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From left to right: Jake, Marco, Ax in human morph, Rachel, Cassie, and Tobias. You probably needed the clarification, even if you're familiar with the books.
The popularity of the Animorphs book series eventually led to a live action television series. This was initially met with a lot of resistance within the fanbase, due to feelings that this would eventually cheapen the great book series, and that the books would translate more easily to an animated adaptation due to their truly alien aliens and the premise of morphing. But even in the midst of the backlash, Scholastic went ahead with production and the series premiered September 15, 1998 on Nickelodeon. The series didn't gain the same popularity as the books, and only lasted 26 episodes. Reruns currently air on Qubo. The series is also now available on Netflix.
Adaptation Name Change: Tom's Yeerk is given the name Iniss Three-Three-Five. Because in the books his first Yeerk has a different name and his second Yeerk has no name at all, 'Iniss Three-Three-Five' is the name sometimes used in fandom for Tom's second Yeerk.
Adaptational Angst Upgrade: Inverted with Tobias. In the flashback episode that bears his name, it's revealed he led a pretty comfortable life before becoming an Animorph, in stark contrast to his rough background in the books.
Adaptational Badass: Marco instantly becomes one in the season one finale when he single-handedly destroys the Yeerk Pool.
Adaptation Species Change: Variation in Animorphs...the short-lived TV series rarely used the signature morphs of the protagonists...Rachel never did her bear morph, Marco never went gorilla...possibly because there wasn't the budget for a ton of CGI and they wanted to use animals they could work with decently well. Then the Transformers toys got into it, with things like a Jake-bear and Jake-stingray toy...
Age Lift: In the book series, the protagonists started off as middle school students, and progressively got older. Here, they're already in high school.
Anachronism Stew: The setting. Ostensibly set during the late nineties, the computers seen recall late eighties tech and the movie they see in one episode is straight out of the fifties.
Animal Motifs: The resistance movement seen in Changes has adopted the hawk as their sigil. Weirdly, this isn't because of Tobias.
Ascended Extra: Crazy Fran from The Forgotten, who is based off a throwaway character seen in the first Megamorphs book. Also Melissa Chapman, who makes more appearances in the show than she does in the books.
Bad Dreams: In a rather bizarre callback to all the nightmares the kids go through in the books, one episode opens with Marco having a nightmare about being chased by a T-Rex. This has nothing to do with anything, and it's been speculated that it was a last minute change from him being chased by a Hork-Bajir (the T-Rex makes the same sounds) when they couldn't use the suit for whatever reason (the T-Rex is stock footage from the stop motion short "Prehistoric Beasts"). This makes it tie with the Magical Negro scene from "Tobias" for worst Big Lipped Alligator Moment in the series.
Bottle Episode: "Tobias," which only features Tobias and Ax, and is nothing but character material. Unfortunately, it didn't work out very well and is one of the worst episodes.
But Not Too Black: Nadia Nascimento, Cassie's actress, is considerably lighter-complexioned than the Cassie depicted in the books' cover art. Not nearly as much as it may seem from the article photo, however.
Complexity Addiction: The Yeerks are pretty good about avoiding this in the first season, but they take it Up to Eleven in the second season, cranking out one hare-brained scheme after another and each more ridiculous than the last.
Compressed Adaptation: Due to Ani-TV's limited budget, numerous factors of the series were scaled down - the Hork-Bajir and Taxxons, the size of the Yeerk Pool, and the battle morphs of the Animorphs all got this treatment.
Crazy Survivalist: Mr. Perkins from The Release, though he's light on the 'crazy' and heavy on the 'survivalist'.
Cut Short: The second season ended after a mere six episodes, and the three-part finale ended with nothing resolved. The Yeerks were still a threat, Ax was still stuck on Earth, and aside from the inexplicable destruction of the Yeerk Pool, the Animorphs had accomplished nothing.
Diminishing Villain Threat: Visser One. In the books, she's a menacing, proactive and dangerous figure. In the show, she spends most of her onscreen time watering plants.
Dirty Coward: The Yeerks run like children at the first sight of any wild animal.
Dull Surprise: Anyone who's infested with a Yeerk. A big departure from the books where Yeerk Controllers are able to make their hosts act perfectly normally, which naturally also greatly lessens the tension when it's obvious who the Controllers are.
Epic Fail: The fight scene between Ax and Visser Three is probably the most triumphant example. Two Andalites, in a glorious wrestling match to the death!
Expy: The Ellimist becomes one of Obi Wan Kenobi. Old man, same cloak, blue and transparent. Similarly, Visser Three's human morph is a dead ringer for Lex Luthor.
Karma Houdini: Visser Three, who ends the series better off than he started - he lost the Yeerk Pool, but he's still leader of the invasion, he just seized Elfangor's disk and neutralized a human resistance, and he's acquired a new human morph to replace his fugitive old one.
