Sea turtles eat jelly fish and not much else because they can't catch anything else. They are one of the very few things that can eat certain jellyfish too as some jellyfish are extremely poisonous.
** On the contrary- sea turtles eat fish and plants also. There are few or no animal that eat only one type of prey. Only if they live in an area with limited food sources such as the polar bear. Polar bears mainly prey on seals because there is no other prey for them to eat.