[[quoteright:350:http://static.tvtropes.org/pmwiki/pub/images/kos_1_cover_8500.jpg]]
[[caption-width-right:350:That white is gonna ''stain.'']]
'''''Knights of Sidonia''''' is the latest (and, as yet, still ongoing) manga from Creator/TsutomuNihei, creator of ''Manga/{{Blame}}'' and ''{{Manga/Biomega}}''.

The story takes place on the gigantic GenerationShip Sidonia, where the last remnants of humanity are engaged in a seemingly HopelessWar against the monstrous Gauna. Nagate Tanikaze has lived in isolation in the bowels of the Sidonia his whole life, with only his recently deceased grandfather and Guardian pilot simulator for company. Facing starvation, Nagate journeys to the surface in an attempt to steal from a food processing plant, before being injured and captured by local authorities. Upon discovering his "grandfather's" storied past as a previously important Guardian pilot, the government puts Nagate on the fast track for elite Guardian pilot training. What follows are Nagate's attempts to fit in with civilized society, his growing relationships with his fellow students, and his growth as a pilot in the face of the horrifying threat of the Gauna.

An [[http://www.animenewsnetwork.com/news/2013-05-16/tsutomu-nihei-knights-of-sidonia-manga-gets-anime anime adaptation]] premiered on April 10, 2014. It is being produced by Polygon Pictures, the [=3DCG=] studio behind ''WesternAnimation/TransformersPrime'' and ''WesternAnimation/StarWarsTheCloneWars'', as its 30th anniversary work. Notably, it was licensed and dubbed by Netflix as one of their "original" shows. It was later announced that Creator/SentaiFilmworks got the license to release the series on home video.

Not to be confused with ''Music/KnightsOfCydonia'' by ''Music/{{Muse}}'', or [[Anime/AldnoahZero the mecha anime about literal knights from Mars.]]

Be warned that this page '''contains unmarked spoilers'''.

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!! This Manga and Anime contain the following tropes:

* AbsurdlySharpBlade: Nagate's blade cuts a Gauna in half when he's forced to use it in episode 11 of season 2 after running out of ammo for his weapon.
* AceCustom:
** The Type 17 counts as this for Tanikaze [[spoiler:and Hiroki]]. While rougher to handle than the Type 18 for most pilots, it's also what the simulator Tanikaze trained on was calibrated for, which means he can make it dance.
** [[spoiler:Izana]] gets a later a specialized Type 19 that enables her to act as Sidonia's AWACS InSpace
** The Crimson Hawk Moth is essentially one for the Gauna, being a unique Gauna form. It's roughly the same size and shape as the human's garde mecha, and is quite a deadly fighter.
* ActionPrologue: The beginning of the anime shows Nagate attacking a Guana, then [[ProsceniumReveal stumbling out of the simulator]] after executing that particularly challenging maneuver.
* AlwaysSaveTheGirl: Deconstructed with the elite strike team in episode 3 -- supposedly the top four pilots aboard the Sidonia. They all try to save their female squadmate, she dies anyway and they all get themselves killed in a situation they should have considered not just a possibility but a ''probability'' in the first place.
* AnyoneCanDie: Large numbers of nameless characters are regularly killed by the Gauna, and it's not uncommon to see squads get completely wiped out by a single threat.
** That doesn't even go into how such things as emergency maneuvers can and will override Sidonian gravity, leading to anyone not protected by PlotArmor to go smashing into whatever's in the way at up to five G (gravties, in other words, five times Earth's gravity) acceleration.
* ApocalypseHow:
** The Earth was cut in half, and humanity's desperate flight to the stars is almost Class 3. Sidonia may be the last refuge of Humanity, because contact with the other ships has been gone for almost 800 years.
** Sidonia itself lost more than 90% of its population in the relatively recent past. The vast majority of the ship's current population are descended from artificial genetic templates or are clones of the survivors, [[spoiler:including the protagonist]].
* TheArk: Sidonia and the other Seed Ships.
* ArtisticLicenseBiology: Humans who could photosynthesize would definitely still need to eat more than once a week. [[http://what-if.xkcd.com/17/ As Randall Munroe points out]] in ''Blog/WhatIf'', photosynthesis doesn't produce all that much energy. There's a reason plants don't move around much, let alone have complex nervous systems.
** Following up on the genetic engineering, "Tiresian" humans, the neutral humans that develop gender after meeting the one the Tiresian finds suitable, would be quite an extreme genetic modification. While certain animals slowly swap gender to maintain a reproductive equity and humans given intensive hormone therapy from infancy can develop the other gender's characteristics, gender neutrality is not exactly easy to attain, much less maintain. The photosynthesis in the setting isn't plant photosynthesis at all--they aren't green. The clues are scattered throughout the episodes, but Sidonian photosynthesis doesn't so much feed the body as enable reprocessing of fluids and foods already ingested, in the manner of bovine multiple stomachs. This is why, in a culture that expects the pilots to stink as a matter of necessity, Takikaze smells distinctly ''worse'' after a deployment as he consumes, perspires, urinates, and defecates rather more than "modern" Sidonians. (Hence the cadets poking fun at him while trying to help by supplying him with collected supplies of deodorant, followed by later filling his locker with rice as he eats a lot and the stress of the war was wrecking his digestion.)
