A Boss Bass is a huge fish found in Super Mario Bros. 3. They swim on the surface of the water trying to lunge out and eat Mario when he gets too close. In some of the levels, platforms rise and fall, making Mario an easy target. Throwing blocks and fireballs can defeat them, but they'll eventually come back.
Boss Basses are possibly related to the identical-looking Big Bertha.
In New Super Mario Bros. Spike Basses took their place. They won't eat him.
Boss Basses eat Cheep-Cheeps, but have trouble distinguishing them from something else, as proven in Super Paper Mario when a Shayde said that he fell into the ocean once, but luckily could swim as well as a Cheep-Cheep. Unfortunately, a Boss Bass thought that since he swam like a Cheep-Cheep, he was a Cheep-Cheep and ate him.