I just noticed that you asked whether the meaning is changed if you leave out "numerous". You would end up with "Jellyfish are probably the most predators on Earth", which doesn't make sense. Anyone hearing/reading that would think that you had misssed out a word after "most".
They wouldn't assume that it was "numerous" that you had left out, either. You could have been saying "the most vicious predators", "the most colourful predators", "the most entertaining predators", in other words just about anything!