What is an Olympia Oyster?
The Olympia oyster (Ostrea lurida
, formerly O. conchaphila
), also known as the native oyster, is the only species of oyster native to British Columbia. The more commonly seen oyster, the Pacific or Japanese oyster (Crassostrea gigas
), was imported for commercial harvest in the early 1900’s and is now widespread along the coast. Olympia oysters are far less commonly found.
Oysters are bivalve mollusks. This class of animals also includes clams, mussels and scallops. Compared to the imported oyster species, Olympia oysters are small, usually less than 60 mm in length, and relatively flat. They are irregularly shaped, and roughly elliptical or circular. The outer shell ranges in colour from white to purplish; the inner shell may be white, iridescent green or purple.
Where do Olympia oysters live?
Olympia oysters are found in estuaries, salt water lagoons, tidal flats and protected areas such as pocket beaches. They are commonly found attached to cobble or other shells but can also attach to submerged underwater structures such as floats and pilings. Olympia oysters live only along the west coast of North America, from northern BC to Panama. However, they are no longer abundant and occur in locally isolated areas.
Some of the largest known populations of Olympia oysters live right here, near the heart of downtown Victoria. Olympia oysters are found primarily in the Gorge Waterway and Portage Inlet. They grow on hard substrate (rock or man-made debris) in the Gorge but are also found growing on clumped oyster shells in Portage Inlet where the substrate is silty sand.
How do Olympia oysters live and reproduce?
Like other bivalves, oysters are filter feeders: they eat microorganisms (dinoflagellates, diatoms, bacteria, etc.) and decaying plant and animal matter that are brought to them by ocean currents
Unlike most bivalves, Olympia oysters are brooding oysters. Female oysters hold their eggs in the mantle cavity inside their shell where fertilization and development occur. Nearby male oysters produce sperm balls, which rupture to release spermatozoa when they contact the sea water. Spermatozoa are dispersed by currents and drawn into the female’s shell during respiration. Fertilized eggs develop into free-swimming larvae that are released from the female a couple of weeks later. After another one to two weeks the larvae settle to the bottom and attach to a hard surface where they will spend the rest of their lives; this process is called spatting.
After about 22 to 30 weeks, the newly ‘spatted’ oysters reach sexual maturity. Olympia oysters are alternating hermaphrodites, meaning they switch between having female and male reproductive organs throughout their lives. In warm conditions, as many as three sexual phase changes may occur in a year. Oysters in a given area undergo these changes at different times, depending on when they settled as larvae.
How have people used Olympia oysters?
Olympia oysters were an important food source for First Nations in BC throughout their history. Oysters were harvested by First Nations around Victoria and sold to the European colonists in the newly formed Fort Victoria. From about 1884 to 1930, native oysters were harvested on a commercial scale in many areas. However, as early as 1913 over-fishing was a concern, and the fishery subsequently declined. Larger and faster-growing species of oysters were imported for aquaculture, and today Olympia oysters are not harvested commercially.
Why are Olympia oysters important?
Olympia oysters are food for many animals such as crabs, sea stars, gastropods (e.g. snails and limpets) and birds including white-winged scoters, black scoters and the greater scaup. All of these birds are found in the CRD harbours.
Like other filter feeding animals, Olympia oysters help to clarify the water. This allows more sunlight to reach marine plants such as kelp and eelgrass, which in turn provide nutrients and habitat for many other species. In clear water, fish are able to respire more effectively, with less sediment to clog their gills, and mammals such as harbour seals and otters can find their prey more easily. Providing people do not overload the water with contamination, oysters can continually provide this important cleaning service for free!
What threatens Olympia oysters?
In 2003 Olympia oysters were listed as a Species of Special Concern under the Canadian Species At Risk Act (SARA), due to their low population numbers and sensitivity to human activities and natural events. Threats to Olympia oysters include:
- Unusually cold or hot temperatures that can exceed the native oyster’s range of tolerance, and cause reduced reproduction or death
- The eastern oyster drill, an introduced snail-like gastropod, that has caused significant mortality in Olympia oysters in some areas
- Excessive sediment, introduced into the marine environment through streams or coastal dredging activities, which can smother oysters
- Pollution such as heavy metals, oils, pesticides and household chemicals may damage or kill oysters; these can be washed off roads or disposed of down storm sewers
As oysters are filter feeders, they can accumulate toxins and bacteria that pose risks for human consumption.
How can I help protect Olympia oysters?
For information on protecting Olympia oysters, please visit our How Can I Help?
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