* ArmorPiercingQuestion: Being the dense rube that he is, Takikaze asks the following of Izana while they are in the male changing room "Hey Izana, what type of catheter do you use?" Izana understandably loses it and utterly pulverizes him. Izana during this time had already changed into a biological female and was hiding this.
* TheAssimilator: The Gauna are capable of assimilating humans and replicating their appearances and memories. Their first attempts at this resulted in grotesque {{Humanoid Abomination}}s, but with Shizuka Hoshijiro and Shou Honoka the replication process becomes exponentially efficient.
* AttackOfTheFiftyFootWhatever: Izana and another pilot run into a gigantic humanoid Gauna in episode 11 of season 2 while exploring the area where the crash landed.
* AuthorAppeal:
** Nihei really likes his talking bears.
** Also his claustrophobic cities and UsedFuture locales.
* BattleHarem: Like it or not, the bulk of the female named characters are always attracted to Nagate.
* TheBattlestar: The Mizuki (type 1) and the Sidonia (type 3).
* {{BFG}}: [[spoiler:The magnetic linear accelerator rifles that fire the Gauna-piercing bullets]].
** The Heavy-Mass cannon would be considered one, even if tiny compared to the Sidonia ship, it still fires projectiles the size of a building dwarfing a group of Guardians.
* BeautyIsNeverTarnished: Oddly in a setting where this trope could be in effect (medical technology has reached a point where limbs can be regrown), people do end up being disfigured and having odd bits stuck on, such as Izana and her inhuman-looking bionic limbs (her grandma admits that while they could regrow her limbs it would take a long time so it'd be better to go bionic) or Lala being stuck in a bear's body when she was once a very pretty young woman.
* BeCarefulWhatYouWishFor: Tsuruuchi jokingly offers to do a clasp formation with Tsumugi while Nagate was out on a "date" with Izana. She takes him up on it, and flies around much faster than normal. He barely manages to survive the ordeal.
* BigDamnHeroes:
** Remaining First Platoon troops do it for Nagate and Izana in chapter 25.
** Nagate and Tsumugi do it for Izana in volume 9.
** Traded between Nagate and Samari in the anime:
*** In Adrift Samari [[spoiler: goes against orders to retrieve a stranded Nagate and Hoshijiro after he destroys a Gauna that was threatening Sidonia - and takes so many pilots with her that Kobyashi couldn't punish them without losing most of Sidonia's defensive squads.]]
*** In Home, Nagate returns the favour by covering Samari and [[spoiler:engaging the Crimson Hawk Moth personally while Samari's team enter the gigantic asteroid Gauna to destroy it.]]
*** And finally and most awesomely, [[spoiler: Samari heads a orbital strike team to save Nagate's life and incidentally kill ''every Gauna on the planet'' while he's facing hordes of the things on the surface without any ammunition or working engines.]]
* {{Bizarrchitecture}}: The interior column of Sidonia [[http://cdn3.denofgeek.us/sites/denofgeekus/files/styles/insert_main_wide_image/public/knights-of-sidonia-0102.jpg looking like a city arranged vertically]] makes no sense at the first glance... but then it is explained that the ArtificialGravity mechanism may only compensate for 1G worth of acceleration or less, so the bottom has to be where the engines are, most of the time. The entire core of the ship is [[spoiler:a weapon, specifically a mass driver magnitudes larger than the standard ones.]]
* BlackBox: The Kabizashi blades are the only thing that can kill the Gauna, but no one knows how they work or how to make new ones at first.
** [[spoiler:At least until they start pumping out artificial versions of the material the Kabizashis are made form, which leads to the creation of ranged weapons. The Gauna death tolls begin predictably skyrocketing.]]
** No one has any idea how the Gauna's copies of Sidona's weapons work. This gets lampshaded with [[spoiler:Tsumugi, as no one can figure out how she flies, not even herself. Kanata, a younger chimera, is used to create a Gravitational Beam Emitter, a seemingly impossible weapon.]]
* BlueWithShock: Izana after being manhandled by Tsumugi, who wanted to know what their physical proportions were.
* BodyHorror: The fate of those who are captured by the Gauna.
* BreakOutTheMuseumPiece: Tanikaze initially pilots the Type 17, which was on display as a monument. Due to [[RagnarokProofing diligent maintenance,]] the older model still manages to hold its own.
