It's a fish-eat-fish world out there. But the big Versurgia jellyfish helpfully provides a respite. Small, tasty slender yellowtail kingfish can hide in his bell, out of the reach of larger trumpetfish which lurk just a few inches away.
The Versurgia doesn't sting the kingfish because it doesn't have tentacles. It has spongy arms that filter-feed plankton. It does, however, have stinging threads which the kingfish have to avoid as they hang out. Earth Touch News Network describes the relationship:
The jelly also has stinging thread cells at its disposal, which the baby kingfish are careful to avoid. Their sting isn't harmful to humans, but it's enough to deter some species of fish from getting too close (apparently these trumpetfish are willing to take their chances for a quick meal). [...]
As for the kingfish, they'll stay hidden amongst the stinging threads, feeding on the jelly's leftovers until they're big enough to venture into the open ocean. Just what (if anything) the jellyfish gets out of the situation remains a mystery. In captivity, these fish often turn to snacking on their gelatinous host – likely because they don't have enough plankton.
-via The Presurfer