My next book of Ysabel the Summoner is set deep underwater in the realm of mermaids. Their dark world is lit by bioluminescent creatures.
The jellyfish can be gorgeous:
This is a siphonophore, a colony of hundreds of jellies living along a central core. The fireworks display are the toxic, bioluminiscent tentacles used to lure and snare prey.
Abralia veranyl looks like an art deco brooch.
There also are scary creatures with gigantic eyes and fanged mouths, opened wide to suck in any passing prey.
And its friend, the Pacific viperfish.