Most oysters reproduce by broadcast spawning: when the water warms, males and females release sperm and eggs into the water, and fertilization happens there. A single female Eastern oyster can release over 100 million eggs in one spawning event.[1] The larvae drift for about two to three weeks, then cement themselves to a hard surface, usually an oyster shell, and stay there for life.[2][3] Many oysters start out male and become female as they get older.[1]
Not every oyster does it this way. Flat oysters such as the Olympia and the European flat hold their eggs inside the shell and brood the young. Both methods are covered below, along with why spawning is the reason some oysters taste thin in summer.
What triggers spawning
An oyster spends weeks getting ready. In early spring, rising water temperature, together with changes in salinity or in the amount of plankton in the water, prompts it to start building gonad, the tissue that becomes eggs or sperm. That build-up can take up to two months.[3]
Once oysters are ripe, they need the right conditions to release. Eastern oysters prefer to spawn in water between 68°F and 86°F (20°C to 30°C) and at salinity above 10 parts per thousand.[3] In the Chesapeake Bay that means early summer.[2]
Spawning is contagious. It takes one oyster releasing into the water to set off its neighbors: oysters nearby filter in some of the sperm, detect it, and begin releasing their own eggs or sperm.[3] In Pacific oysters, males release sperm by clapping their shells together, often when the water is warm and the tide is coming in, and ripe females nearby then release their eggs.[4] Because oysters live packed together on reefs and farms, this chain reaction puts enormous numbers of eggs and sperm into the same water at the same time.
How many eggs an oyster releases
| Oyster | Eggs released | Source |
|---|---|---|
| Eastern oyster (Crassostrea virginica) | Over 100 million in one spawning event | NOAA Fisheries[1] |
| Pacific oyster (Crassostrea gigas) | 50 to 200 million | NOAA Fisheries[4] |
Larger oysters produce more eggs and sperm than smaller ones.[3] The numbers are high because the odds for any one egg are poor. Larvae are eaten by anemones, sea nettles, and other filter feeders, and newly settled oysters are eaten by flatworms, mud crabs, and blue crabs.[2]
Male first, female later
Oysters do not stay one sex for life. Eastern oysters first mature as males and later develop female reproductive capability.[1] In the Chesapeake Bay, most oysters under a year old are male and most older oysters are female.[2] Pacific oysters follow the same pattern: they are male in their first year, and some later develop female organs while the male organs shrink.[4]
At any given moment a cupped oyster of the genus Crassostrea, which includes the Eastern, Pacific, and Kumamoto, is either male or female, though it may switch after a breeding season. Flat oysters of the genus Ostrea are hermaphrodites.[5] The Olympia oyster alternates between sexes.[6]
Oysters also start young. They can spawn within their first year, although in Maryland's part of the Chesapeake most reach prime spawning size by about three years old.[3]
The oyster life cycle, stage by stage
| Stage | Timing | What happens |
|---|---|---|
| Spawning | Warm months | Eggs and sperm are released and meet in the water[3] |
| Free-swimming larva | Within 24 hours of fertilization | The fertilized egg develops into a larva that swims[2] |
| Planktonic larva | About two to three weeks | Larvae drift with the currents, feeding on plankton; they can move up and down in the water but cannot swim horizontally[2][3] |
| Pediveliger | Around two weeks old | The larva grows a foot and crawls along the bottom looking for a hard surface[3] |
| Spat | At settlement | The larva cements its left shell in place, reorganizes its body into the adult form, and puts its energy into shell growth[2][3] |
| Juvenile | From one year old | Still growing; already capable of spawning[3] |
| Adult | From about three years old | About three inches long in the Chesapeake, the legal harvest size in Maryland[3] |
For Pacific oysters the drifting stage lasts about 20 to 22 days before the larvae settle on a shell, rock, or other hard surface.[4]
Growth after that depends on the water. In the Chesapeake, oysters typically grow up to an inch a year, faster in saltier water.[3] Eastern oysters can live up to 20 years.[1]
The settling step is why oysters form reefs. Larvae usually settle on oyster shell, so oysters end up attached to one another in dense reefs.[2][3] It is also why shell recycling matters: returned shell gives larvae something to land on. See what to do with oyster shells.
