damn I'm really surprised people are telling you it's unrealistic- I am also a science person and was delighted by how conveniently it worked out with your choice for Longarm to breath Nitrogen
So focusing on the breathing-through-skin-thing, the number one problem is the Square Cube Law; Volume increases at a greater rate than surface area. This is what limits the size of most amphibians and insects on earth, as they can only get so big before they have too much inside that requires oxygen vs area through which to get oxygen.
Life on earth found two solutions for this:
1- Efficiency. This is the evolution of lungs and vascular systems, and just generally being able to move oxygen around the body. Mammals love it, fish love it, birds got weird with it, and moved on to bigger sizes.
(Side note cause you mention Longarm being specifically like an amphibian; amphibians technically have shitty lungs that really on throat muscles/gulping to breath which is what lets them live on land. But this is your alien species so do whatever the fuck you want.)
2- More air per air. While people seem to often forget it, Earth is an extremely dynamic environment. Just as the continents have moved around a bunch over the eons, the composition of the atmosphere has changed drastically. The Current composition is roughly 80% Nitrogen, 20% Oxygen, and a marginal percentage of other bullshit, but this was not always the case.
The Permian period (right before dinosaurs, big fuck-ass crocodile period) had almost 30% of its atmosphere composed of oxygen, while the following Triassic had 15% oxygen (may have helped the rise of dinosaurs, avians are really good at breathing).
And looking back at the Permian period, we find giant insects and amphibians.
So 30% oxygen could allow amphibians to get Several Meters in length on earth- Longarm is living in an insane 80% Nitrogen atmosphere; Things much bigger than him could live on his planet and be fine.
Now that atmosphere is done, we can get a little more in depth with the aquatics side. So things in water breath dissolved O2. Most fresh water in which amphibians live has around 7 ppm (parts per million) of dissolved O2 in it. River water has roughly .25 ppm Nitrogen. Looks like a bit of a problem, but it's important to understand where dissolved oxygen, and O2 in general, comes from.
O2 wasn't originally in Earth's atmosphere- the evolution of plants and their process of converting CO2 to O2 is why it's in our atmosphere at all. It was actually toxic to most earlier life, and it caused a major extinction event when plants started producing so much of it. Life eventually switched to using because O2 happens to be very energy efficient while plants remained on their older system.
So lets say maybe some natural process saturates Longarm's home world with Nitrogen as it does on earth, or maybe some plant organism breaks down something- ammonia, anything- to make N2. This still has problems.
N2 is a nonpolar gas (triple bonded), so has extremely limited solubility in water (and even lower at decreased temperatures). This can be ignored for creativity, as science should be, but is not without solutions.
1- If a lab is trying to create a supersaturated N2 solution, they use a pressure infusion method. Maybe some natural process (or animal? plant thing??) on Longarm's mimics this process.
2- Like dissolves like. Instead of water, Longarm's planet has some covalent liquid.
So dissolving N2 is a little more tricky and interesting, but not all impossible- just an opportunity for more worldbuilding. All this to say that its also not necessary, of course. The science is fun and interesting to pick apart but should come secondary to just fucking around and having fun.
Also Unicron exist. Maybe whatever makes a giant star eating planet thingy possible also dissolves N2 in water. Whos to say?
