- Marine algae species large enough to be recognized with the naked eye are commonly called seaweeds. Seaweeds may be green, red or brown algae.
- One of the most familiar green seaweeds is ulva, commonly known as "sea lettuce." Various species of ulva are found in shallow water from the polar regions to the equator. Another common green seaweed is caulerpa, of which several species occur, mostly in tropical waters. One Caulerpa species has become a pest in the Mediterranean and elsewhere.
- Between 5,000 and 10,000 species of red algae are known, and many are large enough to be called seaweeds. Nori, the seaweed traditionally used to wrap sushi, is a red seaweed, as is dulse, a seaweed often included in European cuisine. The vast majority of red seaweeds live in marine habitats, from the tropics to the polar regions. About 200 red algae species, mostly small ones, live in freshwater habitats.
- The most widely known of the approximately 1,500 to 2,000 brown seaweeds are the kelps. Giant kelps form extensive underwater forests off the coast of California. Kelps occur more frequently in cold waters than in warmer habitats. In the Atlantic, the brown seaweed sargassum forms a vast floating mass known as the Sargasso Sea. Surrounded by currents on all sides, this huge seaweed raft is some 700 by 2000 miles in extent.
- Carageenan, used to thicken ice cream and many other foods, is made from a red seaweed.
