What Do Oysters Eat? Everything You Need to Know

Oysters are bivalve mollusks that live in salty coastal waters, rivers, and bays around the world. These creatures are covered by hard shells that can be opened for feeding and other biological needs, or closed when necessary. Oysters are filter feeders that process up to 50 gallons of water a day through their gills to trap food. Continue reading to discover everything you need to know about what oysters eat.

What Foods Do Oysters Eat?

Oysters eat algae, phytoplankton, and zooplankton. They are omnivores that consume plants and animals, though they do so in very limited amounts.

Oysters are unlike most other sea creatures. They cannot hunt for food and remain stationary for most of their lives. Early in their lifecycle, an oyster will use a temporary foot to find a suitable substrate upon which to anchor itself for the duration of its life. Once a favorable location is found, the oyster attaches to the substrate and loses its temporary foot.

Instead of hunting food, oysters allow the food to come to them. They filter their food directly from the water that surrounds them, consuming valuable nutrients and helping to clean the water in which they live.

Oysters eat the following foods:

• Algae
• Seagrasses (small bits)
• Bacteria
• Phytoplankton
• Zooplankton
• Dissolved organic matter

An oyster’s available food sources depend on where it lives and the quality of the water. Fortunately, these mollusks are not very picky, filtering dozens of gallons of water each day to source their food while simultaneously cleaning the water.

How Do Oysters Eat?

The process of eating begins when the oyster detects that the conditions are right for feeding in its body of water. They follow cues from ambient temperature and circadian rhythms, and are believed to follow tidal fluctuations as well. When their shells open, the oyster will filter water through its gills, retaining the food and expelling the rest.

Basically, oysters remove algae and plankton from the water through a filtration process.

Using ciliary action, the food is guided to the labial palps that surround the oyster’s mouth. Sometimes, food is rejected by the palps because the food is not fit for consumption. These portions are called pseudo-feces.

Acceptable food is passed into the mouth where it is consumed and processed. On the whole, oysters are not too different from other sea-dwelling animals. They put food in their mouths, which then passes into the stomach before moving into the intestines. Any waste is passed through the cloaca, a chamber located in the posterior portion of the oyster’s shell, through which fecal material is expelled.

Oysters have a unique way of eating, and their feeding process provides significant environmental benefits.

How Does the Oyster’s Diet Affect Other Species?

Although oysters may seem somewhat removed from the rest of the food chain, they play an important role in keeping waters clean. Oysters filter suspended sediments, nitrogen, bacteria, and algae from water while they are in the process of finding nourishment.

By doing this, oysters help clean the water and prevent the onset of eutrophication, commonly known as algae blooms, a process in which increased nutrient levels in the water lead to ecosystem changes. Some of the more problematic elements of this process include:

• Increased harmful algae blooms
• Lower water clarity
• pH changes
• Degradation of water quality
• Oxygen depletion in the water  

Fortunately, oysters, whether native to the area or introduced artificially, can help remediate water quality. Overall, the diet of oysters benefits many species that do not directly interact with oysters.

What Do Oysters Eat During Winter?

During winter, oysters become dormant and eat very little, as cold temperatures and reduced food availability slow their metabolism.

Although oysters lack a central nervous system, they can detect when their habitat is likely to have food. They use a combination of circadian, lunar, and tidal cycles, along with temperature fluctuations, to determine when to open their shells to filter water and when to remain closed.

When the water reaches a low temperature, oysters enter a state of hibernation. They become much less active in the cold waters, saving their energy, and then reopening when the waters start getting warmer again.

During this dormancy period, oysters rarely open their valves and barely pump water, if at all.

What Are Oysters’ Predators?

Making a meal of an oyster can be tough since they can hide within their strong shells. However, oysters lack a means of conveyance, so if they get caught by a predator, they can’t get away.

Here are some of the most common oyster predators that stand a chance of eating these mollusks:

• Humans
• Crabs
• Otters
• Raccoons
• Oyster drills
• Flatworms
• Sea nettles
• American Oystercatcher birds
• Sea anemones
• Starfish

It is important to note that many of these predators capture larvae or juveniles rather than old, established oysters.

Nevertheless, humans are among the most successful oyster predators, having developed specialized tools for finding and harvesting these mollusks. They frequently shuck and then consume live oysters and farm them for sustainable food sources.

Other creatures are also effective at catching and consuming oysters, including oyster drill, snails that use their radula to bore a hole in the shell and consume the prey inside.
Still, it is not easy for most predators to get to these creatures.