These busy filter-feeders clean the water, attract biodiversity, and offer protection from storms.

When English explorer Henry Hudson sailed into New York City harbor in 1609, there were oysters everywhere. Accounts say he had to navigate carefully to avoid running into some 220,000 acres of oyster reefs. Fast forward 400 years and most of these oysters are gone, their population decimated by polluted water.

One group of citizens, however, is on a mission to restore the harbor's oysters to at least a shade of their former glory. The Billion Oyster Project grows baby oysters and replants them on the bottom of the Hudson River in order to kickstart a rejuvenation of the ecosystem. So far the group has planted 28 million pounds of oysters across nine reefs and the water quality is measurably improved.

Oysters are desirable residents in the river because, as Billion Oyster Project co-founder Pete Malinowski explained, they are 'ecosystem regenerators,' a species that has a disproportionately positive effect on its ecosystem, not to mention some crucial benefits for New Yorkers. They also form reefs that are effective breakwaters in storms.

When it comes to filtering water, oysters can't be beat. They filter at a rapid rate, about a gallon of water per hour, which means that each one cleans at least 24 gallons of water daily. (NPR says it's more like 30 and 50 gallons per day.) Because the standing volume of New York Harbor is 74 billion gallons, that means that, theoretically, the one billion oysters that Malinowski's organization hopes to plant would eventually be able to filter all the harbor water once every three days.

The presence of oysters attracts other species, too. Madeline Wachtel, director of strategic projects for One Billion Oysters, told NPR:

"Oyster reefs provide great marine habitat, similar to coral reefs, with nooks and crannies to protect juvenile fish, and are active food for some species. They help to create a thriving ecosystem."

An intriguing part of this restoration project is the collection of empty oyster shells from local restaurants. One Billion Oysters has partnered with over 70 restaurants to do weekly pickups of shells that would otherwise go to landfill. Currently, the organization collects 4 tons of shells each week. The shells are left to cure in the sun for a year, then cleaned and moved to an oyster hatchery. NPR describes what happens next:

"In an aquaculture classroom's hatchery, student-grown oysters produce larvae in an artificially induced springtime environment. In one to two weeks, each larvae grows a 'foot' — a little limb covered in a kind of natural glue — and then is moved to a tank full of the 'cured' restaurant shells, which serve as anchors for all of those sticky feet. This phase is critical: If larvae can't find a place to attach, they die. One reclaimed shell can house 10 to 20 new live oysters, depending on shell size."

Eventually the babies are able to form their own shells, drawing on the calcium carbonate in the surrounding water. Not all larvae survive, but the goal is "to get at least five oysters clustered onto one restaurant shell — think flowering oyster bouquets." These are moved into the river when the water is warm enough, and the oysters will continue self-aggregating and building on top of each other, creating reefs.

It is wonderful to hear about environmental regeneration projects that are flourishing like this one. If you live in the New York region you can volunteer to work on four of the nine oyster reefs or in the hatchery. Learn more at One Billion Oysters or watch the video below.