The Olympia oyster, Ostrea lurida, is the only oyster native to the West Coast of the United States. Around the turn of the 20th century, Olympia oysters were heavily harvested from Washington and Oregon for a booming San Francisco market. These small oysters were renowned for their distinctive, coppery flavor. A combination of unsustainable harvest, habitat degradation, pollution, and disease has caused the Olympia oyster population to plummet and remain very low.
Oysters are bivalves, a type of mollusk characterized by two opposing shells, or valves. They are related to clams, mussels, and other commonly known and often edible mollusks. They feed by filtering small particles from seawater. Many oysters, like other bivalves, release sperm and eggs separately in the water, where they meet and fertilize to form embryos outside the body of the mother. But Olympia oysters retain eggs within the mother’s shell. They “brood” their embryos for several weeks before releasing the young, now called larvae, into the water column. All oysters and most bivalves produce larvae, which are generally less than a millimeter in length. The larvae swim, eat, and develop in the water for several weeks to several months. They then search for a hard surface on which to settle and metamorphose into a juvenile oyster. Once settled, they are cemented to the substrate and remain there for the rest of their lives.
Oysters are important in regulating the health and diversity of the estuarine ecosystem. Because they feed by filtering phytoplankton and bacteria from seawater, a sizeable oyster population can reduce the amount of algae in the bay and help control water quality. The beds that form when oysters settle near each other help stabilize the muddy bottom of the estuary and may improve habitat conditions for eelgrass, an important estuarine plant. The hard, complex surfaces provided by groups of oysters provide a unique habitat in which other estuarine animals can hide, settle, or lay eggs. In this way, a substantial oyster population could increase species diversity. A number of restoration efforts are currently underway to restore Olympia oyster beds along the west coast of the US.
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Ostrea lurida is a species of oyster that occurs on the Pacific coast of North America. This bivalve is approximately 6 to 8 centimetres (2.4 to 3.1 in) in length. The shell can be rounded or elongated and is white to purplish black and may be striped with yellow or brown. Unlike most bivalves, the Olympia oyster's shell lacks the periostracum, which is the outermost coating of shell that prevents erosion of the underlying shell. The color of the oyster's flesh is white to a light olive green. Ostrea lurida lie with their left valve on the substrate, where they are firmly attached. Unlike most bivalves, the oyster does not have a foot in adulthood. They also lack an anterior adductor muscle and do not secrete byssal threads, like mussels do. Olympia oysters, like other bivalves are filter feeders. This means that they filter their surrounding water and screen out the phytoplankton they feed on. Olympia's filter between 9 to 12 quarts of water each day. This is an essential function to keeping marine waters clean. Their beds also provide shelter for anemones, crabs, and other small marine life. This species has been recovered in archaeological excavations along the Central California coast of the Pacific Ocean, demonstrating it was a marine species exploited by the Native American Chumash people.
Ostrea lurida live in estuaries, streams and small rivers. At higher elevations they will live in areas bordered by mudflats and in eel grass beds at lower elevation. The oysters attach to the underside of rocks or onto the shells of old oyster beds. Their habitats must have water depths of 0–71 meters, ranging in temperatures of 6-20 degrees Celsius, with a salinity above 25 ppt. However, the oysters can survive in areas with streams that cause a flux in the salinity. This flux will in fact protect them from parasitic flukes that cannot survive the change in salinity. This is the native oyster to the Puget Sound. It Ranges from Baja California to Southern Alaska.
The Olympia Oyster spawn between the months of May and August, when the water reaches temperatures above 14 degrees Celsius. During the oyster's first spawning cycle they will act as a male and then switch between sexes during their following spawning cycles. The males release their spermatozoa from their mantle cavity in the form of sperm balls. These balls dissolve in the water into free floating sperm. The female's eggs are fertilized in the mantle cavity (brooding chamber) when spermatozoa are filtered into her gill slits from the surrounding water. The fertilized eggs will then move into the branchial chamber (mantle cavity). The fertilized eggs will develop into veliger larvae and will stay in the females mantle cavity for 10–12 days for further development. On the first day the larvae develop into a blastulae (mass of cells with a center cavity), on day two they develop into a gastrulae (hollow two layered sac), on the third day they develop into trochophore (free-swimming, conciliated larvae), on the fourth day the valves on the dorsal surface become defined. During the rest of development in the brooding chamber the valves complete and a straight-hinged veliger larva grows.
When the spat (larvae) leave the brooding chamber, they begin to develop an eye spot and a foot. They then migrate to hard surfaces (usually old oyster shells) were they attach by secreting a "glue" like substance from their byssus gland. Ostrea lurida spat swim with their foot superior to the rest of their body. This swimming position causes the larvae to attach to the underside of horizontal surfaces.
Brood size is between 250,000-300,000, with larvae around 187 micromillimeters long and eggs around 100–105 micromillimeters in diameter. The amount of larvae produced is dependent on the maternal oyster's size and the amount of reserved nutrients she has at the time of egg fertilization.
The once thriving Olympia Oyster has been endangered by pollution from mills and outboard motors. Highway construction and over-harvesting has also affected their substrate by creating a lot of silt. This abundance of silt smothers the oysters. Over-harvesting also takes away the old shells that spat need to grow on.
They oysters are preyed upon by animals such as sea ducks and rock crabs (Cancer productus). They are also affected by a parasitic red worm (which lives in their anus), the Japanese oyster drill, the slipper shell (that competes for space and food), and shrimp. The ghost and blue shrimp stir up sediment that smothers the oysters.
Status and conservation
Ostrea lurida is much more stable now due to the action of conservancy associations and new laws. These have worked to put a stop to the pollution from mills and create restrictions to prevent over-harvesting. During the harvesting seasons, people with permits now have to shuck their oysters on the beach to keep from depleting the oyster beds that the spat grow on.
There is still a market for Olympia Oysters in which farms commercially grow and sell them. This helps prevent the depletion of the native Ostrea lurida.
|This article has an unclear citation style. (March 2012)|
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