Chesapeake Bay Introduced Species Database

Crassostrea gigas

Common name(s):
Pacific Oyster
The Pacific Oyster is native to the Indo-West Pacific and is the most widely transplanted shellfish in the world, introduced to at least 42 countries. Incredibly there are no established populations in western Atlantic waters, in spite of illegal or unofficial introductions in Atlantic waters near Chesapeake Bay and Delaware Bay. But the introduction of Pacific Oysters was considered as a possible means of replacing or supplementing native stocks of the Eastern Oyster (Crassostrea virginica), which has been devastated by disease and overharvest. Both the Pacific Oyster and the Chinese River Oyster (C. ariakensis) were investigated for possible introduction until 1998 following research in the Bay, which including the stocking of sterile oysters. But initial studies found that the Chinese River Oyster had better growth and survival under East Coast conditions than the Pacific Oyster, and so further research and political interest shifted to the Chinese River Oyster. Ironically, early plantings of the Pacific Oyster in the 1950s in Delaware Bay are one of the possible means of introduction of MSX (Haplosporidium nelsoni), one of the diseases that lead to the decline of Eastern Oysters. The Pacific Oyster was, however, successfully introduced to Puget Sound WA in 1902 to replace the Olympic Oyster (Ostreola conchaphila), which was devastated by overfishing.
Image courtesy of Paul Fofonoff.

Economic Impacts

Fisheries; Aesthetics; Habitat Change
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 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

Impacts on Natives:
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
Impacts on Non-natives:
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

This data was last modified on Tuesday, February 4th, 2014.
Databases Overview| Chesapeake Bay |