Description
Synonymy - Crassostrea angulata, the Portuguese Oyster, found in waters of southern Europe has been considered either a distinct species, or by many recent investigators, a population of C. gigas brought from Asian waters ~300-400 years ago (Carriker and Gaffney 1996). Currently, C. angulata is considered to be a separate but closely related species, and has been found in China (O'Foighil et al. 1998; Lapegue et al. 2004
Other Taxonomic Groupings - Several varieties (races, forms) are known, with different growth and breeding characteristics. The 'Miyagi' form is the one most often reared outside Japan, and is the strain being most heavily studied for possible planting in Chesapeake Bay and elsewhere on the Atlantic coast (DeBrosse and Allen 1996; Mann et al. 1991; Quayle 1969).
Taxonomy
| Kingdom | Phylum | Class | Order | Family | Genus |
|---|---|---|---|---|---|
| Animalia | Mollusca | Bivalvia | Ostreina | Ostreidae | Crassostrea |
Synonyms
Invasion History
Chesapeake Bay Status
| First Record | Population | Range | Introduction | Residency | Source Region | Native Region | Vectors |
|---|---|---|---|---|---|---|---|
| 1962 | Unknown | Unknown | Introduced | Regular Resident | Eastern Pacific | Western Pacific | Fisheries(Oysters-intentional) |
History of Spread
Crassostrea gigas (Pacific Oyster) is native to the Indo-West Pacific from Pakistan to Japan and Korea, and the Phillipines, Borneo, and Sumatra (Carriker and Gaffney 1996). It is the most widely transplanted shellfish in the world, introduced to at least 42 countries (Food and Agricultural Organization 1998). Small illegal or unofficial introductions have occurred in Atlantic waters near Chesapeake Bay, but no established populations of this oyster are known in western Atlantic waters. However, introduction of C. gigas has been considered as a possible means of replacing or supplementing native stocks of C. virginica (Eastern Oyster), which has been devastated by diseases (Andrews 1980; Baker 1992; Mann et al. 1991). By 1998, research and political interest had shifted to Crassostrea ariakensis, which showed better growth and survival under East Coast conditions (National Research Council 2003).
In the Eastern Atlantic, C. gigas was imported to Marennes, France in small quantities in 1966. This was followed by a disease epizootic in C. angulata (Portuguese Oyster), then the predominant commercial species (itself imported to supplant the overfished Ostrea edulis, European Flat Oyster). Consequently, large imports of C. gigas were made to replace the lost C. angulata stocks (Andrews 1980; Gizel and Heral 1991). In the United Kingdom, laboratory stocks were imported in 1965 and 1972, and the experimental field plantings, of lab-reared spat, in 1967 and 1973. Spawning and recruitment is rare in British waters, owing to low water temperatures (Walne and Helme 1979).
In North America, C. gigas was first introduced to Puget Sound WA in 1902, following overfishing of the native Olympic Oyster ( Ostrea lurida) and unsuccessful stocking of C. virginica (Eastern Oyster). Early transplants were unsuccessful due to mortality in shipping, but after numerous subsequent imports, large-scale cultivation was underway in WA by 1928 (Chew 1979). In British Columbia, imports began in 1912, but large-scale natural spawning was not seen until 1932 (Quayle 1969). This species is now the basis of the West Coast oyster industry, with commercial harvests taking place from southern British Columbia to Tomales Bay CA (Chew 1979; Quayle 1969).
On the Atlantic Coast of North America, a number of plantings of small numbers of C. gigas have been made, either as authorized experiments, unofficially or illegally. Most of the latter two types of introductions are poorly documented.
Gulf of Mexico - An introduction of C. gigas to LA waters failed, reportedly due to heavy Polydora spp. infestations (Kavanaugh 1941).
