Nantucket said:
A Jelly Fish does not die of old age - it just gets reborn over and over again.
Sharks have two penises.
I think I'm too much into this thread but I do love science and I think our trivia should be a proper one. Also, you have the bad luck that I'm a student of biology who just today listened to a lecture about sharks AND jellyfish. So have my apologies.
Jellyfish are not "immortal" as a species, their usual life-cycle only spans between days and a few months, in captive cases and with proper human treatment they might reach a year or so but it won't go much farther generally. In the wild if it doesn't get eaten by predators it at least dies after reproduction.
What you're referring to is the specific <url=http://en.wikipedia.org/wiki/Turritopsis_nutricula>T. nutricula, which has been shown under lab conditions to be able to revert back to polyp stage thus avoiding the inevitable post-reproductive death.
As for the two-penis sharks I'm not gonna correct you there, just adding that they are actually pelvic fins that got modified into what is called
claspers so it is logical they'd have two penis-analogues. I might also say that in two documented cases (one Hammerhead, one Atlantic Blacktip) sharks have been shown to perform "virgin births" or parthenogenesis.