* CallBack: In episode 12 of season 2, Nagate revisits places he was at before, such as the police station and rice factory, before heading back to where he used to live with his grandfather and recalling reliving a FlashBack or two.
* TheCavalry:
** Done in episode 5 of season 1, after Shizuka and Nagate are on their last legs while stranded in space.
** Used again in episode 12 of season 2, [[spoiler:when Samari and the other garde pilots show up to assist him after he barely survives his second encounter with the Crimson Hawk Moth.]]
* CelShading: The characters are actually done in 3D whenever they're not part of the background, but they maintain proportions and conventions of 2D animation. They're literally distorted depending on the viewing angle and often heavily postprocessed; sometimes the omnipresent scratches and other texture details are the only giveaway, but in other shots ConspicuousCG can be noticeable with details like mouths and hair.
* ClonesArePeopleToo:
** The Honoka "sisters" defy CloningBlues and ReiAyanamiExpy with vengeance. Each of them has their own name (Honoka En, Honoka Hou, etc) and is notoriously protective of her individuality. One thing they do have in common is that neither of them is [[ExtremeDoormat timid and complacent]] nor an InnocentFanserviceGirl, as Nagate painfully learns firsthand when accidentally walking on them changing clothes.
** This is also an important part of [[spoiler:Tanikaze's backstory. He was originally grown in a vat as a replacement body for the aging war hero Hiroki Saito, but Saito wanted to die a natural death and believed that his clone should be allowed to live his own life and so he absconded with the baby clone and raised him in seclusion as his own.]]
* ClothingDamage: Izana's pilot suit rips apart in episode 7 of season 2 after noticing some problems with it. [[spoiler:It was around this point that she realized her body was taking on a female form, due to spending so much time around Nagate and being interested in him.]]
* ComicallyMissingThePoint: Tanikaze was a little freaked out the first time he met Lala. She thought that this was because of her claw-like Artifical Arm. ''He'' was more concerned with the fact that a ''[[EverythingsWorseWithBears talking bear]]'' had just walked up behind him. Though it is possible that Lala was doing this on purpose to keep Tanikaze from freaking out.
* ComicBookTime: A meeting between Eiko Yamano's younger brother and Norio reveals that several years have passed since her death at the beginning of the series, though none of the other characters have visibly aged.
* CompressedAdaptation: The anime skips the Garde vs. Garde-type Gauna dogfight in the manga and jumps right to when the Gauna try to ram an asteroid into the Sidonia, which didn't happen until after Kunato was taken over by Ochiai in the manga. However, this is shown in season 2.
* ConservationOfNinjutsu: Used regularly in the show:
** Made pretty blatant in episode 11 of season 2, when Nagate blasts the Gauna with little difficulty. After running out of ammo, he still has three left to deal with. The first two are cut down effortlessly, but the third one gives him a little struggle before he ultimately defeats it. He also has trouble against the Crimson Hawk Moth
** Also tragically played straight when multiple humans fight against one Gauna. Many of them end up being a RedShirt and die rather easily. Only named characters that have been shown longer than one episode tend to have enough PlotArmor to survive, and even then, they usually get hurt badly, such as Izana suffering AnArmAndALeg after barely surviving her first major battle.
* CoolStarship:
** The Sidonia, of course.
** The later introduced Mizuki. Yuhata has a {{Nerdgasm}} when she describes the completed warship and its armament.
* CreatorProvincialism: Many countries built their own ships but the Japanese ship, Sidonia, is the only one we see as the rest have either been eaten by Gaunas or gone off in different directions and are too far away to communicate with.
** WriteWhatYouKnow: Nihei explained in an interview why Sidonia is a Japanese ship.
-->'''Nihei:''' Sidonia has a lot of "Japanese scenes". If I drew foreign culture [[CrossCulturalKerfluffle it would look wrong from the eyes of foreign people]], like {{Ninja}} depicted by American people.
* TheCuckoolanderWasRight:
** There is a significant proportion of Sidonia's population that comes to believe that [[spoiler:the Gauna are specifically targeting the ship because of the Kabizashi spears. They are dismissed as insane, up until they realize the Hoshijiro clone is [[OhCrap always looking in the direction of the Kabizashi...]]]]
** Later subverted. [[spoiler:Even though the Gauna do seem to be drawn to the Kabizashi, that didn't stop them from attacking a ship of demilitarized colonists.]]
** There's a crazy street preacher WavingSignsAround and a Sidonia mystery book claiming, that Sidonia is secretly ruled by a council of immortals. Nobody knows it, but it's the truth (and in chapter 67, [[spoiler:Nagate]] is gonna be a member of it, just like [[spoiler:his so-called grandfather or, rather, genetic template and surrogate father and teacher]] was).
* CurbStompBattle:
** The Gauna are on the giving end of quite a few of these, often killing various characters with relative ease.
** Nagate dishes one out in episode 4, after said Gauna had killed four pilots who were supposedly better trained and equipped than he was. His actions are played up quite a bit, though considering how deadly the Gauna usually are, it was a much needed morale booster.