Mid Atlantic Bight - Davis (personal communication, cited by Dean 1979) noted that 'numerous shipments were introduced into Long Island Sound at one time or another' (Dean 1979). A bushel of C. gigas was planted in Barnegat Bay NJ in the early 1930's, but failed to grow (Andrews 1980). 'An oysterman from Delaware saw impressive specimens of C. gigas at the Seattle Worlds Fair in 1962, and he had some sent to his home state for planting. The oysters were confiscated by a biologist who held them in trays in open waters in Rehoboth Bay, Del. for several years without serious mortality or apparent successful reproduction (Andrews 1980). 'Recently (1970's), C. gigas from the West Coast of North America was planted in Maryland waters by a seafood dealer, which resulted in a specific law prohibiting the species. The oysters were removed as soon as possible by scuba diving.' (Andrews 1980).
Although none of these documented introductions quite match in timing, introductions of Crassostrea gigas are considered a likely route for introduction of Haplosporidium nelsoni (MSX) to the Atlantic Coast of the United States (Andrews1980; Burreson et al. 2000; Kern 1998). The disease first appeared in lower Delaware Bay in 1958 (Ford and Haskin 1982). Morphologically similar organisms have been identified in C. gigas from Korea, Japan, and CA (Friedman 1996; Katkansky and Warner 1970; Kern 1976b) . Genetic comparisons between the Korean organism and Western Atlantic H. nelsoni suggest that the two organisms may be identical (Burreson et al. 2000).
Buzzards Bay, Cape Cod Bay - Six bushels of C. gigas seed were introduced to Barnstable Harbor in 1949. The oysters grew, some surviving for 5 years, but no spawning was observed (Dean 1979). In 1977, growth studies with C. gigas from CA were conducted in Fresh Pond, Falmouth ME, a tidal lagoon adjacent to Buzzards Bay. (The outlet of the pond was blocked during this experiment.) Spawning was observed in June 1977, but no settlement occurred (Hickey 1979).
Gulf of Maine - Growth studies on juvenile C. gigas in coastal ME waters were made from 1971 to 1973, but were stopped based on concern about ecological impacts (Dean 1979). Introduction of C. gigas is still a topic of study and dispute there.
The potential introduction of C. gigas to Chesapeake Bay began to be studied in the late 1970's as sharp decreases in oyster harvests were taking place, especailly in VA, leaving oyster packers in that state with a diminishing share of the market, as Gulf Coast and West Coast producers became increasingly important (Andrews 1980; DuPaul 1992). A comparison with other known species of Crassostrea (10- spp.) indicate that C. gigas had the widest temperature and salinity ranges for growth and reproduction. In addition, as a widely cultured species, its requirements for optimum growth were well-known (Mann et al. 1991). It was believed to be largely resistant to H. nelsoni and Perkinsus marinus (Dermo) (Andrews 1979b; Andrews 1980). Numerous culture experiments have been undertaken with diploid and triploid (sterile) C. gigas in order to assess the disease resistance of the Pacific Oyster and its adaptability to the Chesapeake Bay environment. Experiments in quarantined flumes indicate that C. gigas had lower prevalence and intensity of P. marinus and H. nelsoni infections (Barber 1996; Barber and Mann 1994; Chu et al. 1996; Krantz 1992). Benefits of a disease-resistant oyster would include restoration of the oyster-reef environment and of a filter-feeding biomass in at least part of Chesapeake Bay, as well as revival of oystering (Gottlieb and Schweighofer 1996; Lipton et al. 1992; Mann et al. 1991). By the late 1990s, the poor survival and performance of C. gigas in experimental trials in Chesapeake Bay, led to a loss of interest in this species. Research and political interest shifted to Crassostrea ariakensis, which appeared better adapted to local conditions. (National Research Council 2003)
However, legally sanctioned C. gigas introductions were prevented because of substantial risks to native C. virginica populations, as well as poor performance by the introduced oyster under Chesapeake bay conditions. Potential risks include (1) accidental introduction of epifaunal biota, pests, and parasites (Andrews 1980; Galtsoff 1932; Grizel and Heral 1991); (2) competition with C. virginica (Andrews 1980; Galtsoff 1932); (3) hybridization or genetic exchange (Andrews 1980; Gaffney and Allen 1992). These impacts will be discussed in more detail in the 'Community Ecology' section. Parasite and disease introductions can be minimized by following the ICES (International Council for Exploration of the Seas), using stock reared for several generations under quarantined conditions (Carlton 1992; Kern 1998. The other two types of impacts are more difficult to assess.
