From the Super Mario Wiki
The Spike Bass is an enemy in New Super Mario Bros.. It is a giant fish, brown in color with a pale underbelly. They also have beaming yellow eyes, sharp teeth and spikes all over their bodies.
Only appearing in World 2-A and World 3-C, a Spike Bass jumps from the water and tries to attack Mario, extremely similar to Boss Bass (sans swallowing Mario whole). This enemy can be defeated with a Fireball, a Koopa Shell, a Starman or a Mega Mushroom. However, another Spike Bass will return after a few seconds to continue the pursuit.