Barbeled dragonfish use invisible light
“Prey species cannot see the predatory dragonfish conning”
The light this fish produces is on the border of the infrared spectrum. This is outside the visible spectrum of most fish, so prey species cannot see the predatory dragonfish coming. Along with hunting by invisible light, the dragonfish uses it to recognise friends. The dangling barbel structure plays a role in identification, and members of a shoal can distinguish one another simply by seeing the light.
At depths of 2,000 metres (6,500 feet) there are few other options for visual communication, and as their light is invisible to other species, they can shine to their friends as bright as they like.

Flashlight fish borrow bioluminescent bacteria
Without the ability to talk like humans, flashlight fish communicate by dazzling one another with repeated bursts of light. Their glow actually comes from a species of bacteria that lives in the fish’s light organ, located beneath each eye socket. They change the message they are conveying by increasing or decreasing the frequency of the flashes, covering or exposing the light organ by moving an eyelid-like flap. These headlights can also be used to escape attack; by shining at full brightness in front of a predator’s eyes, the fish can then dart away while the hunter is stunned.

Comb jellies reflect light like an underwater rainbow
Also known as sea gooseberries, comb jellies aren’t actually that closely related to jellyfish. They belong to a family of their own, known as ctenophores.
Glowing bright neon in the dark depths of the ocean, these oval jellies propel themselves through the water using a group of eight combs, or cilia, which they operate like the paddles of an oar. The combs shimmer in all colours of the rainbow in the dark as light hits and refracts from the surface.