Plant growing in the sea, especially marine algae.
Any of a vast collection of marine and freshwater, simple, multicellular plant forms belonging to the algae and found growing from about high-water mark to depths of 100–200 m/300–600 ft. Some have holdfasts, stalks, and fronds, sometimes with air bladders to keep them afloat, and are green, blue-green, red, or brown.
Many have traditionally been gathered for food, such as purple laver Porphyra umbilicalis, green laver Ulva lactuca, and carragheen moss Chondrus crispus. From the 1960s, seaweeds have been farmed, and the alginates extracted are used in convenience foods, ice cream, and animal feed, as well as in toothpaste, soap, and the manufacture of iodine and glass.
The ribbonlike seaweed Undaria arrived in the Mediterranean in the 1980s from Japan attached to imported oysters and is now being grown by the French for export to the Asian market.