It is feeding, that is, eating.

These crabs are called sand crabs, and sand crabs are the dominant crab species living in the intertidal and supratidal zones of warm seas, burrowing in deeper holes in sandy beaches? [

These crabs are called sand crabs.]

The holes are generally spiral in shape, with the mouths of the holes forming sand towers, which are unique to sand crabs.

The crab scrapes small particles from the surface of the silty soil with its small chelae and feeds them into its mouth, where they contain a lot of debris, algae, bacteria, and other microorganisms, which are absorbed into the body after being fed into the mouth. There is a special organ in the mouth, food can be sorted and filtered, the residue can not be used by the small chelipeds and then placed on the ground, focused on the formation of the people see the small ball of soil, called "faeces".

Extended information:

The Beihai sand crab has made an appearance in the second season of "Tip of the Tongue: China," in its sixth episode, "The Secret Land.

Beihai sand crabs were featured in the sixth episode of the second season of "Tip of the Tongue China", "Secret Land".

Sand crab juice is a juice made from sand crabs. Sand crabs are found in abundance along the seashore of Beihai City, and they come out of their holes and move on the beach at low tide. Sand crab juice is completely "raw", and there is no heating and cooking step in the production process, so the juice has a fishy flavor, but it tastes very fragrant. It is somewhat similar to "stinky tofu", which is delicious to those who like it and disgusting to those who do not.

Shark crab juice is a very distinctive seasoning, when eating white cut chicken as a dip, eating will bring back memories. Stewed bean curd with sand crab juice is a famous dish in Beihai city of Guangxi, even if you just put a little bit of sand crab juice when drinking white congee, it will be appetizing. It's also an authentic Beihai specialty dipping sauce, which is rarely sold outside of the city. Reference:

People's Daily Online - Beihai Sand Crab Juice Appears in Tongue Tip 2, Pure "Sand Crab Sisters" Goes Viral