Magallana gigas (Thunberg, 1793) Common name(s): Japanese oyster, Pacific oyster, Giant Pacific oyster, Miyagi oyster, Portuguese oyster, Pacific  cupped oyster, Immigrant oyster, Giant oyster, Giant cupped oyster
Synonyms:  Crassostrea gigantea, C. gigas, C. laperousii, C. posjetica,C. talienwhanensis, Dioeciostrea hispaniola, Lopha posjetica, Ostrea chemnitzii var. elongata, O. cymbaeformis, O. gigas, O. gravitesta, O. laperousii, O. posjetica, O. rostralis, O. talienwhanensis
Phylum Mollusca  Class Bivalvia   Subclass Pteriomorpha    Order Ostreoida     Suborder Ostreina      Family Ostreidae
	Crassostrea gigas group from Padilla Bay.  One large individual is on the bottom, and at least 4 smaller individuals are attached to its top (right) valve. Scale is inches.
	(Photo by: Dave Cowles, July 2005)

How to Distinguish from Similar Species: This is the largest local oyster; and usually the most commonly encountered one.  The other introduced oyster (Crassostrea virginica) and the native Olympic oyster (Ostrea lurida) do not have the conspicuous frills on the valves and grow to no larger than 15 cm in height.

Geographical Range: Widely introduced on the west coast (and worldwide) from Japan.  Prince William Sound, AK to Newport Bay, CA.  Scattered in many bays where oyster culture has taken place. Very common in Willapa Bay on the SW Washington coast, and in Departure Bay and the Georgia Strait in British Columbia.

Depth Range: Intertidal to 6m

Habitat: Firm sediment or rocky beaches

Biology/Natural History: The shape of this oyster is quite variable.  The species was introduced from Japan in the early 1900's (to Washington and British Columbia in 1922) and is the most important aquacultured species on the US West Coast.  The japanese littleneck clam Tapes japonica and the Japanese oyster drill Ceratostoma inornatum, as well as the intestinal parasitic copepod Mytilicola orientalis were apparently introduced to our coast along with this species.  May live 20 years or more.  Often contains irregular, non-lustrous pearls.  Predators include predatory oyster drill snails, the crabs Cancer magister, Cancer productus, Cancer oregonensis, Hemigrapsus nudus, and H. oregonensis, some sea stars, and the black oystercatcher (bird).  The blue mud shrimp Upogebia pugettensis is a problem on commercial beds of this oyster because it digs sediment from its burrows and smothers the oysters with sediment.  The oyster typically attaches to hard substrates, such as rocks or the shells of other oysters.  The oysters are imported as very small individuals (spat) and raised in commercial oyster beds.  They are said to poorly reproduce in California, so are found only in the oyster beds.  I have seen many oysters in apparently natural conditions here in Washington, so they must be able to reproduce at least a bit better here.  Sexes are separate in this species, but an individual may change sexes (especially in the winter) and may alternate being male and female.  Natural populations in Japan tend to favor protandry, and the largest individuals are likely to be females, especially if they are alone instead of in an aggregation (Yasuoka and Yusa, 2016). A few are simultaneous hermaphrodites (both male and female at the same time).  Spawning occurs from May to October (Suquet et al., 2016). They outgrow native oysters, probably partly because they are more efficient filter feeders, and can feed on nannoplankton, which native oysters cannot do.  Occasionally they are influenced by a red tide (usually by the dinoflagellate Gonyaulax catanella) and become toxic to eat. Warm temperatures of 25 C or greater and salinities well over 39 ppt increase their production of heat shock proteins.  37 C is sublethal and 44 C is lethal (Yang et al., 2016).

Larvae of this species are very sensitive to increases in dissolved carbon dioxide in the water (Barton et al., 2012). Lower seawater pH triggers smaller size and reduction in survival in the larvae, but increased temperature of the seawater partly mitigates this effect.  However, this mitigation is accompanied with increased production of enzymes used to detoxify reactive oxygen species, suggesting that the larvae are still under increased metabolic stress when at lowered pH and increased temperature (Harney et al., 2016). Exposure to polystyrene microplastics seems to decrease reproductive potential in the species (Sussarellu et al., 2016).

