A large class of plants, commonly called red algae, coextensive with the division Rhodophycota. Most red algae are found in the ocean, growing on rocks, wood, other plants, or animals in the intertidal zone and to depths limited by the availability of light. A few genera and species occur in fresh water, and these are usually found in rapidly flowing, well-aerated, cold streams. Some, however, grow in quiet warm water, while a few are subaerial. Most red algae are photosynthetic, but some grow on other algae with varying degrees of parasitism. Approximately 675 genera and 4100 species are recognized.