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.
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