"The Fish Man of Steel" just doesn't have the same ring to it.
The bottom half of my body doesn't feel right. It feels foreign. When I drop my gaze to my lap, I let out the loudest shriek I've ever heard in my life. I don't have legs, there's a tail there instead ... I'm a mermaid. A mermaid. I wasn't thirsting after water and craving fish because I was pregnant. It was because I was... I was turning into a fish.
— Cassie's narration in Merman's Kiss.
So you want to give your mundane character a Magical Species Transformation to spice up your story, but think turning them into a vampire or werewolf is too passé? Easy — turn them into a merperson! Depending on the character, they may either spend some time Reveling in the New Form or outright panic at their situation.
Many stories that make use of this Plot Device often stress how these characters are now confined to an aquatic environment — which can bring a lot of angst for someone used to life on land, especially if the nearest large body of water is too far away. Sometimes the transformation is immediate (often the ploy of a bad guy wanting to mess with the protagonists), and the rest of episode is spent with our main team of heroes trying to revert the transformation as the victim complains about how there's nothing to do in the pool or bathtub. Alternately, the merfolk-to-be may undergo a Slow Transformation throughout the narrative — often starting with strange cravings for seawater, kelp or raw fish meat — as they slowly develop aquatic traits. A lot of emphasis is given to their distress and perceived loss of their humanity as they first grow gills, then scales, then at last, a tail, until they must leave their childhood hometown forever to live in a strange environment far away from any land-dwelling civilization.
Other works simply have these characters take upon an aquatic form under the sea as a form of Fisher Kingdom, as a means to enable them to visit an Awesome Underwater World in case writers don't want to settle for a temporary water-breathing spell, an Artificial Gill, bulky scuba suits or Oxygenated Underwater Bubbles. This variation is common in Merchandise-Driven shows, as giving (usually female) characters a mermaid design means toy stores can put mermaid doll variations of these characters for sale.
A Rotten Time to Revert may occur if this transformation is temporary and they change back into a human at the worst possible time, with danger of drowning in the process. In more mature works, the Mermaid Problem may be brought up if the character has questions about their new biology (usually if the transformation results in the removal of the "usual equipment"). Making a Splash powers are often included as part of the package.
Note that this trope deals with characters who were already human (or humanoid) gaining aquatic animal traits, not about humans merely gaining the ability to breathe underwater; for the latter, see Water-Breathing Humans. If the character was already aquatic to begin with and becomes human, that's Humanity Ensues. If the character is pretending to be a mermaid without actually transforming into one, that's Mermaiding Swimsuit. Despite the name, this trope also extends to humans or terrestrial cartoon animals being turned into fish of the Funny Animal and Civilized Animal varieties. For cases where humans are turned into non-anthropomorphic fish or other animals, see Animorphism.
Examples:
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Anime & Manga
- Berserk: Isma is believed to be cursed by the people on her island and rumored to have a merrow mother and shunned accordingly. Then it turns out she actually is a merrow, and after that can transform pretty much at will.
- Fairy Tail:
- On occasion, the series demonstrated there were some spells that can turn people into fish. Lisanna transforms into one on Tenrou Island arc, Mirajane transformed into one to teach Natsu and Happy, and in sequel there was an entire town which was charmed to turn everyone underwater into fish.
- Lucy and Brandish are transformed into a pair of mermaids by the former's guardian Aquarius during one story arc. Much Fanservice ensues when they discover they now have fish tails.
- My Bride is a Mermaid: Mawari Zenigata is temporarily transformed into a mermaid after accidentally using a magic seashell meant for Nagasumi. The others pass it off as a dream once she reverts to being human, but the narration hints she kept quiet after realizing what she'd seen was supposed to be a secret.
- Pretty Cure:
- Yes! Pretty Cure 5: In Go Go!, Shiberetta steals Urara's singing voice and transports all six of the girls into the Little Mermaid story, turning them all into mermaids (or, in the case of the fairies, sea creatures) and threatening to keep it unless they surrender the Rose Pact.
- Smile PreCure!: The girls use the Dolphin Decor to gain mermaid tails and fight the Akanabe underwater in the Amazon rain forest.
- Hugtto Precure: When Homare and Harry are pulled into Bicine's virtual world themed around The Little Mermaid, Homare is naturally turned into the titular mermaid.
