I'm thinking of a predatory creature that lives deep inside large caves. Because this environment is so dark, the creature's eyes have disappeared through evolution since the creature relies on echolocation rather than sight. But the thing is that this creature has a large frill around its neck with small bioluminescent marks. But that got me thinking, what is its use? I mean, if the creature is blind then it surely can't use the frill to frighten other members of its species. So why would it evolve something like that?
Just think of that other place where the sun doesn't shine. I mean the ocean depths.
The bioluminescence of a blind animal (such as a bioluminescent jellyfish) can either stun or confuse a predator that is not blind.
The atolla jellyfish in particular is a sore loser - if it gets attacked by a predator that is able to eat it, it gets flashy and its lights end up attracting even bigger predators that will eat its attacker.
And the there are the angler fish. They use bioluminescence to attract prey. Granted, they are not blind. But your blind creature may use its frills to both attract and capture prey.
The chemicals that create bioluminescence in animals are often highly toxic. This could be a warning to predators, which is how it is often used in nature for terrestrial creatures, or a part of its hunting strategy. If the frill is spikey, like the quills of a porcupine, it could use them to poison and kill its prey. Obviously, the creature itself would be immune to this toxicity. This would have the added bonus of being a sort of area-of-attack weapon, which might be good for a blind creature.
The bioluminescence can be a vestige of when the creature's ancestors lived in very dim caves or caves (or other spaces) that were dark part of the time while they were awake. While they still had functional eyesight.
Plenty of species have biological traits that aren't currently needed. If there's no evolutionary advantage to not having them, they may not fade away as quickly as traits that are better off not being there (like eyes that can be a vector for infection if injured).
Once you've established that the bioluminescence is still around, it can prove itself useful in the ways other answerers have mentioned.
Not all animals who live in the cave or who live nearby may not be completely blind like your creature. Maybe there are others who use very acute vision, who could see well with tiny amount of light. Or maybe they have their own sources of light, like the Angler fish which helps them see inside the dark caves.
So while your creature is visually blind, it may still help it to have a intimidating frill to frighten off predators, and it can evolve while the creature itself is blind.
So why would it evolve something like that?
Because it evolved the frill before it became a completely subterranean species.
- The frill is an echolocation receiver (it helps capture the reflected sound waves).
- It's a mating display, possibly with the bio-luminescent spots "looking" different to echolocation than the surrounding frill (the spots served as display while the species was sighted, and developed to remain a display while the creatures lost their sight)
- Appearing larger than they are (anti-cannibalism: small creatures look larger from head-on, so larger members of the species don't try to eat them)