Crustaceans are called crustaceans because they have "armor" outside their bodies. Most crustaceans live in the sea, and a few inhabit fresh water and land; Crustaceans such as shrimps and crabs have five pairs of feet, four of which are used for crawling and swimming, and a pair of pincers to protect against enemies and prey.
There are many species of crustaceans in the world, about 26,000. Shrimp, crab and other crustaceans are rich in nutrition, taste delicious, and have high economic value. Some shells are also bait for economic animals such as fish. Among crustaceans, there are also some species that are harmful, such as fish lice, which are harmful to fish.