Why do chicken eggs have shells and fish eggs do not?

Chicken eggs have hard shells primarily to protect the developing embryo from physical damage and microbial invasion, providing a stable environment for growth. In contrast, fish eggs are usually surrounded by a gelatinous coating that offers some protection but lacks a hard shell, as they are typically laid in water where the surrounding environment is more fluid and less prone to threats. This difference is due to the distinct reproductive strategies and environments of birds and fish.