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I'm writing a story where one of the main races is orca (killer whale) mer-folk. Being orcas need to breath oxygen from the surface so do they, this said they live in a halfly submerged cave...but it's a cave how do they see? I need a light source.

- ive thought of fish that produce their own light but this doesn't seam to be available, orcas can only dive 250m and it appears no fish that produces its own light is that shallow.

- ive also thought of dinoflagellates (often thought to be a kind of algae) but I don't know how feasible they are in a cave system though.

At the same time what would they use to sit on? I'd assume it would be some sort of living rock or a plant but I have no clue What fits that description.

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    $\begingroup$ Creatures who live in water have a density very close to the density of the water they live in. (Otherwise they would have to expend lots and lots of energy to swim up and down.) The point being that it is not clear what is to be understood by "sit on" -- they live effectively in microgravity, they cannot "sit on" anything because the net downward force is very small and insufficient to fix them to a chair or bed or whatever. $\endgroup$
    – AlexP
    Jun 28, 2020 at 17:15
  • $\begingroup$ Orcas don't need light; they have echolocation. $\endgroup$
    – Jan Hudec
    Jun 29, 2020 at 12:17

4 Answers 4

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Cherenkov Radiation.

enter image description here

https://www.studioalicino.it/en/page/čerenkov-light

I know, usually hard radiation and merpeople don't mix. That's why this is great! Bear with me...

Cherenkov radiation is produced by fast moving particles (usually electrons) in water. This phenomenon produces characteristic far blue / UV radiation which looks awesome. These merpeople are already in the water; check. And one would expect their eyesight to do best with far blue and UV frequencies which are the ones that penetrate water to greatest depth!

But where to get sources for Cherenkov radiation? They are not just lying around... or are they? Fortunately this is fantastic merpeople fiction!

Deep under the ocean, radon generated by natural radioactive decay accumulates. Sometimes these accumulations are trapped in metastable clathrate crystals which can survive being brought topside. The radioactive radon will produce blue Cherenkov radiation suitable for illuminating the cave for some time, gradually becoming dimmer as the radon is exhausted. The cave will be illuminated with indigo, violet and UV frequencies by eerie glowing crystals.

How to safely handle the glowing crystals? Probably with shielding, or tongs, or both. Or short exposure times. Old merpeople who specialize in finding and retrieving these crystals might have growths or cancers on their hands from a lifetime of exposure.

I like the idea that these crystals from deep ocean trenches are produced by life forms, which concentrate uranium salts via biology and live via radiobiology on the emissions produced. The crystals and their contents are made by life forms. The Cherenkov emissions are a byproduct. The merpeople know the crystals are alive, in the way that plants are alive, and treat them respectfully. Once the blue glow fades they bring the crystals back to the deep places where they were found, to regenerate themselves.

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  • $\begingroup$ This is an interesting idea however it sounds like these crystals are pretty deep, once again if they're deeper than 250m the orca mers can't get it. Perhaps it could be one of the other subspecies thing though. $\endgroup$
    – Umber Song
    Jun 28, 2020 at 17:33
  • $\begingroup$ They can send the elephant seal merfolk to get them! I hope you have elephant seal merfolk. $\endgroup$
    – Willk
    Jun 28, 2020 at 18:39
  • $\begingroup$ I got silkies (wich are basically seals that can turn into humans) though they're kinda the enemy of the orcas... I mean some orcas do eat seals $\endgroup$
    – Umber Song
    Jun 28, 2020 at 20:07
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    $\begingroup$ I'd like to point out that water is surprisingly good at keeping ionizing radiation at bay. As noted in Randall Munroe's "What If" book, 7cm of water cuts ionizing radiation by half. Handling crystals with tongs/baskets seems viable. $\endgroup$
    – IvanSanchez
    Jun 29, 2020 at 11:23
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You don’t necessarily need a light source. Depending on how human-like you want these merfolk to be, you might consider giving them echolocation in addition to vision. Toothed whales (including orcas) already use echolocation to compensate for limited-visibility conditions underwater.