- Star★Twinkle Pretty Cure: When the girls arrive on Pururun in "The Ocean Planet! Become a Mermaid and Swim Swim", they receive special pearls from Yanyan that allows them to transform into mermaids and swim underwater.
- Tropical-Rouge! Pretty Cure: Implied when Minori and Laura, a mermaid, swap bodies after they are both sucked into the Mermaid Aqua Pot.
- Toilet-Bound Hanako-kun: Nene Yashiro swallows a mermaid's scale, which causes her to turn into a fish when she's submerged in water.
- Tokyo Mew Mew: Mew Lettuce has porpoise DNA, so her legs transform into a mermaid's tail whenever she submerges underwater in the manga and its 2022 anime adaptation. The 2002 anime changes this to where contact with Mew Aqua results in the alteration.
Comic Books
- Aquaman: In Aquaman (2016), the monster Dead Water was an ordinary human who transforms into an underwater creature as a result of extreme distress.
- Hellboy: Abe Sapien turns out to be this trope. At first, he appears he's just an usual fish man the BRPD found and named, but he used to be a human that was a member of a murderous cult, who got transferred into the body of what he is now as part of a grand scheme to wipe out most of the planet. Abe himself is rather horrified when he learns this.
- Superman:
- The Silver Age of Comic Books has stories where Superman, Supergirl, and Lois Lane turn into mermaids, each for completely separate reasons. There was also a story where Jimmy Olsen thought he’d been turned into a fish person, but that was revealed to be All Just a Prank pulled on him by Superman.
- In Superboy (1949) #194, Superboy is turned into a water-breathing, fish-tailed merman by a renegade Atlantean scientist called Harkon, with the purpose of forcing Superboy to work for him.
- The Unknown Supergirl: Exposure to Red Kryptonite causes Kara to undergo a wide variety of whacky transformations, one of which was into a mermaid. She lives several submarine adventures until the effect wears off.
- Supergirl (2025) introduces Turquoise Kryptonite, created by a sea hag, which turns Supergirl's legs into a mermaid tail and causes her to sprout gills on her neck. The magical effect prevents her from breathing when out of water, forcing her into the sea until the effect wears off, which the hag helpfully tells her will be at sundown.
Fan Works
- Just Add Water (Akela_Victoire): An Encanto fanfic where Mirabel spontaneously gains a gift that turns her into a mermaid when she touches water.
- The TV Tropes Character Crossover War: As a result of the merging realities, anyone who visits Atlantica winds up being turned into some sort of merfolk, per the rules of Kingdom Hearts.
- Chapter 14, "Ocean Man" has a scene in Atlantica, which reveals Misty from Pokémon the Series has taken up residence there as a Goldeen-themed mermaid. This was later expanded upon as it's own one-shot, "The Tomboyish Mermaid", where she sought out King Triton's aid in order to become strong enough to face against Makima after Ash goes missing. It's even revealed that she did wear her old mermaid costume just in case of lingering anti-human sentiment, but an encounter with Kyogre triggered Atlantica's Fisher Kingdom rules and she transformed right after.
- When Makima visits to recruit Ursula, she turns into a draconian-mermaid inspired by Japanese mythology. Later, when members of the SDF and A.R.M.E.D. pay a visit to help Ariel find a potential survivor from Makima and Ursula's massacre, they initially avoid this fate because of Bikini Bottom logic (aka, Rule of Funny) — at least until Ariel beams down with them. After that, with the exception of the already amphibious Squidward and the Penguins of Madagascar, the group ends up as merfolk of various kinds, including a Milotic Mermaid (Cynthia), a sawshark merman (Denji), a pirannah mermaid (Power), a Necrocromicon-themed mermaid (Futaba Sakura), a swordfish (Waluigi), and a Uravity-themed mermaid (Ochaco). That said, it's Parodied with Deadpool and Harley Quinn, with the former ending up in swim trunks and a Hawaiian shirt while wearing a cheap scuba mask, while the latter ends up just wearing a Joker fish head over her classic costume (ala Batman: The Animated Series).
- What The Water Gave Me: A How to Train Your Dragon fanfic in which Hiccup, Astrid, and Valka are turned into merfolk by mermaids.
Film — Animated
- Barbie has had a few examples, given its length:
- Barbie Fairytopia second Entry Mermaidia had Elina (Barbie's character) become mermaid via a necklace to save new friend Nori with the catch of needing to exit and remain out of water later to revert to fairy she fails thanks to a last minute trap by the villains but afterwards is given access to a speical berry that not only restores her to fairy but gives her new, more powerful wings.
