I am not aware of any amphibians that bite as a defense. These animals have other defenses that come first, crypsis, escape, then chemical defenses. ALL amphibians have some form of toxic secretion at some lifestage. Other animals that do not bite but have suitable defenses include the hognose snake, that hisses like a rattlesnake, spreads its neck like a cobra, and will even roll over and play dead with blood coming out of its cloaca and mouth. Supposedely, even the musk imitates rotting flesh, but its not THAT bad!