Oyster Anatomy

Oysters feeding actually cleans coastal waters, preventing algal bloom and filtering out toxins

Oysters are bivalves, which is the name for a number of marine and freshwater mollusks that have long, flat bodies made up of a shell consisting of two hinged parts. Oysters use their gills and cilia to process water and feed. Oysters have a small heart and internal organs, but no central nervous system. Lack of a central nervous system makes it unlikely oysters feel pain, one reason some people who otherwise are vegetarians comfortable eating oysters. Some oysters have two sexes, but all have both eggs and sperm. 

How do they feed (and what do they eat)

Oysters are filter feeders. They take in water from their surroundings and extract particulate matter from it. They use a small filter (called latero-frontal cilli) to grab onto potential food before releasing water back into their surroundings. Oysters are selective in the material they take in. They prefer organic material with nutritional value like plankton.

Oysters prevent algal bloom

Oysters serve an important purpose in removing excess organic matter from the coastal ecosystem. Coastal ecosystems rely on nitrogen and phosphorus to support all marine life. But too much of these nutrients can be dangerous. Runoff from Human activities like farms, residential lawn care, and inadequate septic systems can introduce too many nutrients into the water.

Too many nutrients can trigger a process called “Eutrophication”. The high level of nutrients causes a spike in the amount of algae. This is sometimes called an “algal bloom”. When the bloom of algae dies off, its decay consumes oxygen. The water can become low in or devoid of oxygen (called hypoxic). This kills fish, crustaceans, and other marine life, disrupting the entire ecosystem.

Oysters remove the excessive organic particles before they have a chance to cause an algal bloom. A single oyster can filter as much as 50 gallons of water in a single day, consuming the excess nutrients as food.

Do oysters poop?

Oysters are filter feeders, and take in all different types of particles from the water column. As oysters digest food, waste collects in a cavity inside their shell. Besides to actual waste, items called pseudofaeces also collect there. Pseudofaeces are things like sand or large pieces of food the oyster can’t digest. The oyster covers them in mucus and stores the pseudofaeces with the actual feces. Every once in a while, the oyster claps its shell together and pushes out most of the water out of its body, along with any waste.

While oysters do expel feces and pseudofaeces, they ultimately leave water cleaner. A single oyster can clean up to 5 L of water per hour, removing organic and inorganic toxins that could otherwise harm the ecosystem.

The birds and the bees (and the oysters)

Oysters begin their life the same way most organisms do, with the fertilization of an egg by a sperm. The Eastern oyster is protandric, meaning that they are all born male. As time goes on, oysters are able to change to females, and even then change back into males, depending on circumstances. Environmental cues such as temperature, amount of food, and salinity determine if it's time for the oyster to develop sex organs (gonads). This takes place in spring and the gonad takes roughly two months to form. The mating process kicks off in the summer once the water has reached a certain temperature. When the time is right, one oyster will release its sperm/eggs into the water column, followed by all the other oysters in the area. This is broadcast spawning, when all the oysters in an area release their sperm and eggs into the water column at once. The sperm and eggs mix together, forming fertilized eggs.

Within 24 hours, fertilized eggs will develop into free-swimming larvae. They remain this way for 2-3 weeks. At that point, they have developed a “foot” which they can use to move over surfaces in the water, like sand or rocks. Once they find a suitable location to attach themselves to, they secrete a cement-like substance to bond to their new home. At this point, oysters are miniature versions of their adult forms. They are only a few millimeters wide and called “spat”. They will grow larger each month, reaching adult size after a few years.