- Barbie in a Mermaid Tale: The film's lead protagonist, a surfer named Merliah, discovers she's descended from a mermaid queen, but she only gains the ability to transform thanks to a magic necklace, as well as embracing her mermaid heritage.
- Barbie: Mermaid Power: in this sequel to Dolphin Magic both Barbies and Malbu Barbie's Sisters Skipper, Stacie and Chelsea use the necklace Isla gifted the Melbu Roberts family to become mermaids to help Isla by getting involved in a contest involving Avatar: The Last Airbender style powers though neither of the Barbies or Malbu's sisters are the Avatar Expy however.
- Help! I'm a Fish: A trio of humans named Fly, Stella, and Chuck are turned into fish (a regular fish, a starfish, and a jellyfish respectively) thanks to a potion invented by a scientist named MacKrill. The plot hinges around them getting it back from a nasty pilot-fish named Joe, and they have 48 hours to get it back lest they trapped in that form forever.
- The Little Mermaid II: Return to the Sea: Princess Melody, the daughter of the mermaid-turned-human Ariel and the human Prince Eric, long dreamt of becoming a mermaid (ironically the opposite dream of her mother, who sought to become human). She's given that chance when Morgana (the sister of the late Ursula) uses a spell that turns Melody into a pink-tailed mermaid on the condition she steal King Triton's trident. She nearly drowns when the spell wears off, but is luckily saved.
- Moana: Maui manages to finally get his magic hook back from Tamatoa, but he's so out of practice that he can't transform properly. At one point, he turns into a Shark Man (as in the shark half is on top), then turns into a Mer-Shark when he goofs it up again before returning to normal.
- My Little Pony: The Movie (2017): The Mane Six are temporarily turned into seaponies while Spike becomes a pufferfish though Queen Novo's Pearl of Transformation. It is also revealed the sea ponies themselves were originally hippogriffs, and only went underwater to hide from the Storm King.
- Scooby-Doo: Camp Scare: There's a local legend about a camper who spent all day swimming in the lake so often that he mutated into a fish man. Said legend is just one of three around the camp that come to life and start attacking Scooby and the gang. Subverted when it turns out to be another "Scooby-Doo" Hoax.
Film — Live-Action
- The Adventures of Sharkboy and Lavagirl: The first half of the titular duo, Sharkboy, was an ordinary human who was Raised By Sharks, and gained shark characteristics...somehow.
- Aquaman (2018): Ancient Atlantis was sunk to the bottom of the ocean thanks to a very ill-advised attempt to use the Trident of Atlan as a weapon, with the resulting disaster turning most of its population into water-breathers while also splitting Atlantis into seven kingdoms. The inhabitants of the modern kingdoms of Atlantis and Xebel remained Water-Breathing Humans, but the other Kingdoms became populated by the descendants of other survivors who have developed aquatic animal traits through Hollywood Evolution — one kingdom mutated into fish-like merfolk, another into crab-like humanoids, and another into trench-dwelling monsters (the DCEU's version of the nightmarish Trench).
- Blue My Mind: A 15-year-old girl named Mia has a Slow Transformation into a Mermaid, Played for Drama and Body Horror.
- Dagon: Part of the Twist Ending. Paul discovers that he is part Deep One just like the other residents of Imboca. He attempts to kill himself by setting himself on fire, but Uxia drags him into the water to douse the flames. His gills start to work at that moment, and he follows her towards the underwater city.
- The Incredible Mr. Limpet: The titular character falls off of a dock after having wished to live his life as a fish, and is immediately transformed into a cartoon fish that helps serve the Allied Forces during World War II.
- The Thirteenth Year: The main protagonist, Cody, slowly begins transforming into a merman when he turns 13. It turns out his mother is a mermaid, and this is apparently a normal process for mer-adolescents.
Literature
- And Then I Turned Into a Mermaid: Molly Seabrook is a young girl who discovers she's half mermaid and has to navigate the difficulties of secrets, school, friendships, and a crazy family.
- The Deep (2019): A society of merfolk descends from pregnant women thrown off of slave ships.
- Emily Windsnap: After getting a strange numbing sensation in her legs when starting swimming lessons at school, 12-year-old Emily discovers she can turn into a mermaid when she fully submerges herself in water (something she had never done before because she lived on a houseboat with no tub and her mother's fear of water prevented her from learning to swim sooner). She later discovers this is because her long-lost father was a merman and her mother was enchanted to forget him.