* CyborgHelmsman: [[RidiculouslyHumanRobots Teruru]] becomes the helmsman of the new Sidonian warship, Mizuki. Beforehand, [[{{Cyborg}} Izana]] acts as helmsman of an old Sidonian cargo ship.
* DangerouslyGenreSavvy: The Crimson Hawk Moth knows a lot about the humans tactics in combat, and the weaknesses on them, such as targeting the garde's head to destroy the camera, thus rendering the pilot blind.
* DatePeepers: Yuhata and Tsumugi attempt to do this in episode 9 of season 2 when Nagate is supposedly on a mission to inspect a resort area with Izana.
* DeathFromAbove: Used in episode 12 of season 2 to rescue Nagate and the others on Planet Nine.
* DeusExMachina: A nicely done one happens in the very distant past of the Sidonia. An exploration team from the Sidonia are investigating a giant alien artifact so huge it dwarfs the Sidonia. Once inside they are attacked by a Gauna. Since there is nothing that can harm a Gauna, the team can only flee in horror. Eventually they get cornered into a room full of flying grains of some kind, that even the team acknowledges as bizarre. The team is pondering this when suddenly the Gauna appears and it looks like the only thing the team can do is die horribly. Just as it's about to strike, it gets caught up in the path of some of these grains. The grains cut through the Gauna and one of them bonds with the Gauna's core causing it to be destroyed. The team is amazed by this and gather some samples of the grains for study. Since they were involved in the first instance of humans being victorious over a Gauna, they are declared heroes and awarded by having research into immortality being fast-tracked and are made the first recipients of the anti-agathic drugs. This sets the stage for events centuries into the future.
* DidTheyOrDidntThey: It's implied that [[spoiler:Kobayashi seduced Nagate while at the hot springs, though not outright stated.]]
** In the anime, it is impossible to tell what happened between Izana and Nagate in Senshukyoo all we know is that Nagate and Izana were fighting forcing a probable confession, Tsumugi is embarrassed about what she overheard, and that the next day, when they returned home, they both looked at each other and blushed after almost going to the same room.
* DifficultButAwesome: The Tsugumori Type 17. As the direct predecessor of the Type 18, its controls are more difficult and it lacks a lot of the features which make the Type 18 a simple machine to pilot. However, in the hands of an experienced pilot like Tanikaze [[spoiler:Or his grandfather]], the Tsugumori racked up more Gauna kills than any other single unit in the series.
** More importantly the Type 17 had a feature that the Type l8 dumped: Anti-Higgs particle coating. The Type 18 was made by a different company that thought the coating was an unnecessary feature, since the Gauna didn't have particle beam weaponry at the time. So Tanikaze's Type 17 survived damage that would reduce a Type 18 into atoms, once the Gauna started generating Higgs cannons and Tanikaze's highly aggressive piloting style often meant he was too close to be able to evade.
* DistractedByTheSexy:
** After Izana's pilot suit rips in episode 7 of season 2, Nagate gets a good view of her body. So much so that he doesn't watch where he's floating, and ends up hitting his head on an airlock door.
** Nagate in episode 12 of season 2. [[spoiler:He can't eject the Shizuka ena that infiltrated his cockpit, so has to keep it inside. Unfortunately she's also naked and on top of him making it difficult to control his garde.]]
* DudeWheresMyRespect: Averted with Nagate. He isn't looked too kindly upon when he first arrives at the academy. But after he defeats a Gauna singlehandedly, people around him are much more respectful of him, and whereas his locker was first stuffed with some bad stuff, after he returns, it's instead loaded up with bags of rice as thanks for defeating the Gauna.
* EarthShatteringKaboom: Sidonia is armed with missiles capable of blowing up entire planets, from a dwarf planet to a gas giant.
* EarthThatWas: Earth was lost to the Gauna a long time ago, in what may or may not have been the events of Nihei's earlier manga ''Abara''.
* EldritchAbomination: The Gauna are massive, shapeshifting fetus-like creatures with the ability to assimilate humans with their CombatTentacles. While their placenta bodies can be injured, Gauna are only able to be killed if their True Bodies are destroyed. Gauna are further capable of replicating human technology, something they become increasingly proficient at after assimilating Hoshijiro.
* EndangeredSpecies: The Sidonia's inhabitants are probably the last humans remaining in the galaxy.
* FlawedPrototype: The Type 19 prototype that Tanikaze was force to use was incomplete and not tested yet. As such it lacked thrusters in its jet form and had to stay in robot mode to fly. Worse yet, the prototype didn't have a head cannon built in, something that is a standard feature on all the Gardes. So when the prototype had his arms pinned, the command post kept yelling at him to fire his Higgs cannon only to realize that he didn't have one in his head.
