This is Nereis, taxonomically belonging to annelids, polychaetes, nematodes and Nereistidae, commonly known as sea worms, sea maggots, sea centipedes, sea leeches and sea earthworms. There are about 80 species of Nereis in China, and the main economic species and varieties used for breeding are: Nereis japonica, Perinereis spinulosa, Perinereis bidentata and so on. They like to live in the sand and mud in coastal mudflats, intertidal zones and subtidal zones where fresh water flows in. Larvae feed on plankton and adults feed on humus. Nereis has high nutritional value and is widely used in aquaculture.

Some Nereis have extensive salt tolerance, such as Nereis verrucosa, commonly known as "cereal worm", which can inhabit coastal areas, estuaries or rice fields. Most of them live in the topsoil of rice fields and feed on grass and plants. They are pink, sometimes turning milky yellow or green, and are rich in protein and vitamins.