Although C. gigas shows strong disease resistance, trials in Chesapeake Bay suggest that this oyster was not well adapted to the local environment. In quarantined flumes, C. gigas had high non-disease mortality in summer (Barber and Mann 1994), and heavy Polydora spp. infestations (Burreson and Mann 1994; DeBrose and Allen 1996). While the species' survival ranges for temperature and salinity is broad, substantial recruitment requires salinities above 20 ppt (Mann et al. 1991). Limiting C. gigas to the lower Bay. C. gigas reared in MD waters (Deal Island, 9-16 ppt) did not grow and had meat of poor quality (Krantz 1992). Finally, C. gigas is generally regarded as inferior to C. virginica in texture and flavor (DuPaul 1992).
In 1993, an experimental planting of 300 triploid C. gigas was made in the York River VA (Mann and Burreson 1994). The oysters were removed when 19% of them were found to have reverted to a diploid/triploid state. The cause and significance of this reversion are currently under study (Gottlieb and Schweighofer 1996). Further introductions are unlikely until this problem is resolved. Even in VA, which was likely to benefit most from C. gigas introductions, there was broad support among oyster producers and packers only for the introduction of sterile animals (DuPaul 1992). Deliberate, legally sanctioned establishment of reproducing populations thus does not appear to be an immediate prospect, although illegal planting remains a possibility. After 1998, when triploid C. ariakensis showed good survival and rapid growth in Chesapeake Bay, interest in C. gigas diminished (National Research Council 2003).
History References - Andrews 1979b; Andrews 1980; Baker 1992; Barber 1996; Barber and Mann 1994; Baker 1992; Burreson et al. 2000;; Carlton 1992; Carriker and Gaffney 1996; Chew 1979; Dean 1979; DeBrose and Allen 1996; DuPaul 1992; Food and Agricultural Organization 1998; Friedman 1996; Gaffney and Allen 1992; Galtsoff 1932; Gottlieb and Schweighofer 1996; Grizel and Heral 1991; Hickey 1979; ; Katkansky and Warner 1970; Kavanaugh 1941; Kern 1976b; Kern 1998; Krantz 1992; Lipton et al. 1992; Mann and Burreson 1994; Mann et al. 1991; Quayle 1969; Walne and Helme 1979
Invasion Comments
Population Status - Crassostrea gigas (Pacific Oyster) has had several failed, poorly documented introductions in the Chesapeake Bay region, as well as at least one legally sanctioned experimental introduction of sterile individuals (Mann and Burreson 1994). However establishment of reproducing populations, or planting of sterile triploid oysters on a commercial scale has not been accomplished in Chesapeake Bay or elsewhere on the Atlantic Coast of North America (Gottlieb and Schweighofer 1996; Mann et al. 1991)
Ecology
Environmental Tolerances
| For Survival | For Reproduction | |||
|---|---|---|---|---|
| Minimum | Maximum | Minimum | Maximum | |
| Temperature (ºC) | 3.0 | 35.0 | 16.0 | 30.0 |
| Salinity (‰) | 10.0 | 42.0 | 10.0 | 30.0 |
| Oxygen | ||||
| pH | ||||
| Salinity Range | meso-eu |
Age and Growth
| Male | Female | |
|---|---|---|
| Minimum Adult Size (mm) | 40.0 | 40.0 |
| Typical Adult Size (mm) | 110.0 | 110.0 |
| Maximum Adult Size (mm) | 450.0 | 450.0 |
| Maximum Longevity (yrs) | ||
| Typical Longevity (yrs |
Reproduction
| Start | Peak | End | |
|---|---|---|---|
| Reproductive Season | |||
| Typical Number of Young Per Reproductive Event |
|||
| Sexuality Mode(s) | |||
| Mode(s) of Asexual Reproduction |
|||
| Fertilization Type(s) | |||
| More than One Reproduction Event per Year |
|||
| Reproductive Startegy | |||
| Egg/Seed Form |
Impacts
Economic Impacts in Chesapeake Bay
Established populations of Crassostrea gigas (Pacific Oyster) do not exist in Chesapeake Bay, or elsewhere on the Eastern Seaboard) at this time.
