continued research on culture of C. ariakensis in closed systems (Wheeler 2009)

We will continue to consider the population status of C. ariakensis in Chesapeake Bay as 'unknown' because illegal introductions are still possible, as as reproduction of previously planted 'triploids'.

History References- Allen 1999 personal communication; Blankenship 2001; Blankenship 2003; Breese and Maloof 1977; Burreson and Mann 1994; Calvo et al. 1999; Calvo et al. 2000; Calvo et al. 2001; Carriker and Gaffney 1996; DeBrose and Allen 1996; Hallerman et al. 2001; Langdon and Robinson 1996; Leffler 2003; Luckenbach 1999 personal communication; North Carolina Shellfishgrowers Association 2000

Invasion Comments