River.

The mud snail inhabits the intertidal sand or sandy mud bottom in the inner bay, and the beach is rich in benthic diatoms. It has a strong adaptability to salinity and temperature, and grows well in severe winters and hot summers, but it is not suitable for living in the sea area with high wind and waves and rapid tidal currents. There are a lot of mud snails on the beach after the tide goes out, and when crawling, they use their heads and feet to dig up the mud and sand to cover their bodies, so that they can protect themselves from enemies and reduce the evaporation of water by mimicking the role of the mud snail.

When the weather is rainy or cold, the mud snail dives in the surface of the mud and sand at 1~3cm and is not easy to be found, and then crawls out after sunrise to forage for benthic algae, organic debris, invertebrate eggs, larvae, and small crustaceans, etc. It is also known as the "Mud Snail".

Expanded information;

The mud snail is a kind of mollusc living in the sandy or muddy mudflat, "spit and swallow sand, sand black as iron", the ancient name of "spit iron". Wang Fa period, the island around the shoals densely crawling mud snails, slow, naive, from a distance, like who scattered countless black beans. Closer look, its shell is ovoid, body is rectangular, like a small slippers.

This time to pick up mud snails, guaranteed to return full. In the rainy or cold weather, the mud snail will dive in the sediment surface layer 1 to 3 centimeters, not easy to be found, but experienced people will be based on some of the mud left on the air holes and other "traces" effortlessly dug to them.

Baidu Encyclopedia - Mud Snails