Fisheries - Failed introductions of this species are suspected of having been the vector of introduction of Haplosporidium nelsoni (MSX disease), which has been involved in the decline of the C. virginica (Eastern Oyster) fishery in Delaware Bay and Chesapeake Bay from the late 1950's onward (Andrews 1979a; Andrews 1980; Kern 1998). Possible introduction of C. gigas in Chesapeake Bay has been studied since the late 1970's as a possible way of replacing or supplementing declining stocks of C. virginica with a disease-resistant species (Andrews 1979b; Andrews 1980; Gottlieb and Schweighofer 1996; Lipton et al. 1992; Mann et al. 1991).
The obvious benefit would be revival of oyster fisheries, at least in lower waters of Chesapeake Bay where C. gigas is capable of growing and reproducing (Barber 1996; Mann et al. 1991). Successful reproduction was likely to be confined to waters south of the mouth of the Rappahannock River (Gottlieb and Schweighofer 1996; Mann et al. 1991). Absence of growth, and poor condition was observed in C. gigas held in flumes at Deal Island MD, on the lower Eastern shore (Krantz 1992), so benefits from harvesting would accrue largely to VA. Fisheries benefits were further qualified by the fact that C. gigas, though it grows to a large size, is regarded by oyster processors and packers as being inferior to C. virginica in flavor and texture (DuPaul 1992). If C. virginica continued to be regarded as superior by consumers, a successful introduction of C. gigas to Chesapeake Bay may have simply increased the supply of that species, lowering the price, resulting in few benefits to the local industry (Lipton et al. 1992). Since the largest productive oyster grounds are in MD waters, and C. gigas would be largely restricted to VA waters, risks and benefits are unequally divided between the states (Gottlieb and Schweighofer 1996; Lipton et al. 1992). In MD risks of possible disease introductions, genetic effects, or competition with C. virginica outweighed perceived benefits of C. gigas (Krantz 1992).
A discussion of issues involved in C. gigas introductions emphasized indirect fisheries and environmental benefits of restoring large oyster biomasses in the lower Bay (Gottlieb and Schweighofer 1996). These include maintaining oyster beds as an important habitat for other organisms, including commercially valuable species, and restoring a biomass of filter feeders in at least part of the Bay.
Risks of C. gigas plantings included introduction of parasites and other associated organisms, and competition or other interference with native C. virginica stocks (See 'Community Ecology'). In a legally sanctioned introduction, these effects would be minimized by following the ICES (International Council for Exploration of the Seas), using stock reared for several generations under quarantined conditions (Carlton 1992; Kern 1998), and by using sterile, triploid oysters for initial experimental plantings (Mann et al. 1991). In 1993, an experimental planting of 300 triploid C. gigas was made in the York River VA (Mann and Burreson 1994). The oysters were removed when 19% of them were found to have reverted to a diploid/triploid state. The cause and significance of this reversion are currently under study (Gottlieb and Schweighofer 1996). Further introductions are unlikely until this problem is resolved. In trials in quarantined flumes, high non-disease mortality (Barber 1996; Barber and Mann 1994) make benefits of C. gigas introductions look less certain. While development of disease-resistant stocks of C. virginica has been slow, particularly with regard to Perkinsus marinus (Dermo) (Ford and Tripp 1996), biotechnological research offers some possibilities (Carriker and Gaffney 1996). Krantz (1992) and Kern (1998), among others, favored research into disease resistance over introduction of C. gigas.
