I know things like flashlight fish exist; things with luminescence and all that. But how strong could the light be?

I have this cave dwelling species with three eyes except the middle one isn't exactly an eye but a place that can create a beam of light like a flashlight so the creature can see. It can be turned on/off and the intensity can be adjusted. This creature also comes above ground on occasion.
I thought about just giving it echolocation, but then the three eyes would not be as needed, and the three eyes thing is sort of linked to their triangular architecture and other culture things. How strong of a light can be created biologically, and how?

Or, would having three eyes still be realistic if it also has echolocation?

(Okay, this is a bit embarrassing, but not long after I posted this I realized that giving it a horn for the third point for a triangle themed face would solve all my problems so now my anxiety has flared up cause I asked for help I no longer need and took people's time for nothing)

  • $\begingroup$ Bats and dolphins have echolocation and two eyes. What's so special about echolocation and three eyes? (And the earliest tetrapods, including the reptilian-grade ancestors of the mammals had three eyes. Some extant tetrapods still do.) $\endgroup$
    – AlexP
    May 24, 2020 at 21:10
  • $\begingroup$ Are you asking about sharks with fricking laser beams attached to their heads? Or at least some sea bass? That would be totally cool. :grin: $\endgroup$ May 24, 2020 at 23:57
  • $\begingroup$ Do you know why pirates wore eye patches? So they had one eye for on deck, and the patched eye for below deck where the sun didn't shine. Dark-accustomed eyes do not need a lot of light to see with. $\endgroup$ May 25, 2020 at 2:08
  • $\begingroup$ The problem with echolocation OR bioluminescence is it also alerts the prey to where the predator is, because they are emitting light/sound. That's why you get animals like stoplight loosejaws or spotted bats that try to trick their prey by using unusual frequencies of light or sound to see. That might be a good reason for retaining both, at least in some species (others might lose one or the other). $\endgroup$ May 26, 2020 at 0:03
  • $\begingroup$ IMHO, it'd be much easier to grow more sensitive eyes than to grow a "flashlight". To "focus" a beam, one needs to have a general sense of optics, unlikely for people who rarely see light. $\endgroup$ May 26, 2020 at 9:28

2 Answers 2


There is a nice lecture on bioluminescence at


At around the 11 minute mark they have a video of fish with an organ where it can turn the bioluminescence on and off and direct the light.

Fish with Bioluminescence

As you can see from the photo the light is not that directional. The direction is improved some by the organ having a reflective pigment at the back and the bacteria producing the light in the cavity of the organ having the reflected light directed forward.

In a completely dark environment like a cave even a small amount of light could be useful. Perhaps as a lure, or to provide some background light.

However, there are several examples of animals evolving to lose their eyes as they adapt to a cave environment.


If it's ancestors have 3 eyes and there are no pressures for it to loose one, then it's completely plausible for it to keep them.

Just look at bats. Though they are somewhat poorly developed in a few species (not the rule though, as many bats see decently well, with flying foxes having notably good vision). So if your creature leaves the caves, it will need eyes, unless it also starts to rely on echolocation outside the caves.

Dolphins too are a good example, although they still use their vision, underwater isn't the best place for light to travel compared to air, so to make up for it, they rely on echolocation to find prey, as well as other functions.

Now the flashlight. We have some instances of bright bioluminseclscence, but even the brightest instance (a group of dinoflagelates from Mosquito Bay) clearly don't shine as bright as a flashlight. Additionally, to he able to get something brighter, your 3rd eye will likely have to convert into something less useful at seeing, as it'd need to adapt to become a lightsource.

Additionally, I don't think it'd be an advantage overall. Sure it can see in the dark due to having a flashlight attached to it, but unless it's the only thing with eyes inside the cave, both prey and predator will always be able to see it coming. So yeah, overall echolocation sounds a bit better as an advantage.

Summing up: can it happen? Kinda, angler fish and many abyssal species use it, though more as a lure than as a flashlight, and it won't be nearly as bright as one. It just seems like just adapting it's eyes to be better at seeing in the dark (having a tapetum lucidum for example) and giving it the ability to echolocate, thus granting it 2 ways to navigate, sight when there's enough light, and echolocation for when it finds itself in very dark environments (or should its eyes be wounded/rendered unusable for a period).

  • $\begingroup$ Light emission, sound emission - same potential for detection by the prey as any active detection and ranging method, "unless it's the only thing with sensors inside the cave" $\endgroup$ May 24, 2020 at 23:46
  • $\begingroup$ @Adrian Colomitchi true, however it's not a guarantee that all the creatures it hunts or that hunt it are capable of hearing the specific frequencies it emits, for that'd take a few decades to centuries of natural selection (see how certain moths evolved to hear bats and communicate in even higher frequencies their predators can't hear) while in the case of light, just having eyes or photoreceptors is enough (unless we're talking about ultraviolet or infrared, but still easier than a specific high frequency). $\endgroup$ May 25, 2020 at 2:44
  • $\begingroup$ Yeah, nah, mate... I don't find enough arguments to agree ecolocation is better than vision. The things are too complex to be approached by simple "both have eyes, thus vision is not a competitive advantage". A higher sound freq becomes useless for location (you want wavelen comparable with the details you want to resolve), besides sound attenuation goes up with frequency. There's the reason the moths you mention use hifreq ultrasound to communicate in very close proximity only <2cm. - to continue. $\endgroup$ May 25, 2020 at 4:22
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    $\begingroup$ (Ctnd) UV will be useless in humid or underwater envs, water absorbs strongly in UV. Besides, any bio-luminescence in UV range will require chemistry at energy levels (>2.7eV) the bio matter will start to malfunction (DNA going haywire). Far IR is equally useless, the wavelen becomes to large for the obstacles to reflect it (at best, it will scatter it) and it's hard to emit in that band. $\endgroup$ May 25, 2020 at 4:24
  • $\begingroup$ @Adrian Colomitchi Sorry for forgetting about that regard with uv light. Though I didn't get the "both eyes thus vision" part. What I'm trying to say here is that we have animals which use bioluminescence and those who use echolocation. None of the bioluminescent ones uses it with the intensity the asker wants, while many echolocators can use it in a relatively larger range, depending on the frequencies they use. Additionally, we hunt bats, yet we can't completely hear them, neither can the giant centipedes that prey on them according to what I found. We just go where they live and wait. $\endgroup$ May 25, 2020 at 10:31

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