Biologists estimate that New York City's harbour was once home to half the world's oyster population.
For at least 6,000 years oysters thrived in the Hudson River estuary, becoming "deeply woven into the life of East Coast cities", said BBC Future. But overconsumption and pollution devastated the population, and in the mid-19th century a link was established between eating oysters and typhoid. The Health Department closed all the oyster beds, with New York's last shut in 1927. They are now "long gone".
But over the past decade one of the city's "most ambitious rewilding initiatives" has planted 150 million larvae, aiming at seeding a billion oysters in the city's waterways by 2035. The goal of the Billion Oyster Project: "Restoring the city's coastal habitat, improving water quality and educating the public."
Oysters absorb pollutants when they filter the water, and one adult oyster can filter 50 gallons a day, said The New York Times. Oysters also "enhance biodiversity"; animals like shrimp and snails grow on their shells, providing food for fish. They offer a natural defence against coastal erosion and are "key in the fight against rising seas and flooding".
Restoring the reefs could help to protect the city from extreme storms, according to a recent study in Nature. Wave energy accelerates over the bottom of a river, but reefs absorb much of the momentum.
The Solent Seascape Project, one of the biggest restoration projects in Europe, recently used $5 million (£3.97 million) of funding to introduce more than 14,000 oysters into the River Hamble, said South West Londoner. Other oyster restoration projects are already underway in Bangladesh, Australia and Hong Kong. | 
