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* ''WesternAnimation/ElenaOfAvalor'': Humans who visit the realm of the [[OurMermaidsAreDifferent sirenas]] can wrap their torsos in a magical type of seaweed that allows them to breathe underwater. 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.
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* ''Literature/RealMermaids'': 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 [[PubertySuperpower 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.
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[[caption-width-right:350:"The Fish Man of Steel" just doesn't have the same ring to it.]]
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[[caption-width-right:350:"The Fish Man of Steel" just doesn't have the same ring to it.]]
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* ''VideoGame/PrincessPeachShowtime'': Peach can transform into a mermaid, allowing for underwater-themed levels that utilize singing to progress.
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* ''VideoGame/PrincessPeachShowtime'': Peach can transform into a mermaid, allowing for underwater-themed levels that utilize singing to progress.
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* ''VideoGame/PrincessPeachShowtime'': Peach can transform into a mermaid, allowing for underwater-themed levels that utilize singing to progress.
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** Atlantis became an underwater civilization when Vandal Savage had Klarion sink the continent. A portion of the inhabitants, who were descendants of Savage with dormant metagenes activated by the trauma, developed aquatic mutations to survive.
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* ''Film/TheAdventuresOfSharkBoyAndLavaGirl'': The titular Shark Boy was an ordinary human who was [[RaisedByWolves Raised By Sharks]], and gained shark characteristics...somehow.
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* ''Film/TheAdventuresOfSharkBoyAndLavaGirl'': The first half of the titular Shark Boy duo, Sharkboy, was an ordinary human who was [[RaisedByWolves Raised By Sharks]], and gained shark characteristics...somehow.
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* In ''Literature/TheMermaidsSister'', 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.
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* ''Rescue Sirens'', created by Creator/ChrisSanders 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.
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* ''Rescue Sirens'', ''Literature/RescueSirens'', created by Creator/ChrisSanders 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.
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* ComicBook/{{Superman}} comics from [[UsefulNotes/TheSilverAgeOfComicBooks the Silver Age]] had stories where Superman, Supergirl, and Lois Lane turn into mermaids, each for completely separate reasons. There was also a story where Jimmy Olsen ''thought'' they'd been turned into a fish person, but that was revealed to be AllJustAPrank pulled on them by Superman.
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* ComicBook/{{Superman}} comics from [[UsefulNotes/TheSilverAgeOfComicBooks the Silver Age]] had stories where Superman, Supergirl, and Lois Lane turn into mermaids, each for completely separate reasons. There was also a story where Jimmy Olsen ''thought'' they'd he’d been turned into a fish person, but that was revealed to be AllJustAPrank pulled on them him by Superman.
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* ''VideoGame/{{Shantae}}'': Three of Shantae's many forms she can take on is a mermaid, a crab and a frog, which involves her belly-dancing to change into it.
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* ''VideoGame/{{Shantae}}'': Three of Shantae's many forms she can take on is are a mermaid, a crab and a frog, which involves involve her belly-dancing to change into it.transform.
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* ''Literature/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 LossOfIdentity. Saltwater accelerates the process.
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* ''Literature/{{Animorphs}}'': The titular leads are five young humans who gain powers from an alien that allows them to turn into any animal they can think of. Naturally, some of these include water-based animals.
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* ''Literature/OctoberDaye'': In ''One Salt Sea'', the Luidaeg makes a TransformationTrinket 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.
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* ''Manga/TokyoMewMew'': Mew Lettuce has porpoise DNA, so her legs transform into a mermaid's tail whenever she submerges underwater. ''Anime/TokyoMewMewNew'' changes this to where contact with her teammate Mew Aqua results in the alteration.
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* ''Manga/TokyoMewMew'': Mew Lettuce has porpoise DNA, so her legs transform into a mermaid's tail whenever she submerges underwater. ''Anime/TokyoMewMewNew'' underwater in the manga and its [[Anime/TokyoMewMewNew 2022 anime adaptation]]. The 2002 anime changes this to where contact with her teammate [[AppliedPhlebotinum Mew Aqua Aqua]] results in the alteration.
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* ''Series/RoundTheTwist'' episode "Nails" features a NewTransferStudent and TemporaryLoveInterest 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 [[spoiler:he is [[HalfHumanHybrid half-merfolk on his mother's side]] a fact his father was trying to hide from him]] and the gloves are because [[spoiler: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.]]
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* ''Series/RoundTheTwist'' episode "Nails" features a NewTransferStudent and TemporaryLoveInterest 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 [[ConspicuousGloves suspiciously always wears gloves.gloves]]. It turns out this is because [[spoiler:he is [[HalfHumanHybrid half-merfolk on his mother's side]] a fact his father was trying to hide from him]] and the gloves are because [[spoiler: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.]]
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* ''Series/RoundTheTwist'' episode "Nails" features a NewTransferStudent and TemporaryLoveInterest for Linda Twist, Andrew, who is handsome, polite, charming and a fantastic actor in the school play who has an affinity for the sea and who suspiciously always wears gloves. It turns out this is because [[spoiler:he is [[HalfHumanHybrid half-merfolk on his mother's side]] a fact his father was trying to hide from him]] and the gloves are because [[spoiler: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.]]
