But anyway. The dianoga. Fascinatingly weird little world-building detail. Arguably just a fantasy trope that Lucas couldn't get enough of - between it, and whatever sucks Artoo underwater and then forcibly expectorates him on Dagobah, and the Sarlacc, you get a creepy tentacled submerged/subterranean monster in all three installments of the original trilogy. But the swamp monster and the Sarlacc make a vague kind of ecological sense, and the dianoga ... I mean, come on, you don't have to be that much of a nitpicking killjoy to wonder how the heck a free-floating military installation could have unruly creatures living in its waste management system, right? And being puzzled by that weirdness doesn't make me love Star Wars any less. But it's odd all the same.
You can find some more information about the dianoga on starwars.com and even more on Wookieepedia, some of which goes a little ways toward explaining things. Beyond Episode IV, dianogas have appeared in tie-in novels and video games and comic books, because everybody likes a good callback to the source material and everyone loves a good random fight scene with a tentacled horror. At some point someone exposition-dumped that dianoga's can grow to 10 meters in diameter but start out as microscopic larvae, and someone else established that they are hermaphrodites that can produce offspring in isolation. Which is all well and good, and basically along the lines of what I would expect form one of the largest, dorkiest, most meticulously detail-oriented fandoms around.
See, here's the thing: I get that Star Wars is "just a movie" and a fairy tale at that. I get that almost every piece of speculative fiction ever created will have its "fridge logic" moments. Lucas wanted to have a creepy sewage squid in his escape through the garbage chute sequence, and he made it happen, because wanting it was all the justification he needed and it's not like it's some egregious deus ex machina that resolves the overarching plot or anything. But what I find genuinely surprising is that no one has ever gone back and justified it retroactively, and it's still just sitting there, imponderably, all these decades later.
I mean, microscopic larvae and asexual reproduction and some other stuff about stowing away on garbage ships kinda sorta handwaves away objections to how a non-sentient species could become an invasive pest across multiple planets. So there can be dianogas lurking in the aqueducts running under Coruscant, and also in the sewers on Anoat in the Outer Rim, and also swimming wild and free in the jungles of Vodran. But,specifically, inside the Death Star? With post-ROTJ hindsight we know that a Death Star would be built in space, and in secret. Dianoga larvae spread through organic waste. Why was anyone importing organic waste into the Death Star-in-progress?
It's not completely inconceivable, of course, but that's my point: there's a story there. I've read prose stories about Greedo's childhood, and I've read comics stories about one particular Stormtrooper from the boarding party attacking the Tantive IV (not the first one through the hole cut in the hull, but the one behind him who had the misfortune of being the first Stormtrooper we see killed by a single well-aimed blaster shot). The Expanded Universe canon/fanon is vast and yet, as far as I've been able to tell with some cursory research, no one has ever explained the presence of a scavenger pest on the ultimate orbiting battle station as anything particularly noteworthy. I don't feel I'm owed an explanation, by anyone. I don't think Lucas negligently wrecked the suspension of disbelief in his world. I'm just surprised that my fellow geeks haven't addressed it already.
(Yeah, yeah, this is a classic example of "If you see a story begging to be told and fancy yourself a writer, and no stranger to fanfic at that, why not write the story yourself?" I hear you. Even if I were halfway through a first draft in that very vein, I would still be wondering how I was the first person to get there.)
(UPDATE: I just realized that this is basically the flip side of what I wrote about a couple weeks ago: then I thought of a geeky joke, Googled it, and found someone else had already executed it delightfully; and now I'm thinking about something equally geeky, Googling it, and vaguely disturbed by the lack of of pre-existing similar thoughts. Make of that what you will.)