* FoeRomanceSubtext: The Crimson Hawk Moth does this in episode 12 of season 2 after infiltrating Nagate's cockpit. [[spoiler:It takes on Shizuka's form, rips his helmet off, then kisses him as tentacles from her mouth enter him, while as slowly choking him to death.]]
* ForMassiveDamage: The only weak point on the Gauna is their core, or "True Body", which when sufficiently damaged cause them to explode. Most engagements boil down to the Guardians desperately trying to chip through the Gauna's nigh invulnerable regenerating armor to reach the weakpoint.
* FourIsDeath: All pilots of the Elite Four die in their first real battle against a Gauna. Kunato's four-man squad does not fare much better.
* {{Gainaxing}}: The anime features relatively realistic breast physics, in particular for scenes in null gravity.
* GenerationShip: The eponymous ship. There were others, but contact with them was lost.
* GenreShift: While ''Sidonia'' never stops being about a grim war, as it goes on, its SliceOfLife moments do at times become outright...
* GodzillaThreshold: The Sidonia performs some high-G emergency maneuvers in episode 4 to avoid contact with a Gauna. Captain Koboyashi is warned that casualties would be high based on a computer simulation, but she feels that those deaths are preferable to the Gauna getting onboard the Sidonia. Unfortunately for them, said evasion causes even more casualties than expected. And the Gauna turns around to make a second attempt at reaching the ship...
* ...HaremComedy: It's not easy for Nagate to navigate between the affections of [[spoiler: a variable gender teammate with ArtificialLimbs, a command officer directly responsible for his MissionControl, and an absolutely adorable and distinctly non-humanoid chimera the size of his mech. Nearly every other lady in the cast also has her moments.]]
* HealingFactor: The Gauna have a rather ridiculous one, making fights with them a race against the clock to get to the weak point before they regenerate.
* {{Hermaphrodite}}: Due to some events in the setting's backstory, a third gender was created. Like some species of fish and amphibians, they eventually metamorphose into one sex or the other in order to breed.
* HoistByHisOwnPetard: The Crimson Hawk Moth takes its time to slowly infiltrate and incapacitate Nagate in episode 12 of season 2, rather than just destroying his garde like it would have done normally. [[spoiler:This buys Izana enough time to hack into his mecha, and cause it to force shut the neck seal the Hawk Moth was using to infiltrate his garde, and giving him time to destroy its ena.]]
* {{Homage}}: The camerawork in a lot of the space battle scenes is pretty blatantly done in the style of ''Series/{{Battlestar Galactica|2003}}''
* HonorBeforeReason: Kunato never gets over Tanikaze getting the first chance to pilot the Type 17, nor for Tanikaze getting a Gauna kill before he does. This drives Kunato to screw with Tanikaze in a number of ways, most notably [[spoiler: sabotaging Tanikaze in a fight with a Hive-Type Gauna, which results in a number of his squadmates being killed unnecessarily and [[NiceJobBreakingItHero indirectly enabling the creation of the Benisuzume.]]]]
* HopeSpot:
** The asteroid Gauna's propulsion core needs to be disabled before it can be destroyed, to prevent evasive maneuvers. Ninety percent of it is destroyed...[[spoiler: only for the Crimson Hawkmoth Gauna to break off from the main body and slaughter the squad taking care of it, and the propulsion system quickly regenerates]].
** Used several times in the fight against the Crimson Hawk Moth in season 2. Tsumugi and Nagate always seem to be one attack from destroying it, only for it to then pull an IAmNotLeftHanded attack on them.
* HumanoidAbomination: The Gauna, when they are trying to imitate humans, but they take less humanoid forms in many encounters.
* HumongousMecha: The Guardians are Sidonia's first line of defense against the Gauna.
* HumansAreSmelly: Much is made of how Tanikaze smells bad and it's assumed that it's because he eats so often compared to the new brand of humanity that only eats once a week. However there is no mention about the Captain and other immortals stinking it up, and they are all even more standard old human than Tanikaze. Also at the end of one chapter, Tsutomu Nihei has a note that he has all the pilots stink like mad and have to hit the showers after a mission. The pilots not only sweat like crazy, they also piss inside of their suits and each suit has a biological lining composed of engineered bacteria that process the sweat and urine but generate a foul odour as a side-effect.
** The immortals that don't wear flight suits have significantly less cause to sweat. Nihei does have the cadets sending Tanikaze added supplies (usually just stuffing them into his locker), so the poor kid probably just has the same issues as any other pilot, just more so. Also, he likely did not bathe nearly as often when he was living as an Underdweller.
* HumanPincushion: Well technically Tsumugi isn't human, but she's impaled with lots of projectiles from the Crimson Hawk Moth in episode 11 of season 2.
* IdiosyncraticEpisodeNaming: Most chapters take a possessive form in their naming, usually as "(Someone)'s (Something)": for instance, "Tanikaze Nagate's Choice".