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* ''Series/RoundTheTwist'' episode "Nails" features a NewTransferStudent and TemporaryLoveInterest for Linda Twist, Andrew, who is handsome, polite, charming and a fantastic actor in the school play who play. Andrew has an affinity for the sea sea, and who he lives on an island off the town's coast, and suspiciously always wears gloves. It turns out this is because [[spoiler:he is [[HalfHumanHybrid half-merfolk on his mother's side]] a fact his father was trying to hide from him]] and the gloves are because [[spoiler: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.]]
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* ''TabletopGame/DungeonsAndDragons'':
** 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, but there's always some way to gain a swim speed and the ability to breathe underwater.
** The 5th Edition Pact of the Fathomless warlock gains a permanent ability to breathe and function underwater at 1st level.
** 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, but there's always some way to gain a swim speed and the ability to breathe underwater.
** The 5th Edition Pact of the Fathomless warlock gains a permanent ability to breathe and function underwater at 1st level.
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* ''Rescue Sirens'', created by Creator/ChrisSanders 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.
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** ''WesternAnimation/JusticeLeague'': During the ''Unlimited'' saga, Project Cadmus made a group of [[{{Expy}} expies]] of the characters from ''WesternAnimation/SuperFriends'', including the Wonder Twins in the forms of Shifter and Downpour. Like their counterparts, they could transform into animals (including aquatic ones) and water-based forms, though they could do so voluntarily instead of having to make contact with each other. The latter was dumb enough to try and take on ''Aquaman'' in his natural element.
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* ''WesternAnimation/XiaolinShowdown: Recap/XiaolinShowdownScreamsOfTheSiren'' introduced the Gills of Hamachi a Sheng Gong Wu that grants its user gills, fins and scales so they can breath and swim underwater.
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* ''WesternAnimation/XiaolinShowdown: Recap/XiaolinShowdownScreamsOfTheSiren'' ''WesternAnimation/XiaolinShowdown'': The episode "[[Recap/XiaolinShowdownScreamsOfTheSiren Scream of the Siren]]" introduced the Gills of Hamachi Hamachi, a Sheng Shen Gong Wu that grants its user gills, fins and scales so they can breath and swim underwater.
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-->"Look at this place… a tank! I can't command a ship from inside an aquarium – I can't LIVE in here!"
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* ''Film/XMenFirstClass'' has the character of Armando Muñoz / Darwin, whose mutant ability lets him adapt to any environment. To demonstrate his powers, he sticks his head in a fish tank and immediately grows gills and fins. [[spoiler:He also dies at Sebastian Shaw's hands.]]
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* ''Film/XMenFirstClass'' has the character of Armando Muñoz / Darwin, Muñoz/Darwin, whose mutant ability lets him adapt to any environment. To demonstrate his powers, he sticks his head in a fish tank and immediately grows gills and fins. [[spoiler:He also dies at Sebastian Shaw's hands.]]
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[[folder:Anime and Manga]]
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* ''WesternAnimation/MyLittlePonyTheMovie2017'': The Mane Six are temporarily turned into seaponies while Spike becomes a pufferfish though Queen Novo's Pearl of Transformation. [[spoiler: It is also revealed the sea ponies themselves were originally hippogriffs, and only went underwater to hide from the Storm King]].
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* ''WesternAnimation/MyLittlePonyTheMovie2017'': The Mane Six are temporarily turned into seaponies while Spike becomes a pufferfish though Queen Novo's Pearl of Transformation. [[spoiler: It [[spoiler:It is also revealed the sea ponies themselves were originally hippogriffs, and only went underwater to hide from the Storm King]].
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* ''Series/RoundTheTwist'' episode "Nails" features a NewTransferStudent and TemporaryLoveInterest for Linda Twist, Andrew, who is handsome, polite, charming and a fantastic actor in the school play who has an affinity for the sea and who suspiciously always wears gloves. It turns out this is because [[spoiler:he is [[HalfHumanHybrid half-merfolk on his mother's side]] a fact his father was trying to hide from him]] and the gloves are because [[spoiler: 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.]]
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* ''Series/RoundTheTwist'' episode "Nails" features a NewTransferStudent and TemporaryLoveInterest for Linda Twist, Andrew, who is handsome, polite, charming and a fantastic actor in the school play who has an affinity for the sea and who suspiciously always wears gloves. It turns out this is because [[spoiler:he is [[HalfHumanHybrid half-merfolk on his mother's side]] a fact his father was trying to hide from him]] and the gloves are because [[spoiler: his [[spoiler: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.]]
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** In "[[Recap/KimPossibleS2E19ReturnToWannaweep Return to Camp Wannaweep]]", a seemingly cured and rehabilitated Gil [[spoiler: reveals he actually misses being a mutant, and willingly re-mutates himself with the concentrated Wannaweep muck in the grotto, 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.]]
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** In "[[Recap/KimPossibleS2E19ReturnToWannaweep Return to Camp Wannaweep]]", a seemingly cured and rehabilitated Gil [[spoiler: reveals [[spoiler:reveals he actually misses being a mutant, and willingly re-mutates himself with the concentrated Wannaweep muck in the grotto, 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.]]
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* ''WesternAnimation/SuperFriends'': 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.
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* ''WesternAnimation/SuperFriends'': The Wonder Twins have the ability to transform themselves-- themselves -- Zan into anything water based (as in just water and its various forms), and Jayna into any animal-- animal -- when they make contact with each other. The latter can take on aquatic forms, both real and even fictional ones like mermaids.
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They were certainly not that old.
