Short answers, pulled from the story.
An oyster is the common name for several different families of salt-water bivalve molluscs living in marine or brackish habitats. True oysters belong to the family Ostreidae, while pearl oysters belong to a distinct family, the feathered oysters or Pteriidae. Many oysters fall within the superfamily Ostreoidea.
Under ideal laboratory conditions, a single oyster can filter up to 50 US gallons of water per day, and some studies report up to 190 liters. Under average conditions, mature oysters filter 3 to 12 US gallons. As filter feeders, oysters remove plankton, nitrates, ammonia, phosphates, bacteria, and organic particles from the water.
Oysters are protandric, spawning as males in their first year by releasing sperm, then spawning as females over the next two or three years by releasing eggs. A single female oyster can produce up to 100 million eggs annually. Bay oysters usually spawn from the end of June until mid-August, and the settled young, called spat, measure less than 25 mm long.
In the early 19th century oysters were cheap and eaten mainly by the working class, with New York Harbor becoming the largest source worldwide. Rising demand exhausted the beds, introduced foreign species brought disease, and effluent and sedimentation destroyed most beds by the early 20th century. The resulting scarcity raised prices and turned oysters into an expensive delicacy.
The oyster-tecture movement uses oyster reefs for water purification and wave attenuation. A project at Withers Swash, South Carolina, filters roughly 4.8 million liters of water daily for a $3000 installation cost. The Billion Oyster Project in New York aims to return a billion oysters to the harbor, and recycled oyster shells help rebuild reefs that reduce flooding and protect shorelines.
The folk rule that oysters are safe only in months with the letter 'r' has a kernel of truth, because in the Northern Hemisphere oysters spoil more readily in May, June, July, and August. As filter feeders they concentrate whatever is in the water, including Vibrio vulnificus, the most deadly seafood-borne pathogen, which is most dangerous to immunocompromised individuals. Depuration places oysters in clean, sterilized water for 48 to 72 hours to remove contamination.
Oysters are an excellent source of zinc, iron, calcium, selenium, vitamin A, and vitamin B12. One dozen raw oysters provides only 110 kcal, and 100 g of Pacific oysters contains about 9 g of protein. Just two oysters, 28 g, supply the Reference Daily Intake of zinc and vitamin B12.