Deep-Sea Fish Sight
This is unheard of in vertebrate vision. ~ Finnish sensory biologist Kristian Donner
As an energy efficiency, crickets and cave fish lost their sight generations after their forebears moved into pitch-black caverns. Deep-sea fish took a different tack.
The last glimmer of sunlight is gone at 1,000 meters. Below that are faint flashes of bioluminescence from bacteria, shrimp, octopi, and fish. With the opportunity to communicate and hunt, at least 3 different branches of deep-sea fish independently evolved elaborate arrays of opsins that pick up the specific spectra that ambiently appear, affording color vision through ingenious receptor cell design and unique sensory processing.
Rod opsin photopigments are tuned to different wavelengths of light, allowing color vision in the dark. ~ Czech evolutionary zoologist Zuzana Musilova et al