I think the shark you are refering to is the greenland shark and its about 512 years old, as a matter of fact there are lots of animals with a very long life span, the turtle for example, what makes them different from the immortal jellyfish is the fact that though they age very slowly, at some point age would catch up to them and they'd die, the immortal jellyfish on the other hand wont die of old age and that's why it's technically immortal