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 mollusc Phylum Mollusca -the cephalopods, scaphopods, polyplacophorans, etc. - never made the transition from a fully marine environment to a freshwater environment. A few species of freshwater molluscs are among the most notorious invasive species. In contrast, numerous others have become threatened or have become extinct in the face of anthropogenic change. Typical freshwater species (such as many river mussel species in the family Unionidae) have a range which may consist of a series of adjacent river systems, a series of adjacent tributaries, or part of a single large river system. Large rivers and small tributary creeks typically share few species,