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* ''Series/H2OJustAddWater'' is about three women 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 [[WaterTriggeredChange getting splashed with water turns them into their aquatic forms]].
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* ''Series/H2OJustAddWater'' is about three women 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 [[WaterTriggeredChange getting splashed with water turns them into their aquatic forms]].
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* ''Manga/MyBrideIsAMermaid'': 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.
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* ''WesternAnimation/TheTransformers'': In the episode "[[Recap/TransformersG1SeaChange Sea Change]]", the Autobots learn an entire race had fallen under attack to the Decepticons and immediately move to render aid. They learn these aliens have avoided capture by using a special tree 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 heartless machines). Seaspray falls in love with one, and uses the tree to make himself human to prove he's not a soulless machine, and temporarily takes on a merman form to aid his lover. [[spoiler: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.]]
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* ''WesternAnimation/TheTransformers'': In the episode "[[Recap/TransformersG1SeaChange Sea Change]]", the Autobots learn an entire race had fallen under attack to the Decepticons and immediately move to render aid. They learn these aliens have avoided capture by using a special tree 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 heartless soulless machines). Seaspray falls in love with one, and uses the tree Well to make himself human to prove he's not a soulless machine, and temporarily takes on a merman form to aid his lover. [[spoiler: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.]]
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[[foldercontrol]]
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* ''VideoGame/CoralIsland'': 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.
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-->"Look at this place… a tank! I can't command a ship from inside an aquarium – I can't LIVE in here!"
-->-- '''Captain James T. Kirk''', ''WesternAnimation/StarTrekTheAnimatedSeries'', "[[Recap/StarTrekTheAnimatedSeriesS1E13TheAmbergrisElement The Ambergris Element]]"
A subtrope of OurMermaidsAreDifferent and MagicalSpeciesTransformation, a character finds themselves in a situation where, somehow, they discover they have aquatic traits and become an ApparentlyHumanMerfolk of some sort. Usually, this gives them the ability to survive underwater, if the character isn't outright turned into a mermaid or some kind of fish. How this happens tends to vary from medium to medium, but it usually occurs the following ways:
* VoluntaryShapeshifting: The character can alter their body any way they like, and can take on aquatic characteristics to survive underwater. If they happen to like mermaids, they tend to take on that form as a means of TravelTransformation, or sometimes they just do it for fun.
* ForcedTransformation: Some characters get unlucky and circumstances force them into this form. They could swallow a magic potion without knowing, get cursed, or just happen to be at the wrong place at the wrong time when some catastrophe occurs and wind up being changed.
* HollywoodEvolution: Strange circumstances force the characters underwater, and even stranger circumstances give them the means to survive.
* FisherKingdom: The laws of the universe mean the characters must adopt the shapes of the dimension they're traveling to so they can blend in, and if it happens to be underwater, it's the only way to accomplish it.
* MadScientist: Sometimes that one unlucky soul gets kidnapped and used as part of some twisted experiment to forcibly evolve them to live underwater.
* HalfHumanHybrid: They have the genetic potential to adopt underwater traits. They typically discover this heritage at some point in their lives when they gain powers or traits, sometimes as a result of a traumatic experience, or happen to encounter their people or family members who lost them or gave them up for adoption.
* TraumaticSuperpowerAwakening: The character suffers an incident where their powers kick in, leading them to the astonishing discovery.
* TransformationTrinket: The character possesses a device that lets them turn into an aquatic form.
* InvoluntaryShapeshifting: A character's powerset kicks in unwillingly and they have no choice in the matter.
Once this kicks in, the character may spend some time RevelingInTheNewForm or outright panic at their situation. If they make this discovery early on, the story will usually revolve around them exploring their new powers, trying to find a cure if it's dangerous or they don't want to be stuck that way, or going to find out exactly where they come from. Expect a fair amount of {{Fanservice}} if the character turns into a merfolk of some kind. ARottenTimeToRevert may occur if this transformation is temporary and they change back into a human at the worst possible time, likely drowning in the process. The MermaidProblem might be brought up if the character has questions about their new biology (usually if the transformation results in the removal of the "usual equipment). This trope always involves SuperNotDrowningSkills, though MakingASplash can vary from case to case.
Note that this trope deals with characters who were already human (or humanoid) gaining fish traits. If the character was already aquatic to begin with and becomes human, that's HumanityEnsues. If the character is pretending to be a mermaid without actually transforming into one, that's MermaidingSwimsuit. See {{Animorphism}} for other examples of humans being turned into animals.
!!Examples:[[foldercontrol]]
[[folder:Anime and Manga]]
* ''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.
* ''Manga/FairyTail'':
** 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 [[Manga/FairyTail100YearsQuest 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.
* ''Anime/StarTwinklePrettyCure'': 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.
* ''Manga/ToiletBoundHanakoKun'': Nene Yashiro swallows a mermaid's scale, which causes her to turn into a fish when she's submerged in water.
* ''Manga/TokyoMewMew'': Mew Lettuce has porpoise DNA, so her legs transform into a mermaid's tail whenever she submerges underwater. ''Anime/TokyoMewMewNew'' changes this to where contact with her teammate Mew Aqua results in the alteration.
[[/folder]]
[[folder:Comic Books]]
* ''ComicBook/{{Aquaman}}'':
** The titular character is a HalfHumanHybrid, born to a human father and an Atlantian mother. He discovers his abilities at a young age, though his mother is not around in his life on account of some rather [[{{Understatement}} messy politics]] in Atlantis.
