The puffer fish

Bony fish dominate the oceans. They differ from cartilaginous fish because they have bone skeletons and a swim bladder that helps them float. Although they come in many shapes, most bony fish species have fusiform, hydrodynamic bodies, which allow them to cruise through the water at high speeds. Other species have flat bodies and hide on the bottom of the sea. For instance, puffer fish, which have a rounded shape, inflate themselves to a globe several times their normal size by swallowing water when threatened. 

Over millions of years, fish species have developed effective strategies to adapt to their environments, in order to survive.