Habitat Change, Aesthetics - The need for a large biomass of oysters in order to filter phytoplankton from the water column, as well as providing habitat for reef-inhabiting species, has been used as a justification for the introduction of C. gigas (Gottlieb and Schweighofer 1996; Mann et al. 1991).
In growth trials in 1997-1998, triploid C. gigas grew poorly in low-salinity VA waters in Chesapeake Bay, compared to C. virginica, but outperformed C. virginica at high-salinity sites in Atlantic coastal bays (Calvo et al. 1999). After 1998, further trials with C. gigas were suspended, in favor of C. ariakensis (Suminoe Oyster), owing to better growth results and higher perceived quality of the latter Asian oyster (Hallerman et al. 2001).
References - Andrews 1979b; Andrews 1980; Baker 1992; Barber 1996; Barber and Mann 1994; Burreson 1998; Calvo et al. 1999; Carriker and Gaffney 1996; DuPaul 1992; Gaffney and Allen 1992; Gottlieb and Schweighofer 1996; Hallerman et al. 2001; Kern 1976b; Kern 1976b; Kern 1998; Krantz 1992; Lipton et al. 1992; Mann and Burreson 1994; Mann et al. 1991
Economic Impacts Outside of Chesapeake Bay
Crassostrea gigas (Pacific Oyster) is the most widely cultivated and harvested shellfish in the world, introduced to at least 42 countries (Carriker and Gaffney 1996; Food and Agricultural Organization 1998). Among the more notable introductions have been those to the west coast of North America (Chew 1979; Quayle 1969) and to European waters (Grizel and Heral 1991; Walne and Helme 1979). The disease resistance of this oyster, its adaptability to a wide range of environments, the long development of culture techniques, and its large size are among the reasons for its widespread introduction (Andrews 1980; Mann et al. 1991). Disadvantages include bland flavor compared to other species, including C. virginica (DuPaul 1992), and risks to native oyster populations, including competition, hybridization, and introductions of associated organisms (parasites, fouling organisms and oyster predators) (Galtsoff 1932; Grizel and Heral 1991; Mann et al. 1991).
Numerous unofficial, illegal, and some officially sanctioned 'experiments' have been made on the Atlantic Coast of the United States (Andrews 1979b;Dean 1979; Andrews 1980; Kavanaugh 1941). [The possible role of these in the introduction of Haplosporidium nelsoni (MSX) to the mid-Atlantic region is discussed in more detail the account for that organism.] In the 1960s and 70's, there has been interest in a number of states, particularly MA and ME, outside the Chesapeake region in introduction of C. gigas (Andrews 1980; Dean 1979; Hickey 1979). Growth studies on juvenile C. gigas in coastal ME waters were made from 1971 to 1973, but were stopped based on concern about ecological impacts (Dean 1979). We are unaware of more recent experiments with C. gigas, aside from those in Chesapeake Bay in the 1990s.
References - Andrews 1979b; Andrews 1980; Carriker and Gaffney 1996; Chew 1979; Dean 1979; DeBrose and Allen 1996; DuPaul 1992; Food and Agricultural Organization 1998; Friedman 1996; Gaffney and Allen 1992; Galtsoff 1932; Grizel and Heral 1991; Hickey 1979; Kavanaugh 1941; Mann et al. 1991; Quayle 1969; Walne and Helme 1979
Ecological Impacts on Chesapeake Native Species
Established populations of Crassostrea gigas (Pacific Oyster) do not yet exist in Chesapeake Bay. However, unofficial plantings of this oyster to the mid-Atlantic region are considered a likely vector for introduction of Haplosporidium nelsoni (MSX, Delaware Bay Disease) (Andrews 1979a; Andrews 1980; Kern 1998 ). Importation of this disease constituted a major impact to Eastern Oyster (C. virginica) populations in Chesapeake Bay, and on much of the Atlantic Coast.
