New York City in the United States has an unusual way of cleaning up its polluted Hudson River: oysters (牡蛎). It is part of a plan to create a healthier ecosystem in the river by increasing the oyster population. Oysters may help to restore the waters and bring back more sea life. How do oysters do it?
Oysters act as natural "water purifiers". They are filter feeders (滤食动物) . That means they take in water through their gills and filter out the food in it, such as algae (藻类) ，which they then eat. A single adult oyster can filter up to 189 litres of water in a day. As they feed, oysters also filter other harmful things out of the water.
One of these is nitrogen, a chemical found in septic tanks (化粪池) . Sometimes nitrogen gets washed into rivers and oceans when it rains. Too much nitrogen in the water causes much algae to grow. The algae uses up the oxygen in the water, which can harm other sea life. Oysters filter nitrogen out of the water and use it to grow their shells.
Oysters also build reefs (珊瑚礁) ，which are important to the whole ecosystem. Oysters attach (依附) themselves to hard surfaces underwater like rocks. Then more oysters attach themselves on top of those. As the oysters grow, their shells mix together to form a reef. When the new oysters are placed in the Hudson River, they are attached to metal cages in the water. These become the foundations for new reefs, which will get larger as more oysters build on top of them.
Oyster reefs provide homes for plants and animals. Reefs also help to protect the shoreline. Large reefs reduce the effect of waves coming in from the ocean, and help to reduce flooding and protect the coast.
Over the years, the New Yorkers have done a lot for the river, but it will still take tens of years before the new reefs being created in the Hudson River grow as the reefs that were destroyed long ago, but there are already signs that the ecosystem is healthier.
B . 
C . 