This could have some interesting cultural/psychological implications on your merfolk community as well. If they use biosonar for underwater communication as well as navigation within their cave, they could, for example, have two different languages - one for when they speak through the air, and one for when they speak underwater. Or they could have a single dual-purpose language with which they can communicate either above or underwater.

See also this link, which describes orca vocalizations in greater detail.

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  • $\begingroup$ I see your point, I had planned to give them a language that was clicking and squeaks for underwater, after all we require air to talk and using that underwater to talk would be a waste. I could definitely add in they listen to their own clicks as it comes back. $\endgroup$
    – Umber Song
    Jun 28, 2020 at 17:31
  • $\begingroup$ @UmberSong Right. The only potential problem I see is that merfolk usually have human heads and fish bodies, but orca squeaks are (presumably) produced in their heads. So you’ll have to modify the anatomy of their humanoid heads. But then again, the humanoid part will have to be significantly modified anyways to adapt to underwater life. $\endgroup$
    – Franklin Pezzuti Dyer
    Jun 28, 2020 at 17:33
  • $\begingroup$ Oh I completely forgot about that! Thanks for bringing it up... This definitely makes it harder as I wanted to keep the top looking nearly 100% human (minus webbed hands on some and the occasional orca colored skin) maybe the clicks are actually from an item they all hold ment to mimic orcas sounds? This would also explain why lights could be needed, as like a back up. $\endgroup$
    – Umber Song
    Jun 28, 2020 at 17:40
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    $\begingroup$ @UmberSong Well, “nearly 100% human” isn’t going to work because of other unrelated issues. For example, human tissue doesn’t fare well when submerged in water for extended periods of time. Also, human eyes probably aren’t optimized for seeing underwater anyways, and as mentioned in this answer, human-length hair and noses may be obsolete. Accommodating biosonar might be easy relative to these issues - just modify human vocal chords to also make underwater clicks. $\endgroup$
    – Franklin Pezzuti Dyer
    Jun 28, 2020 at 17:44
  • $\begingroup$ Sound travels very differently in water... they almost surely will have a separate underwater language, and it won't necessarily be audible. The sea dragons from Wings of Fire use bioluminescence, for example. Or, if it is, it will probably use infrasound (which can be used in air, also; see e.g. elephants). $\endgroup$
    – Matthew
    Jun 28, 2020 at 17:44
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As Dyer said, echolocation might do in the dark. If you really want light, though, then they can drill holes to the floor above the cave, then at the bottom of each hole they can place some refractory material such as plastic - might illuminate the place as well as light bulbs by day.

If these merfolk have technology as advancednas our own, they may also use the kind of lamps we use in swimming pools. Otherwise they might trade with the sperm whale merfolk and maybe shark merfolk for the corpses of light producing fish from the abyss. From these they could extract luciferin and luciferase (call it glowfish juice for a primitive feel). Mix these on demand to get minutes to hours lf light in a cave.

Last but not least they might trade with land dwellers for elemental phosphorum. That thing can replace oxygen underwater for some really hot combustion (water kills fire because it doesn't hold as much free oxygen as our atmosphere, as well as being a heat sink). You can light some marine torches with it.

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    $\begingroup$ I like the whole luciferin and luciferase Idea, a bowl of pretty glowing fish guts sounds both gross and funny XD $\endgroup$
    – Umber Song
    Jun 28, 2020 at 17:25
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I got a few more exotic ideas for light:

  • Chemiluminescence - they don't need the organism, just the chemicals. Aequorin from a kind of Jellyfish might be a candidate, there is even a note about gene experiments to use the relevant genes to create glowing animals.

  • Sonoluminescence - light from (very loud) sound. Apparently pistol shrimps manage to create flashes of light by (loudly) clapping. I could imagine a "clapping" device for such flashes.

  • Heat - essentially underwater torches. There are mixtures such as Thermite that burn so hot they are not extinguished by water.

  • Light guides, stationary though. There are low tech ways to guide light into dark places - such as sunlight bottle lights.

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