Parasitism - Organisms similar to H. nelsoni are known to occur at low frequencies in C. gigas populations in Korea, Japan, and North America (CA) (Friedman 1996; Katkansky and Warner 1970; Kern 1976b). Genetic comparisons between the Korean Haplosporidium sp. and Western Atlantic H. nelsoni indicate that that the two organisms are identical (Burreson et al. 2000). Numerous small plantings of C. gigas have occurred on the Atlantic Coast, though none of the known examples exactly coindicides with the H. nelsoni outbreak (Andrews 1980). Further information on H. nelsoni's history is given in the account of that species.
In other regions of the world, parasites, epifauna, and predators have been imported with shipments of C. gigas. Known parasites of C. gigas which are now established on the Pacific coast of North America, or in France, include 3 viruses, 3 bacterial diseases, 3 protistans (other than haplosporidians) (Marteilia refringens, Marteilioides chungmuensis and Microcytos mackini), the copepod Mytilicola orientalis, and at least one disease of unknown etiology (Mann et al. 1991). The first imports of C. gigas to France coincided with a viral epizootic which largely wiped out the then-dominant commercial oyster C. angulata (Portuguese Oyster), but the origin of this disease is unknown (Grizel and Heral 1991). At least 8 species of macrorganisms have been introduced to French waters with C. gigas, and at least 6 in West Coast waters, these include macroalgae, flatworms, barnacles, snails, clams, etc. Some of these species have had serious negative impacts (Cohen and Carlton 1995; Grizel and Heral 1991; Mann et al. 1991).
Legally sanctioned introductions of C. gigas will follow ICES (International Council for Exploration of the Seas) guidelines, intended to minimize the accidental introduction of parasites or other organisms (Mann et al. 1991; Carlton 1992). The oysters which would be introduced have been reared in quarantine conditions for 5 generations without detectable diseases and parasites (Kern 1998). Most of the parasites listed above, except for bacteria and viruses can be assumed to be absent, since they would have been detected in the first generation (Mann et al. 1991). However, one potential risk of official introductions is that they may encourage illegal plantings, without these precautions.
Additional potential impacts of a large-scale C. gigas introduction include:
Competition - Introductions of new oyster species, usually have been motivated by the decline of the previously dominant oyster due to overfishing or disease, but in some cases they have led to further damage to the remaining populations. Introductions of C. angulata in France coincided with the decline of the native Ostrea edulis (European Flat Oyster) in the 19th century (Galtsoff 1932), the replacement of C. angulata by C. gigas in the 1970's seems to have largely a consequence of a disease of unknown origin (Grizel and Heral 1991). In Australia, competition with C. gigas is considered a threat to the native Saccostrea commercialis (Sidney Rock Oyster) (Mann et al. 1991). However, the failure of numerous introductions on the Atlantic Coast (Andrews 1980), high non-disease mortality in flume trials with York River water (Barber and Mann 1994; Chu 1996), and heavy infestations of Polydora sp (DeBrosse and Allen 1996; Kavanaugh 1941) suggest that this oyster may be a poor competitor with C. virginica in Atlantic coast estuaries. In any case, the physiological requirements of C. gigas would restrict this species to the lower parts of Chesapeake Bay (Gottleib and Schweigkofer 1996; Mann et al. 1991; Krantz 1992).
Hybridization - Eggs and sperm of C. gigas and C. virginica readilty produce living larvae (Galtsoff 1932), and gametic reproductive isolating mechanisms appear to be weak or absent. However, few of these hybrids survive to metamorphosis. The consequences of hybridization thus appear to be a waste of gametes for both species, rather than introgression. This 'gametic warfare' may be one cause of the failure of C. gigas introductions in the native range of C. virginica. Hybridization with C. gigas is not considered to be a major threat to C. virginica populations (Gaffney and Allen 1992).