** Most takes on the lost city of Atlantis where Aquaman rules sees the city sink to the bottom of the ocean, as in the myths, which results in its people altering their physiology in order to survive. Most of them are ApparentlyHumanMerfolk, though there are traditional mermaids as well, and the series demonstrates that the fish are sentient (though only those with telepathy like Aquaman can communicate with them).
** The Sub-Diego storyline deals with an entire portion of San Diego, California sinks to the bottom of the ocean after an earthquake. In order for its denizens to survive, Anton Geist alters the populations physiology to make them able to breathe underwater. One of its denizens, Lorena Marquez, becomes the second Aquagirl.
** The monster Dead Water was an ordinary human who transforms into an underwater creature [[spoiler:as a result of extreme distress]].
* ''ComicBook/BrightestDay'': Jackson Hyde was raised in New Mexico and taught to fear water growing up, due to him growing gills and webbing whenever he touches any source of water (including rain). The activation of a White Lantern battery activates his ability to control water while standing in a rainstorm, at which point he is told he was given to his adoptive parents by Mera. He later discovers he is half-Atlantean and joined Aquaman in defeating his father Black Manta.
* ''ComicBook/{{Fathom}}'': Aspen Matthews, who's always had a talent for swimming and was raised by a naval captain, is partially descended from an underwater race known as The Blue, which gives her incredible dexterity underwater in addition to being able to breathe underwater indefinitely. She can also alter herself into living water, or adjust her body to take on more aquatic characteristics, on account of [[spoiler:being part of the other underwater race, The Black.]] She first learned about her true heritage and powers into her adult years, but the hint of who she really was came when she was disqualified from participating in the Olympics on account of having a much higher oxygen count than usual (due to her DNA, which the Olympic Committee thought was her putting more blood into her system).
* ''ComicBook/{{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 [[spoiler: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.
* ''ComicBook/SubMariner'' has a similar version of Atlantis to DC, and it's titular lead is likewise a HalfHumanHybrid, but he also has winged feet ''and'' is Marvel's first Mutant. He's more aware of his heritage at a young age than Aquaman started out as, but all that mixed DNA has made him ''very'' unstable.
* ComicBook/{{Superman}} comics from [[UsefulNotes/TheSilverAgeOfComicBooks the Silver Age]] had stories where Superman, Supergirl, and Lois Lane turn into mermaids, each for completely separate reasons. There was also a story where Jimmy Olsen ''thought'' they'd been turned into a fish person, but that was revealed to be AllJustAPrank pulled on them by Superman.
* ''ComicBook/TeenTitans'':
** Garfield Logan, aka Beast Boy, has the ability to turn into any animal he can think of. If a mission requires going underwater, he can turn into virtually any fish he wants.
** Post-Rebirth Aqualad (Jackson Hyde) grew up knowing he had hydrokinesis, but was unaware he could breathe underwater until he saved Robin from drowning, having grown up in the middle of the New Mexico desert. After joining the Titans, he later discovers he got his powers from his mother, a secret Xebelian refugee, who hid in the desert to avoid Jackson being discovered by his father, Black Manta.
[[/folder]]
[[folder:Fan Works]]
* ''Fanfic/DungeonKeeperAmi'': "Deadline" has the magic of "partial amphibian transformations" on humanoids, for water-breathing purposes.
* ''Fanfic/JustAddWaterAkelaVictoire'': An ''WesternAnimation/{{Encanto}}'' fanfic where Mirabel spontaneously gains a gift that turns her into a mermaid when she touches water.
* ''Fanfic/PokemonResetBloodlines'': In a reality where some humans known as Bloodliners can tap into Pokémon abilities thanks to their unique DNA, Misty is a Water Bloodliner who can use Water Pokémon moves, survive underwater, and perfectly understand Water Types. According to her, she first learned of her powers when she accidentally fell into her family's pool and nearly drowned until they kicked in. Alas, the fact her family was horrifically abusive left her a BrokenBird until Ash came along.
* ''Fanfic/WhatTheWaterGaveMe'': A ''WesternAnimation/HowToTrainYourDragon'' fanfic in which [[spoiler:Hiccup, Astrid, and Valka]] are turned into merfolk by mermaids.
[[/folder]]
[[folder:Film-Animated]]
* ''WesternAnimation/BarbieInAMermaidTale'': 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 [[spoiler:thanks to a magic necklace, as well as embracing her mermaid heritage.]]
* ''WesternAnimation/HelpImAFish'': 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.
* ''WesternAnimation/TheLittleMermaidIIReturnToTheSea'': 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. [[spoiler:She nearly drowns when the spell wears off, but is luckily saved.]]
* ''WesternAnimation/{{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.
* ''WesternAnimation/MyLittlePonyTheMovie2017'': The Mane Six are temporarily turned into seaponies while Spike becomes a pufferfish though Queen Novo's Pearl of Transformation. [[spoiler: It is also revealed the sea ponies themselves were originally hippogriffs, and only went underwater to hide from the Storm King]].
* ''WesternAnimation/ScoobyDooCampScare'': 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. [[spoiler:{{Subverted}} when it turns out to be another ScoobyDooHoax.]]
* In ''WesternAnimation/TheSwordInTheStone'', Merlin turns himself and Wart into fish for a lesson on swimming like a fish.