Oysters that brood their young
The Olympia oyster, Washington's only native oyster species, does not release its eggs. Males release sperm into the water, and the eggs are fertilized inside the female's mantle cavity. She broods the developing larvae inside her shell for about 10 to 12 days, then releases them as shelled, swimming larvae. Olympias generally spawn between May and September.[6]
Other flat oysters of the genus Ostrea fertilize and hold their eggs inside the shell in the same way.[5] On a menu that includes the Olympia and the European flat, sold as Belon or simply as European Flat.
Why spawning changes how an oyster tastes
Reproduction is the reason for the old advice about eating oysters only in months with an R in them.
Before spawning, an oyster pours its stored energy into eggs or sperm, and the gonad can make up as much as half of the weight of its soft tissue. After spawning, the meat softens and looks watery, and it is generally considered poor eating.[7] The Chesapeake Bay Program gives this as one of two reasons for the R-month rule, the other being that oysters spoiled in warm months before refrigeration.[2] Our article on the R month myth covers how much of that still applies, and oyster season by region shows when each coast is at its best.
Farmers have a way around the problem. Triploid oysters have three sets of chromosomes instead of two and are considered sterile, with poor gonad development. The energy a normal oyster would spend on reproduction goes into growth instead, so a triploid keeps its meat quality through the summer.[7] That is a large part of why you can order good oysters in July. See what are triploid oysters for how they are bred.
How hatcheries use the life cycle
Oyster hatcheries run the same life cycle under controlled conditions. Larvae are raised in tanks and fed algae. After two to three weeks they settle onto crushed or whole oyster shell, and the resulting spat are held in tanks for a few more weeks before being moved to grow-out sites on farms.[1] From that point the oysters feed themselves by filtering the water.[1]
Our guide to how oysters are farmed follows the oyster from that point to harvest, and the regional guides for the East Coast, West Coast, and Gulf Coast show what those farms produce.
Frequently Asked Questions
How do oysters reproduce?
Most oysters reproduce by broadcast spawning. When the water warms, males release sperm and females release eggs into the water, where fertilization takes place. The fertilized eggs become swimming larvae within about a day, drift for two to three weeks, and then attach permanently to a hard surface as spat. Flat oysters such as the Olympia instead fertilize and brood their eggs inside the shell.
How many eggs does an oyster produce?
A female Eastern oyster can release over 100 million eggs in a single spawning event, according to NOAA Fisheries. A female Pacific oyster can release 50 to 200 million. Many of the larvae are eaten by filter feeders before they can settle.
Are oysters male or female?
Both, at different times. Eastern and Pacific oysters usually mature first as males and many later become female, so most young oysters are male and most older ones are female. Flat oysters such as the Olympia are hermaphrodites that alternate between sexes.
When do oysters spawn?
Oysters spawn when the water is warm. Eastern oysters prefer water between 68°F and 86°F, which in the Chesapeake Bay means early summer. Olympia oysters in Washington generally spawn between May and September. Timing depends on local water temperature.
How long does it take an oyster to grow?
In the Chesapeake Bay an oyster typically grows up to an inch a year and reaches the three-inch legal harvest size in about three years. Growth is faster in saltier water. Oysters can spawn within their first year, well before they reach market size.
Can you eat oysters when they are spawning?
It is a question of quality rather than a separate safety rule. An oyster that has just spawned is thin, soft, and watery, and is generally considered poor eating. Farmed triploid oysters are considered sterile and keep their meat quality through summer. Food safety in warm months is a different issue, covered in our guide to Vibrio and raw oysters.
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