5. They are growers, not showers.
Most jellyfish are small and unassuming, but holy crap do they grow. So quickly, in fact, that massive schools of them "bloom" in a matter of days. This explains why they aren't diluted by ocean currents but instead wash up all at once and spoil your sexy beach time. Jellies grow so rapidly that they can wreak havoc on fishermen. Japanese shrimp boats have capsized after accidentally scooping up tons of the creatures.
6. They are immortal, kinda.
Most jellyfish live only a few weeks or a couple of months. But a few types can revert to their larval, or "polyp," stage indefinitely, meaning one jelly can more or less live forever.