Marine life appeared in the form of fish, which live exclusively in water. Some of these evolved into amphibians which spend portions of their lives in water and portions on land. Other fish evolved into land mammals and subsequently returned to the ocean as seals, dolphins or whales. Plant forms such as kelp and algae grow in the water and are the basis for some underwater ecosystems. Plankton, and particularly phytoplankton, are key primary producers forming the general foundation of the ocean food chain. Marine invertebrates exhibit a wide range of modifications to survive in poorly oxygenated waters including breathing tubes (see
Freshwater bivalve on a fish or salamander. Many species are utilized as sources of mother-of-pearl. Families: Order Veneroida The Veneroida is a large group of bivalve "clams", most of which are marine. However, several families occur in fresh and brackish waters. Families: Freshwater bivalve Freshwater bivalves are one kind of freshwater molluscs, along with freshwater snails. They are bivalves which live in freshwater, as opposed to saltwater, the main habitat type for bivalves. The majority of species of bivalve molluscs live in the sea, but in addition, a number of different families live in freshwater (and in some cases also in brackish