[[/folder]]
[[folder:Film-Live Action]]
* ''Film/TheAdventuresOfSharkBoyAndLavaGirl'': The titular Shark Boy was an ordinary human who was [[RaisedByWolves Raised By Sharks]], and gained shark characteristics...somehow.
* ''Film/Aquaman2018'': 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, but the resulting disaster turned most of its population into water-breathers. Some further mutated into merfolk, others crabs, and one into the DCEU's version of the nightmarish Trench.
* ''Film/BlackPantherWakandaForever'': The denizens of Talokan (the MCU's version of Atlantis) were turned into underwater denizens when a local village priest discovered a cure to the population's case of smallpox. [[spoiler:The plant itself was infected with Vibranium from a meteor that fell to Earth millions of years ago]].
* ''Film/BlueMyMind'': A 15-year-old girl named Mia has a SlowTransformation into a Mermaid, PlayedForDrama and BodyHorror.
* ''Film/{{Dagon}}'': Part of the TwistEnding. [[spoiler:Paul discovers that he is part [[FishPeople 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.]]
* ''Film/TheIncredibleMrLimpet'': 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.
* ''Film/TheThirteenthYear'': 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.
* ''Film/XMenFirstClass'' has the character of Armando Muñoz / Darwin, whose mutant ability lets him adapt to any environment. To demonstrate his powers, he sticks his head in a fish tank and immediately grows gills and fins. [[spoiler:He also dies at Sebastian Shaw's hands.]]
[[/folder]]
[[folder:Literature]]
* ''Literature/AndThenITurnedIntoAMermaid'' A series of children's drama books focusing on Molly Seabrook a young girl who discovers she's half mermaid and has to navigate the difficulties of secrets, school, friendships, and a crazy family.
* ''Literature/{{Animorphs}}'': The titular leads are five young humans who gain powers from an alien that allows them to turn into any animal they can think of. Naturally, some of these include water-based animals.
* ''Literature/TheDeep2019'' is about a society of merfolk descended from pregnant women thrown off of slave ships.
* ''Literature/EmilyWindsnap'': 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 [[HalfHumanHybrid long-lost father was a merman]] and her mother was enchanted to forget him.
* ''Literature/HarryPotter'': Gillyweed allows a human to grow gills as well as webbing between their fingers and toes. Harry uses this in ''Literature/HarryPotterAndTheGobletOfFire'' to compete in the second task of the Triwizard Tournament. Viktor Krum uses a spell to give him a shark's head, while Cedric and Fleur instead use a bubble charm to help them breathe.
* ''Literature/LostVoices'' 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.
* ''Literature/TheMer'': 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.
* In the Creator/PaulJennings story ''Nails'', Lehman appears to have [[BodyHorror fingernails growing all over his body]], so his father goes to FindTheCure for him. The "nails" are later revealed to be fishy scales, as Lehman's [[MissingMom absent mother]] was actually a mermaid, meaning that he was [[HalfHumanHybrid 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.
* Creator/HPLovecraft gives us "''[[Literature/TheShadowOverInnsmouth The Shadow Over Innsmouth]]''", where, after running from the eponymous town 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 HalfHumanHybrids. 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.
* In ''Literature/ATaleOf'', the story focusing on Ursula, ''Poor Unfortunate Soul: A Tale of the Sea Witch'', revealed Ursula was an orphan raised by humans until her powers kicked in and revealed herself to be a Sea Witch. The resulting trauma from people find out ain't pretty, and kickstarts her down the path towards becoming evil.
* In ''Literature/TheWaterBabies'', abused and neglected children are transformed after death into tiny people who have both lungs and gills and can live in rivers or the ocean.
[[/folder]]
[[folder:Live-Action TV]]
* ''Series/BuffyTheVampireSlayer'' has the episode "[[Recap/BuffyTheVampireSlayerS2E20GoFish Go Fish]]" where a [[FishPeople humanoid fish creature]] is praying on the school swim team. However it turns out [[spoiler:not only is it multiple fish people, they are the former members of the school swim team who are not being killed but permanently transformed into [[DeathOfPersonality mindless]] "Gill Monsters" thanks to the side effects of a concoction of Fish DNA and steroids their coach was feeding them.]]
* ''Series/Charmed1998'': "[[Recap/CharmedS5E1AWitchsTailPart1 A Witch's]] [[Recap/CharmedS5E2AWitchsTailPart2 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.
* ''Series/H2OJustAddWater'' is about three women 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 [[WaterTriggeredChange getting splashed with water turns them into their aquatic forms]].
*''Series/MakoMermaidsAnH2OAdventure'', a sequel to ''[=H2O=]'', deals with a ''merman'' when a human boy named Zack falls into a similar pool and turns into one. As punishment for letting him fall in, the mermaids guarding it have to watch over him.
* ''Series/RoundTheTwist'' episode "Nails" features a NewTransferStudent and TemporaryLoveInterest for Linda Twist, Andrew, who is handsome, polite, charming and a fantastic actor in the school play who has an affinity for the sea and who suspiciously always wears gloves. It turns out this is because [[spoiler:he is [[HalfHumanHybrid half-merfolk on his mother's side]] a fact his father was trying to hide from him]] and the gloves are because [[spoiler: 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.]]
[[/folder]]
[[folder:Tabletop Games]]
* ''TabletopGame/ArkhamHorror'': Both Amanda Sharp and Silas Marsh have the infamous Innsmouth Look originating from ''Literature/TheShadowOverInnsmouth''. Fail their personal stories, and they will become Deep Ones.