The Klutz: Brooke Nevin's Rachel is depicted as notably clumsy, which makes as much sense as anything else on this show considering that she's, you know, a gymnast.
Kudzu Plot: The series three-part finale, Changes, cobbles together a mishmash of plot ideas most likely intended for future episodes and attempts to resolve them all at once. The result, while superior to anything else in the second season, is deeply confusing and disjointed.
La Résistance: In Changes, Tobias is captured by a human resistance made up of former Yeerk hosts. Sadly, they are not explored very deeply and end up getting taken down by the Yeerks in short order.
The Lancer: Marco to Jake, as per his role in the books. Also, the unnamed character played by Shawn Roberts, who appears to be second-in-command of the resistance movement.
Making Use of the Twin: When Ax morphs Jake so that Jake can be tied up while his Yeerk is starved. Shawn Ashmore's twin Aaron Ashmore appears in the one scene with two Jakes.
It's also speculated that we're seeing Aaron in all the other scenes of Ax-as-Jake, but information is scarce.
New Media Are Evil: In The Front the Yeerks concoct a hare-brained scheme to sell cell phones with Yeerks nestled inside them. At the time this episode aired, cell phones were still relatively new.
No Name Given: The resistance fighter played by Shawn Roberts is credited simply as "Skater Dude". Which is strange, seeing as how he is never seen skating.
Oxymoronic Being: Erek the Chee, who in the show is allergic to his own hologram crystals. To put it into perspective, that's like a computer being allergic to its own keyboard.
Power Creep, Power Seep: The Animorphs all have significantly weaker battle morphs than in the books. Rachel goes from her grizzly bear and elephant to a lion (stealing Sixth Ranger Traitor David's battle morph), Marco goes from his gorilla to a wolf, and Cassie loses her own wolf in favor of a horse. Even Jake loses a little; he still gets a tiger morph but he's forced to trade in the formidable Siberian tiger for a smaller, less dangerous white tiger.
Visser Three loses his menagerie of monstrous morphs. Aside from one (mostly offscreen) instance, the only morph he ever uses is his human morph.
In Season 2, Erek the Chee gets to keep his durability and strength but loses the ability to project holograms around anyone but himself.
Rebel Leader: Hugo, the leader of the resistance movement seen in Changes.
Red Eyes, Take Warning: Of a sort. If somebody scratches at their ear in the show, it's a good sign that they're a Controller.
Relationship Compression: Rachel and Tobias. In the books, they barely knew each other before becoming Animorphs. In the show, a flashback episode establishes they were already good friends at the very least.
Refuge in Audacity: Used the heroes and villains alike. The Animorphs at one point memorably defeat the Yeerks with water balloons, and the Yeerk plots in season two range from 'sell people cell phones with Yeerks inside them' to 'air an awful sitcom that will brainwash people'.
Repeat Cut: When the Controllers break open the door of the observatory, the shot is played 3 times.
Marco's dinosaur dream in My Name is Erek is a strange shout-out to the second Megamorphs book.
Sir Not-Appearing-in-This-Show: David. There was a teaser at the end of The Threat which implied he might show up in Ani-TV, but he never did and to add insult to injury, his lion morph was taken by his archnemesis Rachel.
Status Quo Is God: In Changes Ax builds himself a spaceship out of spare parts, pre-empting Tony Stark by a decade. Though this was the series finale, Ax's spaceship still failed to get off the ground, presumably because he was never able to leave Earth in the books.
Stupid Evil: The Yeerks always had elements of this, but it went into overdrive in the mercifully-brief second season.
Tall, Dark and Snarky: Christopher Ralph's Tobias, in a considerable departure from the meek bully-magnet book-Tobias started out as.
Unexplained Recovery: In the episode based on "The Underground," Tobias is shot with a dracon beam and falls down completely stiff, and even if he somehow survived, there's no indication that the other Animorphs took him with them in their escape. In the next episode, he's back to normal with no explanation.
Villains Blend in Better: Averted. In the books, it's said that Yeerks can perfectly imitate their host's behavior so that not even those closest to them can tell the difference. In the TV series, Controllers show clear differences in behavior from their hosts. For example, Principal Chapman says that he hates a pet cat that he once loved, which makes his daughter suspicious. Ax also says that he can tell who's being Controlled by looking in the host's eyes.
Oddly enough, it's averted in the It's a Wonderful Plot episode "Not My Problem," which actually has two surprising twists about characters being Controllers. It's a good thing they weren't this effective in the real world.
The Controllers also finger their ears a lot, which receives a Take That in one of the later books.
Villain Decay: The Yeerks were never particularly effective, but in season two they threw all pretense of competence to the wind.