Nagamo is a type of hondawara (a brown seaweed), and its official name is akamoku. It attaches itself to rocks and other objects on the sandy bottom of shallow seas and grows to a size of 4 to 7 meters in one year. Nagamo grows more abundantly in the deeper parts of the ocean and in relatively calm waters. The depths of Ryotsu Bay and Mano Bay, with their accumulation of sand, stone and rocks from nearby mountains, are ideal environments for the growth of nagamo. This seaweed sometimes gets entangled the propellers of boats and ships and so is sometimes called jamamoku (jama means nuisance in Japanese).

This seaweed has a crunchy texture, a unique consistency, and is rich in dietary fiber. When dipped in boiling water, it turns bright green and becomes sticky. On Sado Island, nagamo is used in miso soup, in soba noodles or served on rice with soy sauce.