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
Culture of microalgae in hatcheries Culture of microalgae in hatcheries Microalgae or microscopic algae grow in either marine or freshwater systems. They are primary producers in the oceans that convert water and carbon dioxide to biomass and oxygen in the presence of sunlight. The oldest documented use of microalgae was 2000 years ago, when the Chinese used the cyanobacteria "Nostoc" as a food source during a famine. Another type of microalgae, the cyanobacteria "Arthrospira" (Spirulina), was a common food source among populations in Chad and Aztecs in Mexico as far back as the 16th century. Today cultured microalgae is used as direct feed for humans