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I'm designing a world similar to Earth (liquid water, temperate climate, there is an atmosphere, oxygen and complex life everywhere).

The only big difference is that there is a giant (not glowing!) moon with an impossible orbit, and its shadow is always on the same area on the ground. So I basically have an area with good life conditions, but no sunlight (We can assume that some river will cross the dark area and that wind will bring oxygen in it from illuminated areas).

I'm aware that such an orbit is not realistic, let's ignore this point.

What would life (plants, animals) look like and evolve? Could light be used as another tool by evolution?

I though about mushrooms, and I looked for similar conditions that could exist in real life (caves?), but I didn't find anything that could be interesting for a story.

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    $\begingroup$ How large is your dark area, and where is it located? $\endgroup$ – clem steredenn Aug 27 '15 at 14:08
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    $\begingroup$ You realize that sunlight is literally what makes a planet temperate? If you take that away, you're going to end up with a snowball. $\endgroup$ – Dan Smolinske Aug 27 '15 at 17:40
  • $\begingroup$ IFLS posted an article about what a hypothetical mermaid that dreamed of becoming human and marrying a prince would realistically look like if she had evolved under the sea. That's probably a real good starting point for you to reference. $\endgroup$ – Ellesedil Aug 27 '15 at 18:54
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    $\begingroup$ You should clarify whether or not the rest of the planet receives direct sunlight. It sounds to me that it does, but answers/comments are being made under the assumption that it does not. $\endgroup$ – Alec Gilliland Aug 27 '15 at 19:09
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    $\begingroup$ As Alec suggests, if the rest of the planet receives direct sunlight, then life would evolve there from whatever initiating event brings life into being. From there, certain life forms would adapt to increasing levels of darkness until they didn't need light at all (and probably are allergic to it). $\endgroup$ – J.D. Ray Aug 27 '15 at 19:40
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Your answer lies in the deep places of Earth because those are the exact same conditions you've described. Life in these areas have developed sensitive chemical sensors or very sensitive eyes (or no eyes) to locate food. Often they have no coloring because there's no use for it in absolute darkness.

In the darkness, creatures sometimes form symbiotic relationships with bacteria. Tube worms rely on bacteria in their gut to transform the hydrogen sulfide in the water into something the tube worm can use for food. Others feed on detritus that falls from higher up in the the water column.

Terrestrial Life

Amphipod Cave Shrimp

Aquatic Life

Hatchetfish Anglerfish

Just don't go too deep. Balrog

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    $\begingroup$ That's weird. Why would there by a guy in robes with a glowing stick deep down? $\endgroup$ – Frostfyre Aug 27 '15 at 16:48
  • $\begingroup$ -1 It seems reasonable except for the part where most of the energy (not all, some comes from hydrothermic vents) comes from up above in the deep sea. It just slowly progresses down (either through animals eating eachother, plankton dying and slowly dropping down, etc. etc.) $\endgroup$ – David Mulder Aug 27 '15 at 21:46
  • $\begingroup$ @DavidMulder I added a note indicating the falling detritus method you described. $\endgroup$ – Green Aug 27 '15 at 21:50
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    $\begingroup$ @Frostfyre Don't you know? It's Gandalfs all the way down. $\endgroup$ – jedd.ahyoung Aug 28 '15 at 1:06
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Look deep down below the water the life would be mostly anaerobic (not breathing oxygen, not producing oxygen.) and on cell level. For complex life you need more energy = light

Add light, otherwise say good bye to complex animals :)

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The problem with this whole scenario is that you need a source of energy for life. On earth that is almost always the sun, the only exception being certain geothermal vents. Even the deep oceans get food coming down from above which originally used sunlight.

In other words there would be nothing living in this area, it would be a dead zone.

The only way to correct that is to have stuff coming into the zone - possibly using geothermal vents as suggested in the other answers - or possibly being brought in through wind or water.

For example if you had a massive river flowing into the dead zone then it would sweep life in with it. Eyeless scavengers could then live alongside the river grabbing anything edible that comes floating down.

