High Quality Content by WIKIPEDIA articles! The red algae 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 10,000 species.The red algae form a distinct group characterized by the following attributes: eukaryotic cells without flagella and centrioles, using floridean starch as food reserve, 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.