Oysters can change sex multiple times during their lives.
Source: Original photo by ES3N/ iStock
Next Fact

Oysters can change sex multiple times during their lives.

Within 12 hours of their birth, oysters begin pulling calcium out of the water to create their signature shells. For the first few weeks of their lives, these newborn bivalves zoom around in a current until they eventually settle on some hard substrate, whether it’s a rock, pier, or another oyster. This place of protection is where the oysters will spend the rest of their lives (which can be as long as 20 years). Eventually, usually a year after birth, it’ll be time for the oysters to breed, and that’s where things get interesting. 

Only oysters can make pearls.
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It's a Fib
When debris gets caught inside an oyster so that it can’t be flushed out, some species encase the unwanted irritant in a material called nacre, also known as mother-of-pearl. Although oysters make pearls, most commercial pearls come from saltwater clams or freshwater mussels.

Although born male, oysters have the impressive ability to switch their sex, seemingly at will. Every season, females can release up to 100 million eggs, and the amount of sperm released is so high it’s essentially incalculable. Once the egg and sperm are released, the oysters rely on pure chance for fertilization to take place, as the egg and sperm meet in the open water. Because any resulting larvae are extremely vulnerable to predators (especially filter feeders), oysters have evolutionarily compensated by being one of the most virile and sexually flexible species in the world — meaning that their ability to change sex likely evolved as a matter of survival. This impressive fecundity means that natural oyster reefs can grow to tremendous size; as little as 10 square feet of reef can house up to 500 oysters. Scientists theorize that water temperature could play a role in triggering whatever causes an oyster to change its sex, but many aspects of the process remain a mystery. 

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Numbers Don’t Lie
Length (in inches) of the world’s largest oysters
Amount of energy an oyster bed can absorb from a wave, protecting coastlines from storms
Gallons of water a 1-acre reef of oysters filters every day
24 million
Number of species of oysters found in U.S. waters
Juvenile oysters are known as _______.
Juvenile oysters are known as spat.
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Think Twice
New York City was once the oyster capital of the world.

Before the 17th century, the island of Manahatta (as the Indigenous Lenape called it) was absolutely inundated with oysters. With their impressive filtering abilities, these oysters kept the surrounding estuary clean, and they also became a staple of the Lenape diet. When Henry Hudson’s ships sailed the river that would one day bear his name in 1609, the New York estuary was estimated to be home to 350 square miles of oyster reef — roughly half the world’s entire oyster population. The original names for Ellis and Liberty islands were “Little Oyster Island” and “Great Oyster Island,” respectively, and one of the oldest streets in Manhattan — Pearl Street — is named after an Indigenous oyster shell midden located along the shore (it was later paved, fittingly, with oyster shells). New Yorkers also began eating lots of oysters, upwards of 1 million a day at the industry’s height, while also shipping millions abroad. Sadly, overharvesting and environmental degradation caused oysters to severely decline in New York’s waters, and by 1927 they were deemed too contaminated to eat. Today, groups are reintroducing oysters to New York Harbor, and wild populations are beginning to return. Although these oysters are already hard at work cleaning the estuary while providing important aquatic habitats, it’ll likely be a century until New York oysters are once again safe for human consumption.

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