* {{Immortality}}: The captain and other members of the crew deemed essential to Sidonia's mission are kept alive indefinitely with biotechnology but the rest of the population is allowed to age and die at a normal rate. They hide it and convince anyone who must be in the know that the reason this discovery isn't shared among the entire ship is because it requires limited resources, but the truth is that they simply don't want to, as being the only immortals puts them in an unique position of control over the ship's inhabitants.
* ImpaledWithExtremePrejudice:
** Tsumugi suffers a particularly brutal form of this in episode 11 of season 2, after the Crimson Hawk Moth stabs her [[spoiler:through her throat.]]
** Nagate does this to the second Crimson Hawk Moth in episode 12 of season 2 after he manages to regain control of his garde. He also later does it to random Gauna attacking him.
* InertialDampening: The Sidonia has only a limited degree of it, unable to fully compensate for the ship's acceleration at full thruster throughput. When they have to make sudden evasive maneuvers, massive internal damage results and people die, even though they used an EmergencyBroadcast to warn people.
* InfiniteSupplies: Despite placing much emphasis on conserving resources and recycling, the Sidonia has the ability to outfit massive numbers of Guardians and pilots. Later on, they start creating large numbers of smaller combat ships to ferry Guardians into battle. Explained by the ship posessing limited supplies of raw materials but significant manufacturing capability. Any asteroid with valuable materials, from water to metals, that they encounter, is quickly strip-mined and put to good use.
* LastStand: Nagate attempts this in episode 12 of season 2. After barely surviving against the Crimson Hawk Moth and rescuing an injured Tsumugi, lots of Gauna show up. He pulls out his katana awaiting the impending doom... [[spoiler:Only for Samari and the others to arrive and help him.]]
* LensmanArmsRace: The Gauna continually adapt and evolve after every battle, constantly growing more sophisticated and even managing to replicate Guardian technology and tactics. Sidonia meanwhile is constantly developing new technologies and weapons in order to stay ahead of the Gauna.
** Much is made of the issue of communication with gauna, but the aforementioned adaptation shows the possible motive for the eldritch beings out of space falling upon and shattering the Earth in the first place. The gauna combine material and act as hive entities, implying communion and learning through shared genetic memory. It should be noted that the growing sophistication and technological equivalence comes as Sidonians are absorbed by gauna, organic Higgs-boson cannon and the Benisuzume being fairly blatant examples.
* LostAtSea: Technically they're in space, but otherwise played straight. Nagate and Shizuka are both stranded after he goes to rescue her in her escape pod. It takes three weeks before they're finally rescued, and only because all of the cadets disobeyed orders and went on a rescue mission for them.
* LudicrousGibs: Many of the inhabitants of the Sidonia unfortunately end up this way after the ship is forced into some emergency high-G maneuvers to evade an attacking Gauna. The unlucky ones who couldn't find a place to strap themselves into get splattered along the walls of the ship.
* ManInWhite: Most of the Guardian pilots have white flight suits. [[WhiteShirtOfDeath Considering their survival rate]], they might as well [[RedShirtArmy wear red]].
* MarshmallowHell: Sasaki unintentionally does this to one of the shorter mechanics in celebration of [[spoiler: Nagate's success in using a prototype railgun to save the civilian colony ship.]]
* MeaningfulName: ''Gauna'' is an ancient Japanese word for a ''hermit crab''. ''Kabi'' means the germ of a rice grain.
* MileLongShip: The eponymous ship is 28 km long.
* MilitaryCoup: The supervising immortal ship committee [[spoiler:is murdered by the Ochiai clone at Kobayashi's command after its members decided to impeach her.]]
* MinovskyPhysics: Higgs/Hyggs particles play this role in the story.
* MirrorMatch: The Guardians versus Benisuzume, [[spoiler:a replica of a Guardian eaten by the Gauna]].
* MohsScaleOfScienceFictionHardness: A solid 4, at least for the humans' daily hardware.
* MookHorrorShow: Pretty much any time Nagate or Tsumugi are attacking a larger group of them, causing them to suffer from ConservationOfNinjutsu.
* TheMothership: The Gauna Mass-Union ships.
* MythologyGag: Nihei recycles some elements of his [[{{Manga/Blame}} previous]] [[{{Manga/Biomega}} works]] in Sidonia, either as references or actual existing elements, such as Toha Heavy Industries, MSCF, and the Gravitational Beam Emitter, among other things.
* NeuroVault: Ochiai created one (named his 'auxiliary brain'), containing most of Sidonia's accumulated knowledge, which could only be accessed by using his brain. He then destroyed most of the original information to assure his safety (since killing him would destroy any way to access it).