- InCryptid: Both Finfolk and Sirens are born looking like humans, and gradually turn into their adult merfolk forms. For the finfolk, this also leads to Loss of Identity. Saltwater accelerates the process.
- Looming Gaia: Mermaids in this series are women (or sometimes rarely men, which has likely something to do with hormones) who are cursed by undines when they're children to change into an aquatic form whenever their hair gets wet along with growing blue hair and scales on their face and legs even in their terrestrial form. This makes them forget all their memories and the undines use them as soldiers, but they can be freed, though they still lose all their memories. Though there's also the separate species of sirenes, who are born resembling traditional mermaids.
- Lost Voices by Sarah Porter: The main characters are basically humans turned into mermaids. They are given magical voices like the sirens, which they use to lure humans to their deaths. These mermaids/sirens are all formerly human girls who were abused or neglected, and they take vengeance on humanity for the mistreatment in their previous lives by sinking ships and drowning people.
- The Mer: A young woman about to embark on her "perfect" post-college life faces an unexpected and fatal twist during a beach vacation. She is offered a choice between death as a human or immortality as one of the mysterious Mer. The Mer are a race of mermaid-like creatures that are all former humans. Choosing the name Val for herself, she soon finds herself embroiled in a mystery and a love-triangle.
- The Mermaid's Sister: The Sea King's daughter is raised by humans on a mountain in Pennsylvania. When she's in her late teens, she starts to transform into a mermaid. If a mermaid is away from the sea for too long, she'll sicken and die, so her adopted family has to transport her to the ocean.
- Merman's Kiss:
- The main character Cassie Price is transformed into a mermaid after she has sex/mates with her lover Damarian. This is because merpeople in the setting mate for life, so the natural order makes them more compatible. She's freaked out at first but gets over it pretty quickly once she finds out how much fun being a mermaid is.
- Cassie isn't the only one who goes through this in the series, as her younger friend Timmy eventually goes through the process himself after he falls in love with Damarian's younger sister Zarya.
- It happens to Cassie and Damarian's daughter Marella (who was born human) after she comes of age and her mermaid half emerges.
- In the Paul Jennings story Nails, Lehman appears to have fingernails growing all over his body, so his father goes to Find the Cure! for him. The "nails" are later revealed to be fishy scales, as Lehman's absent mother was actually a mermaid, meaning that he was a mermaid-human hybrid all along. After his father drowns, Lehman fully becomes a merman, and goes to join all the other mermaids in the sea.
- October Daye: In One Salt Sea, the Luidaeg makes a Transformation Trinket for Toby so she'll be able to breathe underwater. It also has the side effect of temporarily turning her into a merrow, which Toby doesn't find out until she uses it.
- Real Mermaids: Because Jade is half mer and half human, she can transform from human to mer and back just by taking a few breaths of water or air, an ability she discovers four hours after getting her first period when she falls asleep in the bath and her face slips under the surface. Full-blooded mers have to spend anywhere from hours to weeks partially immersed in water with just the right mixture of oxygen and salt in order to become human.
- Rescue Sirens, created by Chris Sanders and his wife Jessica Steele-Sanders, reveals that humans are the descendants of land-locked mermaids, with Kelby being a rare example of an atavistic human able to take on a mermaid form.
- The Sea Fairies: Trot and Cap'n Bill are temporarily transformed by some friendly mermaids. Real mermaids are a kind of fairy, which means they can't be harmed in water, though Trot and Cap'n Bill don't get that advantage.
- The Shadow over Innsmouth: After running from Innsmouth and having the Navy attack a point off of the coast with torpedoes, the narrator of the story learns that he was actually descended from the lineage of Obed Marsh, a corrupt sea-captain who struck a deal with the Deep Ones to provide them human mates to create grotesque Half-Human Hybrids. The narrator notes at the end of the story that he has begun to take on "The Innsmouth Look", which is a sign that the latent fishman heredity is beginning to take hold and transform him.
- A Tale of...: Poor Unfortunate Soul: A Tale of the Sea Witch: Ursula is an orphan raised by humans until her powers kicks in and reveal her to be a Sea Witch. The resulting trauma over how people suddenly treat her kickstarts her down the path towards becoming evil.
