Oysters may seem like humble shellfish, but ecologically, they tower above many other species. They are a tasty food source for a variety of creatures, including humans. But more importantly, oysters help to filter pollutants from coastal estuaries, places where fresh river waters and ocean salt waters co-mingle.

Fourteen out of 20 of the world's largest cities are built near estuaries, and urban pollution, development and other factors are now threatening these delicate ecosystems. In response, some grassroots groups, are using oyster beds to help restore and stabilize damaged estuaries. New York and New Jersey Baykeeper is one of them.

On an unseasonably blustery spring day on the shores of the Navesink River in the town of Red Bank, New Jersey, a group of girl scouts counts the oysters they've grown from seeds over the winter. They are among the more than 400 grassroots volunteers for the organization, which is committed to the restoration of the region's vast estuary, which includes about 1100 kilometers of shoreline.

An oyster, which has a soft body within two hard shells is a bivalve. "It basically looks like a big beige blob inside," says Katie McCrone, the group's oyster program coordinator. "But within that blob, are all the organs that an oyster needs to breathe and eat and grow. It's got a heart in there, intestines; it's got a muscle that holds the two shells together. It's a whole animal inside there!"

This animal also serves as a highly efficient water-filtering machine. An average adult oyster filters as much as 190 liters of water a day. "That might not seem like much," says one girl scout named Jackie, "but when you put a million together it makes a big difference. And it helps the ecosystem."

Julia and Gabby, both 11-years old, have enjoyed doing their part. "You get a lot of responsibility with the oysters," says Julia, "remembering all your data charts, getting there on time, taking care of them, getting the proper things you need to scrub off the cage and all the muck and stuff."

"If they are not cleaned," explains Gabby, "they won't grow enough. After they are big enough - about the size of two baby fists - then they dump them in this big reef so that they are all together."

Delighted shrieks fill the air as these girls discover with their own eyes and (gloved) hands that the beds and the reefs they form are home to many kinds of marine life that non-experts might consider a bit "yucky."

"Oyster reefs are at the bottom of bays and estuaries and they provide homes for lots of other species of crabs, fish, worms - you name it," says McCrone. "Any invertebrate living down there will hide under a shell, anything that needs to attach to a shell to grow -- anemones, barnacles, sea grasses and things like that that will attach to hard surfaces will maybe attach to the oyster reef."

Indeed, "Three out of every four fish depend on an estuary for some part of their life," says Mark Wolf Armstrong, president of Restore America's Estuaries, a coalition of 11 environmental groups in cities around the country. "It's usually the juvenile stage. It's usually when they're young and they're trying to grow up."

Throughout history, humans have also loved estuary habitats. "That's where we love to be," says Armstrong, "because it's protected. It's also rich in food. We like to eat! We move into an estuary and we blossom ourselves. Our populations grow. Look at our cities. They are huge and getting huger!"

A casual visitor to today's New York would never know it, but until the 1920s, New York City was home to some of the world's most productive oyster beds.

"The Dutch colonists and the English that followed were really oyster gourmands, and the native Americans who lived here at the time were also," says Andrew Willner, director of New York and New Jersey Baykeeper. "Oysters were so ubiquitous [common] that they were a penny apiece on a wagon on the corner of downtown Manhattan and people would eat three, four or five dozen for lunch."

Typhoid from the raw human sewage dumped into New York waters killed off the local oyster industry in the 1920s, demonstrating the sensitive link between humans and their natural environment. Estuary restoration and protection is a challenge that is finally being addressed globally, from Britain to Africa, and from South America to Bangladesh.

"And it's really gratifying to see that people are finally 'getting it!'" says Mark Wolf Armstrong. "Otherwise," he adds, "we won't be able to enjoy the bountiful nature of this kind of habitat."