From the Super Mario Wiki
The Spike Bass is an enemy in New Super Mario Bros..
Only appearing in World 2-A and World 3-C, a Spike Bass 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. Spike Bass jumps from the water and tries to attack Mario. The fish can only reach a certain level though, if Mario is on high grounds, he will have a good chance of being safe. If Mario is to swim to complete the level, a Spike Bass will come to attack the plumber. Its habit is quite similar to Boss Bass, but a Spike Bass will not try to gulp Mario whole.