I'm sorry in for double-posting in advance, but I feel like I have to get this off my chest as it was something that was really nagging at me for quite some time now, but I held off since I was afraid there was going to be some mean jerk saying I'm objectively wrong and bullying me because of it, but now I think it is finally time to clear up the confusion on how Octolings reproduce, and off course, both in-game and out-of-game evidence to support my claim. This is probably going to be me explaining the birds and the bees of Octolings as clean as possible, I will try my hardest at least. So here we go!
Now to begin, there's a page of the original Splatoon Artbook that states that Octarians are created by cutting off a tentacle from the host and cannot reproduce on their own. Now while this statement does explain why most Octarians appear to be severed tentacles or a mass of tentacles, it doesn't list examples of high-class Octarians, which are Octarians with more tentacles like Octobomber and Octohurler. What does this have to do with the Octoling, you might ask. Well, I believe that the "cut tentacles" are actually the other Octarians and not referring to Octolings at all. But I do believe that the Octoling is the host in which the rest of the Octarians come from. While that statement might have been true for Octolings in Splatoon 1 since all the enemy Octolings from that game look exactly the same aside from tentacle color, but it stopped being true since they indirectly confirmed that Octolings can have different skin tones when they have shown Marina.
So here is where I can draw conclusions, things that are gender, skin tone and eye color are genetics and thus, set in stone. I do believe that things that can only (keyword only) be changed in the separate options menu from clothes entirely (out of battle only) aren't part of lore and is just simply there for players' convenience, especially now that you're able to choose which species to play as, which I just can't see an Inkling just suddenly becoming an Octoling out of nowhere. My point is that Octolings can reproduce both sexually and asexually, now I won't go into detail of what they would have to do to reproduce sexually for obvious reasons. When Octolings reproduce sexually, only new Octolings are born (unless an Octoling and an Inkling reproduce instead, but we don't know if that's a possibility within canon yet), but when they reproduce asexually, only those tentacled Octarians are formed and none of them develop into Octolings (I know there's the Octo Samurai, but I'll get to him later in the theory). To sum it up with evidence, the Squid Research Lab use real world biology when it comes to the species in Splatoon i.e. Jellyfish reproduce by splitting (look up their page about Jelfonzo and you'll know what I mean) just like their real life counterparts, Salmonids prey on Inklings just like how salmon prey on squids in the real world.
Asexual reproductions that Octolings perform is based off how if you cut an octopus's tentacle, the tentacle will keep moving on its own for sometime (not a long time). This specific type of asexual reproduction is called Fragmentation (which can easily be confused with Budding if a difference is not drawn). However, real octopus reproduce sexually. Another piece of evidence is SashiMori's Octoling member, he already has all 8 tentacles and resembles a developing Inkling (as an Octoling instead), now while this could be evidence for both sides, one crucial part that no one seems to pay attention to is the Octarian lips, the Octo Samurai is actually a highly advanced tentacle that managed to have the capability of respawning, a rarity among the tentacled Octarian subspecies. So what does the Octarian lips have to do with this, you might ask. Well if we look at SashiMori's Octoling again, he is 10 years old, however when we look at something called Apoptosis, we would see that he would still have the Octarian lips if Octolings would've started as tentacles, just look at how a frog who has just come out of the tadpole stage still have the tail which is then absorbed into the body through Apoptosis, eventually losing it completely. Also, there is just no way Octolings start as tentacles if they have differing skin tones and eye colors from each other, as they would have the same exact genetic material as their hosts. To conclude, Octolings can only be born when a male and female Octoling mate, where as the other Octarians come from their severed tentacles, which the tentacles themselves cannot grow into Octolings.
Phew! That was quite a long explanation, if there are more interesting lore in the future, I hope I don't have to type as much as I did.