The usual answer is that oysters eat plankton.
Here we have some magnified phytoplankton and zooplankton, aka oyster food [1]. If you've never had the pleasure, be warned that looking through a microscope at live seawater is terrifying, like descending into an intergalactic horror story:
Some tasty looking diatoms...
...and a few microscopic monsters. (Although the green dude on right is clearly a plant).
The word 'plankton' derives from a Greek word meaning “to wander” and encompasses a big group of stuff that drifts around in the ocean. Microalgae is a common type, and if there’s enough algae in the water you can see it, like with this noctiluca bloom a few years back [2]:
The more algae in the water, the more food for the oysters (usually). If the water is super clear, it’s generally Christmas Dinner at the Cratchits.
To rephrase our question and ask, "what do you feed oysters?" illustrates why shellfish farming is sustainable: We don't feed them anything [2.5]. The tide comes in and the oysters eat. A few hours later, the tide goes out and we harvest. This repeats twice a day, every day, in a cycle older than dirt. There's no getting up at 5 am to milk the cows. No arranging someone to toss hay to the goats when we leave the farm. No overharvest of a bait fishery, or worries about protein conversion rates. Shellfish farming requires "nothing more than seawater and sunshine."
This is not to say that we don't think about what they eat. We do, pretty much constantly. Diet determines their flavor, growth rate, seasonality, shell strength, safety, and even color. Some kinds of algae are harmful to humans [3]. An oyster can starve in an estuary full of the wrong kind of food. We wonder if the coccolithophore we've had the past three years imparts a chalky flavor, and suspect the blooms are interfering with oyster reproduction [4]. It's just that we can't control what they eat... we can only change our farming and harvest practices in reaction.
Left, hungry, wee-bit skinny. Right, well-fed oyster, fat and happy. These photos were taken on the same day, but the oysters are from different estuaries.
But if you only say "oysters eat plankton," you're leaving out a lot of the story. An oyster food diary would show that they also consume dissolved organic material like leaf litter from upstream forests or small pieces of sea grasses [5]. This is one reason why location is so important to oyster flavor: they are what they eat, and they can't move to greener pastures. It's also why economics and environment are closely linked on an oyster farm. Anything that cleans up storm water runoff or improves water treatment facilities protects both the shellfish industry and the overall health of the ecosystem.
Final answer: oysters are sedentary filter feeders that dine like well-behaved children: they'll eat what they get, and they're [usually] happy about it. [6].
Q: What’s for dinner? A: Food.[7]
Notes
1. These photos come from a microscope on loan from Sound Toxins, a Sea Grant program that works as an early warning system for harmful algal blooms. They were taken by our good pals and former colleagues Tiffany & Brady.
2. It’s red! It’s a tide! IT’S NOT RED TIDE. Noctiluca is harmless and super cool… it’s bioluminescent, baby… and the real bad boys on the street (alexandrium, psuedo nietsche, etc) aren’t red. Who named it red tide?
2.5. Jumping in here to say that oysters are fed as juveniles in hatcheries. Much of the work in a hatchery centers around growing algae in tanks, and getting the food mixture just right for happy mommy, daddy, and baby oysters. Also, about the tides: know that not every place on earth has two tides a day. It's pretty complicated. Read this beautiful book to learn more!
3. We're fortunate to farm in an incredibly clean estuary in Washington State, which has an extensive monitoring program to warn shellfish communities about toxic algae blooms far in advance.
4. Total guess. But we’re sticking with it until someone informs us otherwise. And also: these cocco blooms, and the fact that our grandpa never saw one, illustrate one small part of the human struggle: It’s hard to tell if things are scary changes or normal [long term] trends. (This is why we need science.)
5. We could list some unappetizing stuff here, alongside this poetic flora, but we’re not gonna.
6. Of course, the obvious drawback to this is that sometimes it pays to be a picky eater. For more info read about the history of the New York oyster industry. Moral of that story: trust your gut.
7. Direct quote from Hama Hama Oyster Mama the 2nd, circa 1991.