- TF High: One of the many victims of the TF virus is Melody Perez who gets turned into a mermaid during "Season 3" of the story. Since the transformation also heals her of a crippling injury that left her unable to walk and she loved swimming already, she's more than okay with this development.
- A Twisted Tale: In “The Secret Exchange“ from Anthology, the story ends with King Triton turning Eric into a Merman so that he can be with Ariel.
- Whateley Universe: After an otherwise minor injury, hydrokinetic Vic Rivera starts growing scales as a result of pushing his powers. With further injuries (He has a rough Christmas Break), the area covered by scales increases and his hands now have webbed fingers. All of this points to an increasing connection to his 'neotriton' avatar spirit, slowly remaking him in her image to some extent.
Live-Action TV
- Buffy the Vampire Slayer: In "Go Fish", a humanoid fish creature is preying on the school swim team. However, it turns out that not only is it multiple fish people, they are the former members of the school swim team, who were not being killed but permanently transformed into mindless "gill monsters" thanks to the side effects of a concoction of fish DNA and steroids their coach was feeding them.
- The Bureau of Magical Things: Lily's attempt at magically fixing a cake she ruined causes anyone who eats part of the cake to turn into one of the three cake ornaments, one of which is a mermaid. Imogen is the one to get the mermaid transformation, which happens next to a pool and with the effects from H₂O: Just Add Water.
- Charmed (1998): "A Witch's Tail" features human witch Phoebe getting turned into a mermaid by her sisters' spell, since only a mermaid can track a sea hag. Unfortunately, this also gives her a mermaid's strong attraction to the sea, which, in the wake of her break up with Cole, makes her not want to turn back, risking her heart turning cold forever.
- H₂O: Just Add Water is about three girls from Australia who fall into a magic pool under the full moon and wind up being transformed into mermaids. They spend the series trying to hide their secret, which isn't easy when getting splashed with water turns them into their aquatic forms.
- Mako Mermaids: An H₂O Adventure, a sequel to H₂O, deals with a merman when a human boy named Zac falls into the Moon pool during a full moon and turns into one. As punishment for letting him fall in, the mermaids guarding it have to watch over him. The second season then subverts this by revealing Zac was always a Merman but was magically bound to human form by his mother. On the flipside, his love interest Evie ends up becoming a mermaid just like the H₂O girls.
- Once Upon a Time (2011): In the episode titled "Ariel", Snow White was temporarily turned into a mermaid by Ariel thanks to the latter putting an item known as a sea bracelet onto the former.
- Round the Twist episode "Nails" features a New Transfer Student and Temporary Love Interest for Linda Twist, Andrew, who is handsome, polite, charming and a fantastic actor in the school play. Andrew has an affinity for the sea, and he lives on an island off the town's coast, and suspiciously always wears gloves. It turns out this is because he is half-merfolk on his mother's side a fact his father was trying to hide from him and the gloves are because his fingernails are actually scales that have grown all over his arms as he is slowly is transforming permanently into a merman. Whilst he develops feelings for Linda he leaves at the end of the episode to live in the ocean with his mother, but not before giving Linda a goodbye kiss.
Mythology
- The Syrian goddess Atargatis, also known as "Decerto" in Ancient Greek, flings herself into a lake out of shame of an illicit affair, and the water transforms her lower half into that of a fish, preserving her divine beauty from the waist up.
- Classical Mythology has a few examples:
- Some variations of Glaucus, a Greek sea god, depict him as a human fisherman who became immortal by consuming a magical herb, and his appearance became fishier as a result with a merman being a common way to visualize him.
- Leucothea and her son Palaemon were originally mortals who leapt into the sea to escape a case of divine-insanity-induced Pater Familicide, and were turned into sea gods. While Leucothea usually retains her human appearance in depictions, Palaemon was sometimes depicted as a merboy (or "triton," as the ancient Greeks called them).
Tabletop Games
- Arkham Horror: Both Amanda Sharp and Silas Marsh have the infamous Innsmouth Look originating from The Shadow over Innsmouth. Fail their personal stories, and they will become Deep Ones.
- Dungeons & Dragons: The spell "Alter Self" allows the caster to change their body in a variety of ways, depending on which edition's rules are being used — one of the possible options is growing aquatic animal features.
Toys
- Turtles of Grayskull: Mer-Man becomes even more fishlike after being mutated by the Ooze, growing a powerful fish tail in place of his legs.
