|Description||The red algae, or Rhodophyta, are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000?6,000 species of mostly multicellular, marine algae, including many notable seaweeds. Other references indicate as many as 10,000 species; more detailed counts indicate about 4,000 in about 600 genera (3,738 marine species in 546 genera and 10 orders (plus the unclassifiable); 164 freshwater species in 30 genera in eight orders).
The red algae form a distinct group characterized by these attributes: eukaryotic cells without flagella and centrioles, using floridean polysaccharides as food reserves, with phycobiliproteins as accessory pigments (giving them their red color), and with chloroplasts lacking external endoplasmic reticulum and containing unstacked thylakoids. Most red algae are also multicellular, macroscopic, marine, and have sexual reproduction. They often have alternation of generations and may have three generations rather than two.|