By Chris Lim
Birds do it. Bees do it. But how do oysters do it?
How does an animal reproduce when it spends its adult life attached to a hard surface and unable to move? An oyster has no eyes to see its potential mates. Oysters have no calendar or clock to coordinate a mating time. To make the story even more interesting, Olympia oysters (Ostrea lurida), the only oyster native to the West Coast and thus San Francisco Bay, is a hermaphrodite, meaning they have both male and female sex organs.
So why would an oyster need both sex organs and how does that help an oyster continue to reproduce generation after generation? Olympia oysters begin their lives as males and sexually mature in about a year. Then the oysters change to female after spawning. A regular alternation of sexes between male and female apparently continues throughout life. This alternating lifestyle is advantageous because at any given time an oyster should have potential mates nearby.
When the waters of the San Francisco Bay estuary warm in the summer, male Olympia oysters release sperm balls and on contact with seawater, the sperm balls disintegrate, releasing the sperm. Other male oysters sense this and soon follow suit, releasing their sperm balls into the water. A male oyster releases hundreds of thousands of sperm balls, each containing approximately 2,000 sperm. Females then bring the sperm into their shells through respiratory action and fertilize their eggs internally. The fertilized eggs develop inside the female for about 10 days before being released into the environment.
The larvae then become part of the planktonic community, floating with the currents and tides. After approximately three to four weeks, the larvae metamorphose to their juvenile form and are ready to settle on and attach to a hard substrate, such as oyster reef balls, rip rap along the shoreline, or scientific monitoring devices. If the oysters are able to find a hard substrate (which can be difficult and thus the reason for our native oyster restoration efforts), they attach themselves and will hopefully remain there to live out their lives.
If the attached baby oyster, or “spat,” can survive a number of factors–including predation, low salinity, exposure during high temperatures, and sedimentation–then the oyster should be able to produce the next generation of native oysters in the Bay who will also participate in the odd and unique sexual life of oysters!
Photo credits (from top): Robert Sisson, National Geographic; Robert Downey, Pacific Coast Shellfish; Chris Lim, The Watershed Project
