|Toothy, clownish smile.|
Oysters are filter feeders. They "inhale" water, pumping it over their short tentacles and their gills, trapping small particles and swimmers in mucus, which they pass on to the mouth. The "teeth" in this photo are tentacles. The gills are right behind them.
|Oyster anatomy. Gif from East Hampton Aquaculture.|
The food travels through the stomach and intestine. What is rejected out of hand, before it enters the intestine, such as bits of sand, is wrapped in mucus and expelled, much as we spit out fish bones without swallowing them. Of the rest, indigestible food particles travel through the digestive system, and are also expelled into the surrounding water, where the busy janitors (aka hermit crabs) collect them and reprocess them.
An oyster can filter up to 5 l (1.3 US gal) of water per hour. ... Excess sediment, nutrients, and algae ... Oyster filtration can mitigate these pollutants. (Wikipedia)
So the three oysters now in my tank may be filtering 15 litres of the water every hour, 180 litres a day. The anemones will be happy; they hate polluted water.