Habitat Change - Maintenance and restoration of oyster bed habitat and filtering abilities is regarded as a major potential benefit of C. gigas' introduction, as compensation for the decline of C. virginica populations (Gottleib and Schweigkofer 1996; Mann et al. 1991). Gottleib and and Schweigkofer's discussion emphasizes the desirability of re-establishing a large filter-feeding biomass to control increasing phytoplankton biomasses. However, these benefits would be limited to the more saline parts of Chesapeake Bay (Gottleib and Schweigkofer 1996; Mann et al. 1991).
References - Andrews 1979a; Andrews 1980; Barber and Mann 1994; Burreson 1998; Cohen and Carlton 1995; Chu 1996, DeBrosse and Allen 1996; Friedman 1996; Gaffney and Allen 1992; Galtsoff 1932; Gottleib and Schweigkofer 1996; Grizel and Heral 1991; Katkansky and Warner 1970; Kavanaugh 1941; Kern 1976b; Kern 1998; Krantz 1992; Mann et al. 1991
Ecological Impacts on Other Chesapeake Non-Native Species
Established populations of Crassostrea gigas (Pacific Oyster) do not yet exist in Chesapeake Bay. However, unofficial plantings of this oyster to the mid-Atlantic region are considered a likely vector for introduction of Haplosporidium nelsoni(MSX, Delaware Bay Disease) (Andrews 1979a; Andrews 1980; Kern 1998; Burreson et al. 2004 ).
Parasitism - Crassostrea gigas' (Pacific Oyster's) chief impact in Chesapeake Bay is as a probable vector for introduction of Haplosporidium nelsoni (MSX, Delaware Bay Disease) (Andrews 1979a; Andrews 1980; Kern 1998; Burreson et al. 2000). C. gigas serves as a host for one or more haplosporidian species in the Pacific, but these parasites are present only at very low prevalences, and do not seem to be a major cause of mortality (Friedman 1996; Katkansky and Warner 1970; Kern 1976b). In flumes of York River water, C. gigas had no detectable H. nelsoni infections (Barber 1996). This oyster is also highly resistant to Perkinsus marinus (Dermo), with no infections occurring in natural water (Barber 1996), and only light ones following artifical exposure (Barber and Mann 1991).
References - Andrews 1979a; Andrews 1980; Barber 1996; Barber and Mann 1991; Friedman 1996; Katkansky and Warner 1970; Kern 1976b; Kern 1998
References
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Barber, Bruce J. (1996) Gametogenesis of Eastern oysters, Crassostrea virginica (Gmelin, 1791), and Pacific oysters, Crassostrea gigas (Thunberg, 1793) in disease-endemic lower Chesapeake Bay, Journal of Shellfish Research 15: 285-290
Barber, Bruce J.; Mann, Roger (1994) Growth and mortality of Eastern oysters Crassostrea virginica (Gmelin 1871) and Pacific oysters Crassostrea gigas (Thunberg 1793) under challenge from the parasite Perkinsus marinus, Journal of Shellfish Research 13: 109-114
Burreson, E.M.; Stokes, N.A.; Friedman, C.S. (2000) Increased virulence in an introduced pathogen: Haplosporidium nelsoni (MSX) in the Eastern Oyster Crassostrea virginica., Journal of Aquatic Animal Health 12: 1-8
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Calvo, Gustavo; Luckenbach, Mark W.; Allen, Standish K.; Burreson, Eugene M. (1999) Comparative field study of Crassostrea gigas (Thunberg, 1793) and Crassostrea virginica (Gmelin 1791) in relation to salinity in Virginia., Journal of Shellfish Research 18: 465-473
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