In addition to strengthening the bottom, mollusks purify water from pollution and help marine vegetation.

In the US, scientists have begun to strengthen the shores of San Diego Bay in California with concrete structures that are used as the foundation for oyster reefs.

Each of these structures weighs 135 kilograms and consists of a mixture of cement, sand and crushed shells of mollusks. The last component attracts live oysters, which attach themselves to objects and set up their colonies there.

After about a year, these hollow balls of concrete are covered with many shells and silt.

Eileen Maher, director of environmental protection at the Port of San Diego, said the reefs protect the shores from erosion, and the molluscs that inhabit them filter the water and help the marine ecosystem.

One oyster filters about 190 liters of water per day, making it less cloudy. This helps to develop underwater vegetation.

Both shellfish and algae also remain important food sources for the 80 fish species and 300 bird species that live in the San Diego Bay area.

The $1.3 million oyster reef project will take five years. Similar systems also appeared in San Francisco and New York.