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  • $\begingroup$ Thanks for your answer. I updated my question because there would be sunlight and life outside the dark area. There would be a river crossing the area, wind that can bring oxygen, and complex life forms could even hunt outside during the night. $\endgroup$ – Sebastien C. Aug 28 '15 at 10:43
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I would start looking first at hydrothermal vents in the ocean. They feed microorganisms in colonies creating a 'bacterial mat' which in turn attract "Snails, shrimp crabs, tube worms, and fish feed on the bacterial mat". These in turn draw larger animals like octopus and squid.

On top of that. The there will still be energy being applied to nearby ecosystems from the sun and light as as such there will be other things that could live there. Just not 'plants'.

Now there is a problem with your imagined situation. Physics won't allow things to work just how you've said. A moon has to travel around (orbit) the planet, not it could be in a geostationary orbit which would keep it exactly over the same spot on the planet, however, it's shadow would move around. It would prevent direct sunlight from ever reaching a certain area, but would not prevent all light forever. Any moon planet at an inside orbit between the sun and the planet itself, would orbit the sun at a faster speed and thus only block the planet for 'short' periods of time. If they were close enough to have a very similar orbit they would interfere with each other.

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  • $\begingroup$ Thanks. I'm aware that is an impossible orbit, but I decided to ignore this fact because I like the idea and it's a fantasy world :) . $\endgroup$ – Sebastien C. Aug 28 '15 at 10:45
  • $\begingroup$ You could put the moon at the Lagrange point between earth and sun. This is still problematic because the Lagrange point is unstable (that is, over time the moon would tend to leave that position), but apart from that it's an orbit that moves the right way for your purposes. $\endgroup$ – celtschk Sep 12 '15 at 20:26
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Actually this already does exist in deep waters.

Near the bottom of the ocean (specifically, below the Photic zone [sunlight zone]) there are creatures that survive with effectively no sunlight at all. They've adapted to either create their own light, or use other methods to detect their prey and eat.

How would they look?

The barreleye

Source: http://www.mbari.org/news/news_releases/2009/barreleye/barreleye.html

Rainbow Jellyfish

Source: http://news.nationalgeographic.com/news/2009/03/090317-new-rainbow-jellyfish-picture.html

Abraliopsis Squid

http://www.wired.com/2011/01/bioluminescent-sea-creatures/

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    $\begingroup$ I'd just like to point out that they wouldn't 'look' like anything, since there is no light to see with :P $\endgroup$ – DaaaahWhoosh Aug 27 '15 at 14:12
  • $\begingroup$ +1 For giving me an idea (transparent creatures are interesting for a fantasy world) :) . $\endgroup$ – Sebastien C. Aug 28 '15 at 10:48
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According to origins of life, life originated without need of light, and only much later photosynthesis evolved. So yes, life could, and did, evolved without light.

Plants and higher life forms would have hard time to evolve though, because without light energy there is no photosynthesis. Without light, all your life would likely be bacteria processing chemical energy resources like volcanic vents.

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I think you should explain how the moon came to a fixed orbit. Was it always like that, or id it halt due to a catastrophe of cosmic proportions? If it's the first case, then you could use a lot of the answers already provided to create a basis for the creatures living there. But if it's the second case, you can say that the complex, even sentient creatures already living there have evolved and adapted to the new circumstances, even if it was over the course of centuries.

Also, you need to specify how large this moon is. Let's say that your world is the size of Earth and this moon is the size of Australia. Even if it was always on the sky, the area it hovers over would not be completely dark, as you could see light at the edge of the horizon. Perhaps the creatures inhabiting that area can adapt to live in a world with less light.

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if the moon-thing actually is too unrealistic you could accomplish something similar with an Eyeball Planet. https://www.space.com/20856-alien-planets-eyeball-earths.html

The life living further towards the shadow-side could just reference deep-sea creatures, arctic creatures and troglobites.

https://en.wikipedia.org/wiki/Deep_sea_creature

https://en.wikipedia.org/wiki/Wildlife_of_Antarctica

https://en.wikipedia.org/wiki/List_of_troglobites

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  • $\begingroup$ Hello Fred! It looks like those links could help the OP and future readers with the question asked, but links can get out-dated, which would leave your answer basically useless. If you are using links in your post please make sure to summarize their content and only post the link for readers who want to know more about the topic than they need to know to answer the question. Otherwise your answer might get deleted as a "link-only"-answer. And deep-sea creatures have been discussed in other answers to this question. $\endgroup$ – Sec SE - clear Monica's name Jun 26 '17 at 9:09

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