Video Games
- 9 Years of Shadows: The Siren Form power-up gives Europa the ability to turn into a mermaid at will while she's wearing the Poseidon Armor and fully immersed in water. This transformation allows her to swim freely in all directions, even up waterfalls, and it replaces her basic halberd attacks with a Dash Attack.
- Call of the Sea: The protagonist Norah gradually discovers that she's descended from an ancient culture inspired by the Deep Ones of Lovecraftian mythos; halfway through the game, she gains the ability to transform into an amphibious form, which from the game's first-person view is conveyed by her arms gaining scaly skin and webbed hands, to traverse the island's undersea chasms. The Acceptance ending has Norah embrace her heritage and join her people and their god under the sea. It's implied from the murals that she becomes something like a fishier variant of a classic fish-tailed mermaid.
- Coral Island: After cleaning up all the coral sites in the ocean, the player character has the opportunity to do a side-quest for Cho Oyu the oracle that lets them turn into a merfolk while underwater, increasing their movement speed and allowing them to dash while underwater.
- Cute Knight Kingdom: One of the more magical endings involves the protagonist turning into a mermaid, to make a romance work.
- Dislyte: Biondina's transformation into an Esper changed her from a human into a mermaid.
- Enchanted Portals: The third boss is fought underwater, so the two main characters turn into mermaids for the fight. They can be controlled freely, and can still shoot their wands as usual.
- Help Im Turning Into A Mermaid, set in the same universe as Max's Big Bust: A Captain Nekorai Tale and Alluna and Brie, is about the protagonist, Hunter, turning into a mermaid.
- Kingdom Hearts: In order to survive in Atlantica, Donald Duck uses his magic to turn Sora's party into merfolk. Sora becomes a blue-tailed dolphin merboy, Donald becomes a duck-cecelia not unlike Ursula, and Goofy becomes a sea turtle. When the trio meet Ariel, they lie and tell her they're from another ocean (since there's a World Order that prevents them from telling her they're from another world).
- Pokémon:
- The Normal type Eevee can evolve into the Water type Vaporeon if exposed to a water stone, giving it fins and a mermaid-like tail.
- Princess Peach: Showtime!: Peach can transform into a mermaid, allowing for underwater-themed levels that utilize singing to progress.
- Shantae:
- Three of Shantae's many forms she can take on are a mermaid, a crab and a frog, which involve her belly-dancing to transform.
- Shantae and the Seven Sirens: Shantae's mermaid transformation of the game, as said in the art book, is more of a mix with a Plant Person form and isn't used for underwater travel, but healing.
- The Sims: In the third game's Island Paradise expansion and the fourth game's Island Living expansion, a Sim can become a mermaid by getting and eating mermadic kelp. But if they go too long without swimming in salt water in 3, or eat two mermadic kelps in 4, then the trope is Inverted as the mermaid becomes human as a result.
Webcomics
- Realta: When Capricorn like Amelia submerge their legs in water, they transform into fish tails, a nod to the classic depiction of the mer-goat.
Western Animation
- Alienators: Evolution Continues: Wayne Green has a mutation power that allows him to gain mutant forms for different situations. In the episode "Meltdown", he grows gills and a fish tail that enable him to swim and breathe underwater, basically turning him into a merman.
- Dora the Explorer: Twice in the show's history, Dora has become a mermaid in different ways. The first time she became one, she used Mariana's Mermaid Crown, and all subsequent times, she uses a mermaid necklace given to her by Mariana.
- Elena of Avalor: Humans who visit the realm of the sirenas can wrap their torsos in a magical type of seaweed that allows them to survive under the sea. Usually this just grants them the ability to breathe underwater, but more extensive use of the seaweed can transform a human into a sirena permanently. This was done to one character in the backstory in order to save her from drowning.
- Futurama: In "The Deep South", it's revealed that the entire city of Atlanta, Georgia sunk underwater (after being moved into the ocean so the airport could expand) before the series began because they overbuilt, and the populous eventually became merfolk. How? The nearby Coca-Cola plant leaked out and turned everyone into mermaids.
- G.I. Joe: A Real American Hero: In "Memories of Mara", the Joes learn that Cobra had kidnapped an innocent woman in actuality a Cobra Eel and genetically modified her with fish DNA. This not only made her grows gills meaning she's only able to survive underwater but it caused her skin to turn blue. She's rescued by the Joes, and winds up falling for Shipwreck.
