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Let's assume the planet has only one half the mass of the moon and is covered by shallow ocean. To eliminate water pressure at extreme depths, assume the ocean is also quite shallow, with a maximum depth of 100 meters.

How would critters living in this ocean be affected? How would life evolve? For the purpose of the question, the planet somehow has an earth-like atmosphere so the alien fish can breathe.

Edit: the ocean MUST be no more then 100 meters deep at any given point. Assume we are dealing with very shallow water here.

Edit 2: for the purpose of simplicity, the mass AND volume are exactly 1/2 of the moon.

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    $\begingroup$ Armok - keep in mind that a planet with lower gravity also means a much lower water pressure as you go deeper - is the 100m max depth limit important to you on its own - or are you actually interested in a "low G ocean with pressure similar to Earth's top few hundred meters of ocean"?. (If it's the second, consider changing your question to reflect that :) ) $\endgroup$ – G0BLiN Feb 25 '18 at 8:40
  • $\begingroup$ Either or is fine, really, but for clarity lets make it 100m depth max. Editing now. $\endgroup$ – Armok Feb 25 '18 at 8:43
  • $\begingroup$ Also - is this scenario about aquatic creatures which naturally evolved on this planet? or about the ability of already evolved creatures (imported from Earth or some alien but more Earth-like planet) to survive in such environment? $\endgroup$ – G0BLiN Feb 25 '18 at 8:54
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    $\begingroup$ This planet seems likely to lose it's ocean and atmosphere fairly quickly on an evolutionary scale. How much heat can we expect in this environment and does the thing keeping the atmosphere in block any light? $\endgroup$ – Muuski Feb 26 '18 at 15:53
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    $\begingroup$ "How would life evolve?" is always extremely broad topic. $\endgroup$ – Mołot Feb 26 '18 at 18:20
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Though I believe the other answer is right, in that a planet as small as you're suggesting would be unable to keep an atmosphere (and therefore an ocean) due to lack of gravity, there are ways for a smaller planet to maintain a liquid ocean that supports life:

"Owing to a greater depth of hydrothermal circulation from thermal cracking in brittle mantle material, small ocean planets in the Solar System may have the capacity to support ecosystems that are stable on geologic timescales, with greater amounts of bioavailable energy than previously suggested. " http://online.liebertpub.com/doi/pdf/10.1089/ast.2007.0075

Assuming there is some sort of force keeping the water on the surface (not to mention the atmosphere), you'd be dealing with several things:

  1. Reduced water pressure - If you've got 1/2 moon gravity, that's about 8% earth gravity, so the water pressure would be reduced as the weight of the water would be less. I'd guess that would mean that creatures would either more easily traverse greater depth ranges or be confined to smaller ranges of pressure than earth creatures. Interestingly, this doesn't mean less dense water, as water is essentially incompressible, and the difference in ocean water density has more to do with temperature and salinity than pressure.
  2. Lack of light - There's a possibility of oceans existing underground or beneath ice on smaller planets, so organisms would likely have to exist either without or with very limited exposure to sunlight. In that case, this probably limits the food chain to very small, efficient organisms. Bacteria, etc. See the above-quoted article for more information on the placement of oceans.
  3. Ice - If your ocean is too deep, the high pressure would turn the water to ice. So watch for that.
  4. Ocean coverage - If the ocean covers the entire planet, some of the precursors to earth-like life would be difficult to come by. On earth, phosphorous is washed by rainwater into oceans from rocks on exposed land, and that's a necessary element for the development of plankton and similar life, so depending on your planet, those might not exist.

    "It turns out that water worlds may be some of the worst places to look for living things. One study presented at the meeting shows how a planet covered in oceans could be starved of phosphorus, a nutrient without which earthly life cannot thrive. Other work concludes that a planet swamped in even deeper water would be geologically dead, lacking any of the planetary processes that nurture life on Earth." https://www.nature.com/news/exoplanet-hunters-rethink-search-for-alien-life-1.23023

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Being the Moon, with all its mass, unable to keep an atmosphere, it is straightforward that also your planet with half a moon mass won't have any atmosphere.

In such conditions liquid water is not going to last long and it will quickly evaporate.

So, your critters won't live at all.

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  • $\begingroup$ As stated in the question, the planet has an atmosphere. If you absolutely 100% need a method without hand-waving, there is an artificial sphere keeping the atmosphere in. $\endgroup$ – Armok Feb 25 '18 at 8:27
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    $\begingroup$ -1. This observation is absolutely correct and important to point out - but this seems more suited as a comment to the question rather than an answer on it's own right (basically, OP asked about "A" given "conditions X, Y and Z", this posts states that "Z" isn't naturally possible, but isn't addressing "A" - the main question - marine life in such environment, at all) $\endgroup$ – G0BLiN Feb 25 '18 at 8:47
  • $\begingroup$ Europa has liquid water and a thin layer of gaseous oxyge over it. $\endgroup$ – Renan Feb 26 '18 at 18:27

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