References:

General References:
Brusca and Brusca, 1978
Harbo, 1997
Harbo, 1999
McConnaughey and McConnaughey, 1985
Morris, 1966
Morris et al., 1980
Niesen, 1997
Ricketts et al., 1985
Sept, 1999

Scientific Articles:
 Barton, Alan, Burke Hales, George G. Waldbusser, Chris Langdon, and Richard A. Feely, 2012.  The Pacific Oyster, Crassostrea gigas, shows negative correlation to naturally elevated carbon dioxide levels: Implications for near-term ocean acidification effects.  Limnology and Oceanography 57:3 pp 698-710

Charifi, Mohcine, Mohamedou Sow, Pierre Ciret, Soumaya Benomar, and Jean-Charles Massabau, 2017. The sense of hearing in the Pacific oyster, Magallana gigas. PLOS one 12(10): e0185353. https://doi.ort/10.1371/journal.pone.0185353

Harney, Ewan, Sebastien Artigaud, Pierrick Le Souchu, Philippe Miner, Charlotte Corporeau, Hafida Essid, Vianney Pichereau, and Flavia L.D. Nunes, 2016.  Non-additive effects of ocean acidification in combination with warming on the larval proteome of the Pacific oyster, Crassostrea gigas.  Journal of Proteomics 135: pp 151-161

Ruesink, Jennifer L., B.E. Feist, C.J. Harvey, J.S. Hong, A.C. Trimble, and L.M. Wisehart, 2006.  Changes in productivity associated with four introduced species: ecosystem transformation of a 'pristine' estuary.  Marine Ecology Progress Series 311: pp 203-215. doi 10.3354/meps311203

Salvi, D. and P. Mariottini, 2016. Molecular taxonomy in 2D: a novel ITS2 rRNA sequence-structure approach guides the description of the oysters' subfamily Sacconstreinae and the genus Magallana (Bivalvia: Ostreidae). Zoological Journal of the Linnean Society 179:2 pp 263-276. https://doi.org/10.1111/zoj.12455

Suquet, Mark; Florent Malo, Isabelle Queau, Dominique Ratiskol, Claudie Quere, Jacqueline Le Grand, and Christian Fauvel, 2016.  Seasonal variation of sperm quality in Pacific Oyster (Crassostrea gigas).  Aquaculture 464: pp 638-641

Sussarellu, Rossana, Marc Suquet, Yoann Thomas, Christophe Lambert, Caroline Fabioux, Marie Eve Julie Pernet, Nelly Le Goic, Virgile Quillien, Christian Mingant, Yanouk Epelboin, Charlotte Corporeau, Julien Guyomarch, Johan Robbens, Ika Paul-Pont, Philippe Soudant, and Arnaud Huvet, 2016. Oyster reproduction is affected by exposure to polystyrene microplastics. Proceedings of the National Academy of Sciences 113:9 pp. 2430-2435. www.pnas.org/cgi/doi/10.1073/pnas.1519019113

Yang, C.-Y.; M.T. Sierp, C.A. Abbott, Li Yan, and J.G. Qin, 2016.  Responses to thermal and salinity stress in wild and farmed Pacific Oysters Crassostrea gigas.  Comparative Biochemistry and Physiology A-Molecular & Integrative Physiology 201: pp 22-29

Yasuoka, Noriko and Yoichi Yusa, 2016.  Effects of size and gregariousness on individual sex in a natural population of the Pacific oyster Crassostrea gigas.  Journal of Molluscan Studies 82: pp 485-491

These two individuals were raised in a local oyster farm (Taylor Shellfish Farms). This view is of the right valves. A few barnacles are attached to the valves.

This ventral view shows the fluting which is common on the ventral end of the shell.  Some of the edges of the mantle of the living oyster can still be seen inside.

In this ventral view the valve has been opened further and the animal tissue removed. The brown hinge on the far (dorsal) side can be clearly seen.

In this lateral view, the dorsal end (hinge) is to the left and ventral to the right.  The right (flat) valve is on top and the left (cupped) valve is below.  The animal tissue has been removed.
 

In this view the valves have been fully opened.  The valve on the left is the left (cupped) valve which is normally attached to a substrate. The valve on the right is the right (flat) valve.  The broken hinge ligament can be seen on the right valve.  The large purple stain inside the shell is the attachment of the single large adductor muscle. Note that the valve height (vertical dimension in this photo) is greater than the length (lateral dimension in this photo), and that there are no winglike extensions near the hinge as are seen in scallops.