It's hard for me to think of anything more voracious than a sea star. I like to refer to them as the walking stomachs of the sea. If you open one up, you will see the digestive gland and little else. This particular sea star is a bat star (Asterina miniata), and it eats plants (like surf grass), animals (anything it can catch, dead or alive), and even the thinnest biofilms (bacterial, protozoan). It basically everts its stomach on anything organic and digests it. The thin tissue in the picture is the bat star's stomach.
Bat stars will fight each other whenever they come into contact. A battle consists of one star trying to put its arm on another. So both stars will rear an arm up and very slowly try to lay it on top, like a slow-motion thumb war (you remember that game, right?). It's rather amusing to watch, if you have the time.