* NiceJobBreakingItHero: If Kunato hadn't been so obsessed with his vendetta against Tanikaze, it's likely that [[spoiler: Toshijiro would not have been integrated by the Gauna, which would also mean the Benisuzume wouldn't exist either.]] An unusual example in that Kunato ''knew'' he was breaking it; he just wanted to pin the blame on Tanikaze.
* NobodyPoops: Averted. Each of the pilots' suits have a strategically placed 'Biological Ureteral Catheter' which is both uncomfortable and embarrassing for the young pilots.
* OffWithHisHead: [[spoiler:Shizuka]] suffers this after pushing Nagate out of the way of a tentacle attack.
* OlderThanTheyLook: All immortals except [[spoiler:Nagate]].
* TheOmniscientCouncilOfVagueness: Sidonia's immortal ship committee.
* TheOutsideWorld: Played with. For Nagate, who was raised in the innermost parts of Sidonia, TheOutsideWorld is Society. However, since Sidonia is a spaceship, it can be considered itself the inner world for all of its inhabitants, with space and the Gauna threat being TheOutsideWorld. This is part of why Nagate's such a great pilot: he's used to space and Gauna due to training a lot, so they're not so unknown to him; his real struggles come from adapting to society and other people.
* OrganicTechnology: [[spoiler:The Chimerae, biomechs grown from Gauna and human cells by Ochiai as a replacement for the Guardians]].
* PhlebotinumDuJour: Both Sidonia and the Guardians are powered by Higgs Bosons (renamed Haigus Particles in the anime).
* PinnedToTheWall: Tsumugi suffers this in episode 11 of season 2 after the Crimson Hawk Moth impales her with a large bolt attack.
* PlotArmor: Only Nagate seems to have a heavy enough suit on to survive the brunt of damage he sustains throughout the show. Many other characters are either a RedShirt or MauveShirt, or at best sustain severe injuries if they do survive such as Izana and Tsumugi.
* PosthumousCharacter: Hiroki Saito.
* RealRobot: The Guardians count as this. Their spindly frames can't even stand up in gravity (with the exception of the [[SuperPrototype Type 19]]).
* RedShirtArmy: Any guardian without a name. Dozens die with every space battle.
* RobotGirl: Teruru Ichigaya.
* RocketPunch: The Guardians can launch their hands off on a towline and grapple onto things with them. Tanikaze manages to weaponize this to defeat the Red Sphingida after it destroys his gun by taking a Gauna-piercing bullet between his fingers and launching it through her body at point blank range.
* SceneryPorn: It's a Creator/TsutomuNihei manga. It's his trademark. In fact, chapters often start with travel-guide style images titled "Sidonia's 100 Sights" that show off the scenery.
** SceneryGorn: Among them is a sketch of a possible Gauna-occupied Sidonia. [[spoiler:It's not too far from what the Gauna recreates in their collective ship in Ch.57, though that is quite beautiful]]. Chapter 0 shows the successful Gauna attack on Sidonia's residential area which happened a century ago.
** The anime does not let down either, and features highly impressive background art. [[http://imgur.com/a/H1DdS Example gallery]]
* ScrewTheRulesImDoingWhatsRight: Nagate and Shizuka are rescued at the end of episode 5 by all 256 of the cadets in the defense squadron. Despite orders not to go looking for them due to them being outside of the retrievable range of the Sidonia, they end up finding the two and bringing them home. None of them are punished for it, as Captain Kobayashi states that they don't have a jail big enough to hold all of those cadets. On top of that, they're also able to use Nagate's victory against the Gauna, and the successful, subsequent rescue mission for much needed morale boost amongst the citizens.
* SnipeHunt: Yure sends Nagate out on a mission to inspect a resort area in episode 8 of season 2. She tells him to take someone along, perhaps Izana. She also threatens him if he reveals her role in this mission to Izana, she would punish him. It later turns out to be an excuse to put them on a date, which Izana isn't happy about after accidentally finding out her grandmother sent him on this "secret" mission.
* SequelHook: Both seasons of the anime end with some unresolved plots, allowing for much more to be seen in future seasons.
* ShoutOut:
** A star system visited by Sidonia during the plot is named "Lem", likely in reference to the works of the Polish science fiction writer StanislawLem, whose novels often feature humans struggling to understand alien intelligence and BlueAndOrangeMorality. Main influence on the series seems to be from ''Literature/{{Solaris}}'', where a planet-wide sentient ocean tries to communicate with humans using memories drawn from their subconscious to manifest solid images, but causes much suffering to the protagonist by manifesting in the form of his former girlfriend who was DrivenToSuicide. Likewise, the Gauna here are made all the more unnerving by mimicking the appearance and personalities of those they assimilated.
** In the episode where they decide to destroy a Gauna-infested planet, one fansub group translated the command as [[TabletopGame/{{Warhammer 40000}} "Exterminatus"]].
