They have to be quick to avoid tentacle contact but get to use the mobile jelly as a safe oasis in the open water. The jellyfish, in return, perhaps are aided in capturing food. Larger fish that are trying to eat the juvenile fish may accidentally get caught in the jelly's protective tentacles. No, they belong to a different group or phylum of gelatinous drifters, the Ctenophores. Most comb jellies do not sting, although one species, in the approximately 100 that exist, does have stinging hairs.
Comb jellies use sticky adhesive to capture their food.
Yes, they are bioluminescent, but they do not do this during the day. The chemicals break down when exposed to light. Light refracts off of their eight rows of comb-like hairs, which beat to move the animal. This refraction is similar to a prism breaking white light into all the colors of the rainbow. The special kreisal tanks are round and the pseudo-kreisals are modified designs with rounded edges.