[[/folder]]
[[folder:Video Games]]
* ''VideoGame/CuteKnightKingdom'': One of the more magical endings involves the protagonist turning into a mermaid, to [[spoiler:make a romance work]].
* ''VideoGame/EnchantedPortals'': 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.
* ''Franchise/KingdomHearts'': In order to survive in [[WesternAnimation/TheLittleMermaid1989 Atlantica]], Donald Duck uses his magic to turn Sora's party into merfolk. Sora becomes a blue-tailed merboy dolphin, 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 [[AlienNonInterferenceClause World Order]] that prevents them from telling her they're from another world).
* ''Franchise/{{Pokemon}}'':
** 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.
** Castform changes into a Water type when the weather gets rainy.
** Azurill starts out as a Normal/Fairy type, but as it evolves, it gains Water typing.
** Arceus, being the god of all Pokémon, has one of 18 plates that lets it alter its typing from Normal, Water being one of them.
* ''VideoGame/{{Shantae}}'': Three of Shantae's many forms she can take on is a mermaid, a crab and a frog, which involves her belly-dancing to change into it.
* ''VideoGame/TheSims'': 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.
[[/folder]]
[[folder:Western Animation]]
* ''WesternAnimation/DoraTheExplorer'': 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.
* ''WesternAnimation/{{Futurama}}'': The entire city of Atlanta, Georgia sunk underwater before the series began because they overbuilt, and the populous eventually became merfolk. How? [[HollywoodEvolution The nearby Coca-Cola plant leaked out and turned everyone into mermaids]]. Of course, [[VoodooShark that just raises further questions!]] It can be seen in "[[Recap/FuturamaS2E12TheDeepSouth The Deep South]]".
* ''WesternAnimation/GIJoeARealAmericanHero'': In "Memories of Mara", the Joes learn that Cobra had kidnapped an innocent woman and genetically modified her to make her only able to survive underwater. She's rescued by the Joes, and winds up falling for Shipwreck.
* ''WesternAnimation/HighGuardianSpice'': In "[[Recap/HighGuardianSpiceTheSchypith The Scypith]]", episode has the main four girls turning into mermaids while trying to complete a mission.
** ''WesternAnimation/JusticeLeague'': During the ''Unlimited'' saga, Project Cadmus made a group of [[{{Expy}} expies]] of the characters from ''WesternAnimation/SuperFriends'', including the Wonder Twins in the forms of Shifter and Downpour. Like their counterparts, they could transform into animals (including aquatic ones) and water-based forms, though they could do so voluntarily instead of having to make contact with each other. The latter was dumb enough to try and take on ''Aquaman'' in his natural element.
-->'''Aquaman:''' King of the seas, remember?
* ''WesternAnimation/KimPossible'':
** In "[[Recap/KimPossibleS1E2SinkOrSwim 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 [[NeverMyFault 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 "[[Recap/KimPossibleS2E19ReturnToWannaweep Return to Camp Wannaweep]]", a seemingly cured and rehabilitated Gil [[spoiler: reveals he actually misses being a mutant, and willingly re-mutates himself with the concentrated Wannaweep muck in the grotto, 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.]]
* ''WesternAnimation/MyLittlePonyFriendshipIsMagic'': In "[[Recap/MyLittlePonyFriendshipIsMagicS8E6SurfAndOrTurf 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.
* ''WesternAnimation/SabrinaTheAnimatedSeries'': 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.
* ''WesternAnimation/SofiaTheFirst'': In "[[Recap/SofiaTheFirstS1E22TheFloatingPalace 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.
* The ''WesternAnimation/StarTrekTheAnimatedSeries'' episode "[[Recap/StarTrekTheAnimatedSeriesS1E13TheAmbergrisElement The Ambergris Element]]" has an ''Enterprise'' shuttlecraft visit the watery planet Argo. A sursnake attacks the craft, and pulls Captain Kirk and First Officer Spock into Argo's depths. Five days later, a rescue party finds the chief officers alive, but changed into water-breathing mermen, including webbed digits. Determined to reverse this process (he can't command a starship from an aquarium after all!), Kirk and Spock return to Argo, and discover that its populace also transformed into aquatic people, to survive after their land masses sank into the seas.
* ''WesternAnimation/SuperFriends'': 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.
* ''WesternAnimation/TheTransformers'': In the episode "[[Recap/TransformersG1SeaChange Sea Change]]", the Autobots learn an entire race had fallen under attack to the Decepticons and immediately move to render aid. They learn these aliens have avoided capture by using a special tree 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 heartless machines). Seaspray falls in love with one, and uses the tree to make himself human to prove he's not a soulless machine, and temporarily takes on a merman form to aid his lover. [[spoiler: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.]]
* ''WesternAnimation/WinxClub'': Fairies are able to gain a mermaid form when transformed by a magic spell used by the mermaid queen.
* ''WesternAnimation/XiaolinShowdown: Recap/XiaolinShowdownScreamsOfTheSiren'' introduced the Gills of Hamachi a Sheng Gong Wu that grants its user gills, fins and scales so they can breath and swim underwater.
* ''WesternAnimation/YoungJustice'':
** Miss Martian is a VoluntaryShapeshifter, 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.
** Delphis was an Indian teenager trafficked to an abandoned factory in Cuba where Klarion [[ForcedTransformation activated her meta-gene, resulting in her growing gills and webbing on her hands]], along with blue body marks and eye and hair color changes. No longer able to breathe on land, once she was rescued, Kaldur arranged for her to stay in Atlantis, where she chose her new name.
