Hemp thread is rope algae, also known as hemp rope vegetables and sea gazi. It is named because it looks like a rope. It grows on underwater rocks and is distributed along the coast of the Yellow Sea and Bohai Sea in China. The algae body is grayish black to dark brown, slender, rope-shaped, with a diameter of 1.5-3.0cm, with no branches on one stem and narrowing at both ends. The upper part is hollow, the lower part is solid, and the hollow part is divided into many cavities by a diaphragm.