Why do shrimps and crustaceans have hard shells?

Shrimps, crabs, lobsters, and many other crustaceans are covered by a hard exoskeleton made largely of chitin and minerals such as calcium carbonate. This shell protects them from predators and physical damage while also serving as an attachment point for muscles. However, because it is rigid, it cannot grow continuously with the animal. To increase in size, crustaceans must molt, shedding the old shell and forming a new one. Molting is a vulnerable period, but it is essential for growth.