The phylum Rhodophyta, commonly known as the red algae, is a large group of mostly marine, multicellular algae found in oceans worldwide, especially in tropical and warm temperate waters. They are easily recognized by their red, purple, or reddish-brown coloration, which comes from the pigment phycoerythrin that masks their green chlorophyll. Red algae include familiar types such as Irish moss, dulse, and nori (used in sushi). They play important ecological roles as primary producers in marine food webs and contribute to reef building through calcified forms like coralline algae. Economically, they are a major source of agar and carrageenan, substances widely used in food, biotechnology, and pharmaceuticals.