Over 50,000 living species have been named, making Mollusca second only to the Phylum Arthropoda in number of species known. The majority of molluscs are marine, but large numbers of species occupy freshwater and terrestrial habitats. Molluscs are also extremely diverse in their food habits, ranging from species that graze on microscopic algae, to those that eat the leaves of terrestrial plants, to predators that capture fish and other molluscs. Many species of molluscs are important to humans. A large number of bivalves and some snails are important sources of protein.
Oysters produce pearls.
Other species are pests in gardens and to crops. Some are essential components in the life cycles of human parasites, including devastating diseases such as schistosomiasis. And a few, such as oyster drills, cause problems by preying on other molluscs that are important to us. Finally, molluscs play a wide variety of essential ecological roles. As common herbivores, they can have a significant impact on the plant species present in an area.