** Calvin Durham was a human experimented on by Black Manta to be able to live underwater in order to infiltrate Atlantis, but he fell in love with Sha'lain'a and gave up his mission, raising her son Kaldur'ahm with her.
[[/folder]]
----
-->-- '''Captain James T. Kirk''', ''WesternAnimation/StarTrekTheAnimatedSeries'', "[[Recap/StarTrekTheAnimatedSeriesS1E13TheAmbergrisElement The Ambergris Element]]"
A subtrope of OurMermaidsAreDifferent and MagicalSpeciesTransformation, a character finds themselves in a situation where, somehow, they discover they have aquatic traits and become an ApparentlyHumanMerfolk of some sort. Usually, this gives them the ability to survive underwater, if the character isn't outright turned into a mermaid or some kind of fish. How this happens tends to vary from medium to medium, but it usually occurs the following ways:
* VoluntaryShapeshifting: The character can alter their body any way they like, and can take on aquatic characteristics to survive underwater. If they happen to like mermaids, they tend to take on that form as a means of TravelTransformation, or sometimes they just do it for fun.
* ForcedTransformation: Some characters get unlucky and circumstances force them into this form. They could swallow a magic potion without knowing, get cursed, or just happen to be at the wrong place at the wrong time when some catastrophe occurs and wind up being changed.
* HollywoodEvolution: Strange circumstances force the characters underwater, and even stranger circumstances give them the means to survive.
* FisherKingdom: The laws of the universe mean the characters must adopt the shapes of the dimension they're traveling to so they can blend in, and if it happens to be underwater, it's the only way to accomplish it.
* MadScientist: Sometimes that one unlucky soul gets kidnapped and used as part of some twisted experiment to forcibly evolve them to live underwater.
* HalfHumanHybrid: They have the genetic potential to adopt underwater traits. They typically discover this heritage at some point in their lives when they gain powers or traits, sometimes as a result of a traumatic experience, or happen to encounter their people or family members who lost them or gave them up for adoption.
* TraumaticSuperpowerAwakening: The character suffers an incident where their powers kick in, leading them to the astonishing discovery.
* TransformationTrinket: The character possesses a device that lets them turn into an aquatic form.
* InvoluntaryShapeshifting: A character's powerset kicks in unwillingly and they have no choice in the matter.
Once this kicks in, the character may spend some time RevelingInTheNewForm or outright panic at their situation. If they make this discovery early on, the story will usually revolve around them exploring their new powers, trying to find a cure if it's dangerous or they don't want to be stuck that way, or going to find out exactly where they come from. Expect a fair amount of {{Fanservice}} if the character turns into a merfolk of some kind. ARottenTimeToRevert may occur if this transformation is temporary and they change back into a human at the worst possible time, likely drowning in the process. The MermaidProblem might be brought up if the character has questions about their new biology (usually if the transformation results in the removal of the "usual equipment). This trope always involves SuperNotDrowningSkills, though MakingASplash can vary from case to case.
Note that this trope deals with characters who were already human (or humanoid) gaining fish traits. If the character was already aquatic to begin with and becomes human, that's HumanityEnsues. If the character is pretending to be a mermaid without actually transforming into one, that's MermaidingSwimsuit. See {{Animorphism}} for other examples of humans being turned into animals.
!!Examples:[[foldercontrol]]
[[folder:Anime and Manga]]
* ''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.
* ''Manga/FairyTail'':
** 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 [[Manga/FairyTail100YearsQuest 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.
* ''Anime/StarTwinklePrettyCure'': 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.
* ''Manga/ToiletBoundHanakoKun'': Nene Yashiro swallows a mermaid's scale, which causes her to turn into a fish when she's submerged in water.
* ''Manga/TokyoMewMew'': Mew Lettuce has porpoise DNA, so her legs transform into a mermaid's tail whenever she submerges underwater. ''Anime/TokyoMewMewNew'' changes this to where contact with her teammate Mew Aqua results in the alteration.
[[/folder]]
[[folder:Comic Books]]
* ''ComicBook/{{Aquaman}}'':
** The titular character is a HalfHumanHybrid, born to a human father and an Atlantian mother. He discovers his abilities at a young age, though his mother is not around in his life on account of some rather [[{{Understatement}} messy politics]] in Atlantis.
** Most takes on the lost city of Atlantis where Aquaman rules sees the city sink to the bottom of the ocean, as in the myths, which results in its people altering their physiology in order to survive. Most of them are ApparentlyHumanMerfolk, though there are traditional mermaids as well, and the series demonstrates that the fish are sentient (though only those with telepathy like Aquaman can communicate with them).
** The Sub-Diego storyline deals with an entire portion of San Diego, California sinks to the bottom of the ocean after an earthquake. In order for its denizens to survive, Anton Geist alters the populations physiology to make them able to breathe underwater. One of its denizens, Lorena Marquez, becomes the second Aquagirl.
** The monster Dead Water was an ordinary human who transforms into an underwater creature [[spoiler:as a result of extreme distress]].
* ''ComicBook/BrightestDay'': Jackson Hyde was raised in New Mexico and taught to fear water growing up, due to him growing gills and webbing whenever he touches any source of water (including rain). The activation of a White Lantern battery activates his ability to control water while standing in a rainstorm, at which point he is told he was given to his adoptive parents by Mera. He later discovers he is half-Atlantean and joined Aquaman in defeating his father Black Manta.
