Any of certain species of red, green, and brown marine algae that generally are anchored to the sea bottom or to a solid structure by rootlike holdfasts that perform the sole function of attachment and do not extract nutrients as do the roots of higher plants. The most obvious seaweeds are brown algae; mosslike carpets of red algae are seen at low tides. Seaweeds are often dense in shallow water. Brown algae commonly found as seaweeds include kelp, which include the largest algae, and sargassum. Some seaweeds have hollow, gas-filled floats that keep their fronds at the surface of the water. Ulva species, commonly called sea lettuce, are among the relatively few green algae that are seaweeds. Seaweeds are used as food, and brown algae are used in fertilizers. The red alga Gelidium is used to make the gelatin-like product called agar.