What Do Oysters Eat?

In my new mini-series, "Bivalve Curious," I'll be asking and answering some questions about oysters that you've always (or maybe never) wanted to know. They say you are what you eat... so what do oysters eat?



What do oysters eat?

Oysters eat phytoplankton or small bits of algae suspended in the water. They are filter feeders, which means that they obtain their food by filtering water in and over their gills. Sometimes they're referred to as bottom feeders, but don't mistake them as detritivores. Whatever they can't eat or digest, they expel as feces and pseudofeces. Not unlike myself, oysters are voracious eaters. Adult Virginica oysters can filter up to 50 gallons of water per day. To demonstrate their filtering power, here is a fascinating time-lapse of some oysters in a tank doing their thing.

You know that saying, "you are what you eat?" It's just as true with oysters as it is with anything or anyone else. While I am highly suspect that algae and water composition impacts the taste of the oyster, it is quite difficult to quantify. Fortunately, there's an easier way to prove the link.

Have you ever come across a green-gilled oyster?


No, they're not sea sick. The greenish-bluish color is caused by a special type of phytoplankton known as Navicula ostrearia. The effect has been studied by scientists as early as 1820, and I found this research paper from 1885 most intriguing. The green tint is temporary and doesn't change the taste. If the presence of this diatom subsides, then the oyster would also turn back to its original color in a few weeks. Green oysters enjoy a positive and desirable reputation in France, where they are specially cultured in Marennes, but they can just as easily occur in nature without any human interference. The photos above are of oysters from the Rhode Island and Long Island Sound, respectively. Green oysters have also shown up as south as Lynnhaven, Virginia.

You're also probably wondering about the darker side of this equation: are oysters also eating things that can be harmful to us? It is possible, but it all depends on the location. Trace metals, chemicals, and bacteria can find their way into oysters if they are present where the oysters live, which is why you don't see any oysters being consumed from New York Harbor anymore. (Btw: I found this blog post by Chris Len for Deep Sea News that is worth a read.) In general though, this shouldn't be a concern. The oysters that you find in today's restaurants and seafood markets are perfectly safe to consume. They are properly harvested from highly regulated waters that contain minimal levels of contaminants. I tend to think that people are more dangerous to your health than the oysters themselves, and this is why you should always buy your oysters from trusted sources.

Got a burning oyster question you want answered? Post a comment below or tweet me at @inahalfshell.

Want more oyster trivia? Check out my extensive Q&A "9 Things to Know About Oysters: Myths, Facts, and Trivia."