The black paste next to the crab roe is the crab's stomach, also known as crab urine bag. It is located in the center of the front edge of the back shell. It is a triangular-shaped bony bag with dirt and sand inside, closely connected to the crab roe. .

When a crab is on an empty stomach, its stomach will appear smaller. However, after a full meal, the stomach will swell and appear to have more black paste. This is normal and can be removed after the black paste is removed. Eat normally.

Extended information:

A small number of crabs grow and reproduce entirely in fresh water, and they mostly live under rocks in mountain streams. There are also a few crabs such as ground crabs that can adapt to life on land. They live in moist mud holes and migrate to the sea during the breeding period.

Most crabs are omnivorous. Some crabs are carnivorous, such as swimming crabs, which can prey on fish, shrimps and molluscs. A few crabs, such as the femoral crab, scrape or filter feed algae and organic debris.