- High Guardian Spice: In "The Scypith", episode has the main four girls turning into mermaids while trying to complete a mission.
- Kim Possible:
- In "Sink or Swim", Ron's childhood camp bully Gil Moss is revealed to have mutated into a fish-like creature over time due to his extra exposure to the toxic water of Camp Wannaweep (which he blames Ron for, since Gil took Ron's unwanted swimming slots). By the time he and Ron meet again in their teen years, Gil has renamed himself Gill (after "these things that grew after I mutated") and has green, scaly skin, webbed, clawed hands, and fins. He tries to mutate the captured Middleton cheerleaders and Barkin as well, with Barkin starting to grow gills before he's rescued.
- In "Return to Camp Wannaweep", a seemingly cured and rehabilitated Gil reveals he actually misses being a mutant, and willingly re-mutates himself with the concentrated Wannaweep muck in a grotto which was missed by clean-up efforts, turning into a bigger version of his former fishlike form. He once again tries to mutate the other campers, with one growing a duck-like bill. Ron also mutates himself to stop Gill, turning into a giant humanoid semi-aquatic beaver.
- In the episode "Menino Sireno" of Jorel's Brother, the titular character turns into a merboy after being bitten by a fish. A local aquarium later tries to turn him into its new star attraction.
- A New Kind Of Magic: In "Fish Out of Water
" Tom turns himself into a merman in an attempt to show off his swimming skills to his crush Sofia, as he normally can't swim. However, he has to keep his tail hidden from her, and by the end ends up turning back at the worst possible time...
- Episodes of Magi-Nation that revolves around Orothe often have the main trio don belts (certericy of Stag) that transforms wearers into aquatic versions of themselves. while Tony and Stag merely gain webbed fingers, feet and ears Edye gains a full tail. Interestingly, in Orothe's debut episode "The Depths of Courage", the city entire population was shown in their aquatic forms all throughout yet later appearances showed them switching like the trio in fairness though they were amist a festival celebrating Tony's grandfather.
- My Little Pony: Friendship Is Magic: In "Surf And/Or Turf", Terramar transforms Twilight and the Cutie Mark Crusaders into seaponies with his crystal. Scootaloo in particular enjoys being in her new form, remarking on how swimming feels a lot like flying and encouraging Terramar to live in the ocean as a seapony on a permanent basis as opposed to Sweetie Belle encouraging him to live on land as a hippogriff.
- PAW Patrol: Whenever the PAW Patrol need to help the Mer-pups (the dog equivalent of mermaids), all they need to do to grow a fish tail and breathe underwater is to get booped on the nose by a mer-pup.
- Sabrina: The Animated Series: The episode "Wiccan of the Sea" has Sabrina, Chloe, and Salem transformed into mermaids and a mer-cat when they want to escape a cold winter for a period of time.
- She-Ra and the Princesses of Power: Mermista, the Princess of Water, can transform her legs into fins to swim faster.
- Sofia the First: In "The Floating Palace", Sofia, after helping to save a young mermaid from a net, is granted the ability to turn into a mermaid by the Amulet of Avalor.
- Super Friends: The Wonder Twins have the ability to transform themselves — Zan into anything water based (as in just water and its various forms), and Jayna into any animal — when they make contact with each other. The latter can take on aquatic forms, both real and even fictional ones like mermaids.
- The Transformers: "Sea Change": The Autobots learn an entire race of Human Aliens had fallen under attack to the Decepticons and immediately move to render aid. They learn these aliens have avoided capture by using the mystical Well of Transformation that can change them into whatever they think, which in this case led them to becoming Merfolk (the Decepticons' drones can't use it because they're soulless machines). Seaspray falls in love with one, and uses the Well to make himself Tlakan to prove he's not a soulless machine, and temporarily takes on a merman form to aid his lover. Said lover takes on a robotic form like his to aid in going toe-to-toe against Megatron, and rather hilariously tricks Rumble into turning himself into a tree.
- Xiaolin Showdown: "Scream of the Siren": The Gills of Hamachi are a Shen Gong Wu that grants its user gills, fins, and scales so they can breathe and swim underwater.
- Young Justice: Miss Martian is a Voluntary Shapeshifter, so she usually turns herself into a mermaid if she's going on underwater missions. The tie-in comic reveals she loves mermaids, and changed herself from a smaller, gilled version of herself into a mermaid upon meeting Lori Lemaris.
