Pacific oyster

Magallana gigas


Photo credit: ©Guy Baker
Magallana gigas – Pacific oyster
  • It is irregular in appearance. It is off-white to yellow to blue-grey in colour; deep purple patches may occur. It may grow up to 30cm in a teardrop shape. The right valve is deeply cupped with 6-7 bold ribs, compared to the flat or slightly convex left valve.
  • It is permanently attached to hard substrates in intertidal and shallow subtidal zones of estuaries and coastal waters. If in muddy or sandy areas, they will settle on small rocks or other oysters, and may create reefs by cementing their shells to each other.
  • Download N.I.E.A. ID guide

Origin and Worldwide Distribution:
  • It is native to Japan.
  • It is commercially farmed in numerous locations in Northern Ireland/Ireland waters which accounts for the escapees now being present in all of our major sea loughs.

Potential or Known Impacts:
  • This species has the ability to be fast reproducers depending on the water temperature (about 18ºC in summer); individuals begin life as males and can transform into females.
  • The larvae are planktonic for up to 4 weeks. Adults may be predated by birds and large crustaceans.
  • The adults encrust many hard surfaces, including rocky surfaces and other shells.

How did it get here?
  • It may be introduced through oyster farms, water column transportation, hull fouling, and intentional/illegal introductions.

Is it found in Ireland or Northern Ireland?
  • It has been recorded in Northern Ireland in Lough Foyle, Strangford Lough and Carlingford Lough. It has also been recorded in Ireland.

You can help by reporting any sightings:

Further Resources:
Methods for Prevention:
  • Ensure that it is not spread from already invaded areas.
  • Ensure that equipment is checked, cleaned and dried before putting into another body of water.
  • Report all sightings. 

Species Related Files:

Invasive Species Ireland

Invasive Species Ireland