They appeared because God said let there be jellyfish.
Nothing is self sustaining. All life is dependant on something. and yes everything would instantly cease if God quit taking care of it.
Where did the water come from to sustain all life? self sustaining: water not needed. Where did the soil come from to sustain roots? Self sustaining: soil not needed. Where did the sun light come from to activate photosynthesis? Self sustaining: sun light not needed.
Nothing on earth can live without dependance on something.
....And man can not live without dependance on God, reguardless if man refuses to realize it. Simple observable fact.