A large, diverse, mostly marine group of algae. Most are red, owing their colour to the presence of large amounts of phycoerythrin, which masks the chlorophyll and other pigments that they contain. Some species are microscopic, others membranous or filamentous, and often much branched. Red seaweeds are relatively small compared with the brown ones and grow at greater depths or in rock pools too shady for the brown to thrive. Calcareous species are common, especially in tropical seas, where they often play a part in the formation of coral reefs. Some yield agar-agar, other are edible.