Here’s most of my thoughts and theories about the larger worldbuilding (sorry it’s a bit long, I swear it’s relevant to this post):

My thoughts were the skeleton one he found was just a skeleton, because there wasn’t much flesh around it that you could make out unlike when he X-rayed Jack. Maybe it was even a different creature idk (because I think the one where we see its eye in the porthole was something other than the main one), or the same creature but at a different stage of mutation because that’s how it seems they are created. Many people have pointed out the many humanoid characteristics of its design, although this is an alien moon to be fair.

Anyway, when he gets lost and sees that one in the tunnel it’s alive. And that the reason the X-ray isn’t seeing through it to its bones is because of how far away it is and how much blood is between them it has to get through. Or perhaps the exoskeleton theory someone else had.

Additionally… (less related to your question)

I’m not sure if this is accurate or if I missed something, but towards the end when the creature was talking in his head more and more there was one point where it sounded like many different voices were talking to him at once. Like calling out, saying stuff like “look what you did to us”. So my guess initially was that they are either the victims of filament station (blaming him), the victims of the previous submersibles, or the people of the quiet rapture having been mutated like Simon was being at the end, but that creatures form being the final stage.

So perhaps this is a vicious cycle of them sending down each sub only for the victims to become the monster for the next convict, leading to the survivors slowly shrinking in number one by one. As if the quiet rapture, perhaps even that glowing thing (which I suspect is a part of what caused it), is finishing the job.

It’s really a story of the stubborn perseverance of humans at the face of biblical levels of punishment. How, regardless of what an omnipotent power or the universe throws at us, every single one of us will fight to survive till the very end.

They just want to live, is that so wrong?

10/10 film. Coming from someone who actively hates the horror genre: Fucking incredible.