** Tsumugi's tail and legs may have been inspired by [[http://www.filmofilia.com/wp-content/uploads/2010/05/splice_m.jpg the chimera from]] ''Film/{{Splice}}''.
* SparedByTheAdaptation: [[spoiler: Izana's limbs were this initially, as she survived intact a battle where she lost AnArmAndALeg in the manga. However, she suffered the very same injury in the second season once it was established that her new ArtificialLimbs were important to the plot]].
* SpiritualSuccessor: The series had an early prototype in the form of an obscure one-shot manga called ''Winged Armor Suzumega, [[EitherOrTitle or]] The Armored Insects Sphingidae''.
* SpitTake: Chapter 34 (or episode 7, season 2)
--> '''Tsumugi''': (talking to Izana) [[spoiler: Since you've taken on female physiology, does that mean you've found a suitable male mate]]?
--> [zoom to Nagate downstairs, who promptly spits out a mouthful of noodles.]
* StarfishAlien: The Gauna are a weird sort of Lovecraftian fetus-like creatures with a central "main body".
* StealthSequel: To ''Abara'', possibly.
* SuperPrototype: Due to being TheAce, Tanikaze gets first run of the Type 19 and (eventually) the high-cost prototype unit for the Type 20 Guardians.
* TranshumanTreachery: Played with. Ochiai aside, almost every member of the cast is genetically and/or cybernetically modified in some way, but they're still pretty normal and ordinary by our standards.
* TheUriahGambit: The Immortal Council wants Tanikaze dead for reasons not immediately explained and to this end has the Captain order him to fly in every single mission against the Gaunas in the hope that they'll eventually kill him. [[spoiler:This backfires as not only Nagate always survives but he eventually becomes Sidonia's greatest hero. The reason for this is that [[spoiler:Tanikaze is an immortal and they couldn't tolerate an immortal outside of the committee hierarchy, so as a compromise to outright killing him, the captain agreed to put him on what should have been a suicide mission as a compromise. As long as he keeps succeeding, he can live. Of course once the committee is offed in a coup by the captain, he's no longer required to attend missions but tends to do so anyway due to having become used to being a hero.]]
* UsedFuture: Interesting in that it is utilitarian rather than dilapidated. Everything from technology to clothing is kept in good enough order to function and recycled once it is not, new items are sparse due to limited resources but made immediately once needed as the ship posesses massive manufacturing capabilities. Nobody quite bothers with fixing up superficial damage and painting over the omnipresent scuffs in the walls as this is considered frivolous, and structural elements like massive pipes run through the place, giving it a rugged industrial town look. All in all, however, the ship is not in disrepair. The residential area of Sidonia is filled with rather ramshackle buildings but those are temporary constructions built independently of the ship's infrastructure. [[spoiler: Sadly, this makes them very vulnerable to the ship's emergency changes in acceleration.]] Much of the clothing the characters wear is patched up but at the same time there is a market for casual and holiday wear. Quarters are spartan but clean and not cramped (by military standards). The Sidonia's hull appears to be cratered and pitted from hundreds of years of minor impacts, but this is in fact superficial: the hull is purposefully coated in a layer of ice hundreds of meters thick, to serve as ablative armor that can be replenished as needed.
* WaveMotionTuningFork: Higgs emitters have a core flanked by two wide rails at the top and bottom. The shape is distinct enough that it can be visually recognized when the Gauna try to replicate it.
* WeHardlyKnewYe: Many characters are briefly introduced and given some slight backstory, only to then get killed later on. And in really egregious cases, their development is seen right before they're killed.
* WhiteShirtOfDeath: Most pilot uniforms are all white, as are the Guardians themselves. Considering the attrition rate versus the Gauna...
* WhoWantsToLiveForever: After the gruesome Fourth Gauna Defense War, Hiroki Saito suffers from longevity-triggered dysfunction and seeks to die in peace. He does not take it well when the captain tries to extend his immortality against his wishes.
** Averted by the his immortal friends, when they were granted immortality every one of them had to swear an oath that their life AND DEATH was not their own. They have a duty to live for the sake of others and as such living didn't feel like such a burden, especially Dr. Yure who has a teenaged grand-daughter to care for. Hiroki got out of this by fulfilling his duty through teaching his immortal clone Takikaze everything he knows, so that the Sidonia will have an Ace who can replace him.
* YouGottaHaveBlueHair: Teruru Ichigaya's hair is literally blue. Justified because she is an artificial life-form.
* YouHaveOutlivedYourUsefulness: The Council of Immortals hold a meeting to forcefully remove Kobayashi as the leader of the Sidonia after hearing about the new hybrid Gauna Tsumugi. [[spoiler:However, just as they choose the new representative, Kobayashi's assistant comes in, and kills them all by shooting them in the head. She then appears from behind, having heard everything they had discussed, and ended up using this trope on them instead.]]
* ZergRush: The Gauna employ this heavily from time to time.
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