Oysters eat phytoplankton: microscopic, single-celled algae that drift in seawater. They are filter feeders, so they do not hunt or graze. An oyster opens its shell, pumps water across its gills, and strains out the food. Along with algae it takes in other small bits of organic matter suspended in the water. Farmed oysters eat exactly the same thing as wild ones, because nobody feeds them.
That diet is also a large part of why oysters from different places taste different.
What is on the menu
| Life stage | What it eats | Where the food comes from |
|---|---|---|
| Larva (first two to three weeks) | Phytoplankton | The water column, where larvae drift and swim. In a hatchery, cultured algae. |
| Spat and juvenile | Phytoplankton | Filtered from the surrounding water once the oyster has attached |
| Adult, wild or farmed | Phytoplankton and other suspended organic matter | Filtered from the surrounding water; farms add no feed |
Algae are the primary food source of oysters.[1] NOAA Fisheries puts it simply for farmed oysters: growing them requires no feed, because they filter phytoplankton directly from the water column. NOAA also describes oysters as removing algae, organic matter, and excess nutrients from the water as they grow.[2]
The cells involved are tiny. The algae grown as oyster food at the University of Maryland's Horn Point Oyster Hatchery measure 2 to 30 microns across, and a single milliliter of water can hold millions of them.[1]
How an oyster eats
An adult oyster cannot move and has no way to chase food. It brings the water to itself.
It pumps. The oyster opens its shell and draws water across its gills.[3] Tiny hairs called cilia on the gills beat to create that current.[4]
It traps. The gills do double duty: they handle breathing and they catch particles. Trapped particles are carried in a string of mucus along a groove at the edge of the gill.[4]
It sorts. The mucus string reaches flaps beside the mouth called labial palps. Their ridged surface works as a sorting field, separating food from mineral particles.[4]
It swallows. Food goes into the mouth, down a short esophagus, and into the stomach, where a rotating gelatinous rod called the crystalline style releases digestive enzymes.[4]
It rejects the rest. Material the oyster filters but does not digest is expelled as pseudofeces. Digested material leaves as feces.[5]
The rejected material is one reason oysters matter to the water around them. As they feed they trap suspended sediment and nutrients as well as food.[6] Our article on whether oysters are good for the environment covers what that does for a bay.
How much water an oyster filters
The number you will see most often is 50 gallons a day. It is real, but it is a ceiling.
| Source | Figure | Conditions |
|---|---|---|
| Chesapeake Bay Program | Up to 50 gallons a day, or more | "Under certain conditions" |
| NOAA Fisheries (Pacific oyster) | Up to 50 gallons a day | Adult oyster |
| University of Florida IFAS Extension | Over 40 gallons a day | Controlled laboratory conditions |
| University of Florida IFAS Extension | 3.0 to 12.5 gallons a day | Natural conditions |
The Chesapeake Bay Program's field guide says an adult oyster can filter up to 50 gallons in one day under certain conditions, and its oyster overview says more than 50 under certain conditions.[3][6] NOAA Fisheries gives up to 50 gallons a day for an adult Pacific oyster.[7] University of Florida extension scientists are more specific about the conditions: over 40 gallons a day in the laboratory, and 3.0 to 12.5 gallons a day in natural settings. They note that salinity, temperature, water flow, and food availability all change the rate.[5]
Multiplied across a reef, even the lower figures add up. The Chesapeake Bay Program estimates that in the late 1800s the Bay's oysters could filter a volume of water equal to the entire Bay in three or four days, and that today's population takes nearly a year.[6]
What baby oysters eat
Oyster larvae spend their first two to three weeks in the water column, feeding on phytoplankton.[2][8] Larvae cannot swim horizontally but can move up and down, and they follow the phytoplankton that is their food.[8]
Hatcheries have to grow that food. Horn Point feeds its larvae a mix of three cultured algae, Isochrysis, Thalassiosira pseudonana, and Chaetoceros, and adds a larger-celled species, Tetraselmis chui, as the larvae get bigger. The hatchery notes that a mixed diet of flagellates and diatoms gives better larval growth than a single species.[1]
Once oysters leave the hatchery, the feeding stops. Horn Point says adult oysters and older spat are generally not fed because of the sheer quantity of algae it would take, and NOAA says that past the larval stage oysters filter their food from the water column.[1][2] Our guide to how oysters are farmed follows them from there, and how oysters reproduce covers the larval stage.
How diet affects flavor
An oyster is built from what it filters, so where it feeds shows up in how it tastes. Texas Sea Grant summarizes it this way: because oysters are filter feeders, the saltiness of the water and the types of phytoplankton and algae they eat determine their flavor.[9]
That is the food half of merroir, the idea that an oyster tastes of the place it grew. The salinity half is easier to taste directly; the algae half is harder to pin to a specific flavor, and this site does not try to match individual flavors to individual algae.
The clearest case of diet you can actually see is the green-gilled oyster. A diatom called Haslea ostrearia produces a blue pigment, marennine, that is water-soluble and fixes onto oyster gills, turning them green. In France these oysters are prized and sell for more than ordinary ones.[10] Our article on why some oysters are green explains the rest, and the Fine de Claire profile describes French oysters that are finished in phytoplankton-rich salt ponds.
To see how much flavor varies with place, compare oyster flavor by region, or taste a few oysters from different waters side by side.
What else oysters take in
Filter feeding is not selective about safety. As oysters pump water they trap contaminants along with food.[6] NOAA notes that shellfish can take up the toxins that cause shellfish poisoning from the algae they feed on, which is why harvest areas are monitored for harmful algal blooms.[2] For the bacterial side of raw-oyster safety, see Vibrio and raw oyster safety.
Frequently Asked Questions
Do oysters eat plankton?
Yes. Phytoplankton, the microscopic algae in plankton, is their main food. Oysters filter it from the water by pumping water across their gills.
How much water does an oyster filter in a day?
Up to about 50 gallons under ideal conditions, according to the Chesapeake Bay Program and NOAA Fisheries. University of Florida extension scientists report over 40 gallons a day in the laboratory and 3.0 to 12.5 gallons a day in natural conditions. The rate depends on temperature, salinity, water flow, and how much food is in the water.
What do farmed oysters eat?
The same thing wild oysters eat: phytoplankton filtered from the water around them. Farmers do not feed them. Only hatchery larvae and very young oysters are fed cultured algae.
Do oysters have mouths?
Yes. An oyster has a mouth, with flaps beside it called labial palps that sort food from mineral particles. Food passes from the mouth through a short esophagus to the stomach.
Do oysters poop?
Yes. Digested material leaves as feces. Oysters also produce pseudofeces, which is material they filtered from the water and expelled without digesting.
Does what an oyster eats change how it tastes?
Yes. Because oysters are filter feeders, the saltiness of the water and the kinds of algae they eat shape their flavor. The most visible example is the green-gilled oyster, which gets its color from a pigment made by a particular diatom.
What do baby oysters eat?
Phytoplankton, the same as adults. Larvae drift in the water for two to three weeks and feed on algae there. In hatcheries they are fed a mix of cultured algae species.