* ''ComicBook/{{Fathom}}'': Aspen Matthews, who's always had a talent for swimming and was raised by a naval captain, is partially descended from an underwater race known as The Blue, which gives her incredible dexterity underwater in addition to being able to breathe underwater indefinitely. She can also alter herself into living water, or adjust her body to take on more aquatic characteristics, on account of [[spoiler:being part of the other underwater race, The Black.]] She first learned about her true heritage and powers into her adult years, but the hint of who she really was came when she was disqualified from participating in the Olympics on account of having a much higher oxygen count than usual (due to her DNA, which the Olympic Committee thought was her putting more blood into her system).
* ''ComicBook/{{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 [[spoiler: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.
* ''ComicBook/SubMariner'' has a similar version of Atlantis to DC, and it's titular lead is likewise a HalfHumanHybrid, but he also has winged feet ''and'' is Marvel's first Mutant. He's more aware of his heritage at a young age than Aquaman started out as, but all that mixed DNA has made him ''very'' unstable.
* ComicBook/{{Superman}} comics from [[UsefulNotes/TheSilverAgeOfComicBooks the Silver Age]] had stories where Superman, Supergirl, and Lois Lane turn into mermaids, each for completely separate reasons. There was also a story where Jimmy Olsen ''thought'' they'd been turned into a fish person, but that was revealed to be AllJustAPrank pulled on them by Superman.
* ''ComicBook/TeenTitans'':
** Garfield Logan, aka Beast Boy, has the ability to turn into any animal he can think of. If a mission requires going underwater, he can turn into virtually any fish he wants.
** Post-Rebirth Aqualad (Jackson Hyde) grew up knowing he had hydrokinesis, but was unaware he could breathe underwater until he saved Robin from drowning, having grown up in the middle of the New Mexico desert. After joining the Titans, he later discovers he got his powers from his mother, a secret Xebelian refugee, who hid in the desert to avoid Jackson being discovered by his father, Black Manta.
[[/folder]]
[[folder:Fan Works]]
* ''Fanfic/DungeonKeeperAmi'': "Deadline" has the magic of "partial amphibian transformations" on humanoids, for water-breathing purposes.
* ''Fanfic/JustAddWaterAkelaVictoire'': An ''WesternAnimation/{{Encanto}}'' fanfic where Mirabel spontaneously gains a gift that turns her into a mermaid when she touches water.
* ''Fanfic/PokemonResetBloodlines'': In a reality where some humans known as Bloodliners can tap into Pokémon abilities thanks to their unique DNA, Misty is a Water Bloodliner who can use Water Pokémon moves, survive underwater, and perfectly understand Water Types. According to her, she first learned of her powers when she accidentally fell into her family's pool and nearly drowned until they kicked in. Alas, the fact her family was horrifically abusive left her a BrokenBird until Ash came along.
* ''Fanfic/WhatTheWaterGaveMe'': A ''WesternAnimation/HowToTrainYourDragon'' fanfic in which [[spoiler:Hiccup, Astrid, and Valka]] are turned into merfolk by mermaids.
[[/folder]]
[[folder:Film-Animated]]
* ''WesternAnimation/BarbieInAMermaidTale'': 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 [[spoiler:thanks to a magic necklace, as well as embracing her mermaid heritage.]]
* ''WesternAnimation/HelpImAFish'': 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.
* ''WesternAnimation/TheLittleMermaidIIReturnToTheSea'': 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. [[spoiler:She nearly drowns when the spell wears off, but is luckily saved.]]
* ''WesternAnimation/{{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.
* ''WesternAnimation/MyLittlePonyTheMovie2017'': The Mane Six are temporarily turned into seaponies while Spike becomes a pufferfish though Queen Novo's Pearl of Transformation. [[spoiler: It is also revealed the sea ponies themselves were originally hippogriffs, and only went underwater to hide from the Storm King]].
* ''WesternAnimation/ScoobyDooCampScare'': 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. [[spoiler:{{Subverted}} when it turns out to be another ScoobyDooHoax.]]
* In ''WesternAnimation/TheSwordInTheStone'', Merlin turns himself and Wart into fish for a lesson on swimming like a fish.
[[/folder]]
[[folder:Film-Live Action]]
* ''Film/TheAdventuresOfSharkBoyAndLavaGirl'': The titular Shark Boy was an ordinary human who was [[RaisedByWolves Raised By Sharks]], and gained shark characteristics...somehow.
* ''Film/Aquaman2018'': 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, but the resulting disaster turned most of its population into water-breathers. Some further mutated into merfolk, others crabs, and one into the DCEU's version of the nightmarish Trench.
* ''Film/BlackPantherWakandaForever'': The denizens of Talokan (the MCU's version of Atlantis) were turned into underwater denizens when a local village priest discovered a cure to the population's case of smallpox. [[spoiler:The plant itself was infected with Vibranium from a meteor that fell to Earth millions of years ago]].
* ''Film/BlueMyMind'': A 15-year-old girl named Mia has a SlowTransformation into a Mermaid, PlayedForDrama and BodyHorror.
* ''Film/{{Dagon}}'': Part of the TwistEnding. [[spoiler:Paul discovers that he is part [[FishPeople 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.]]
* ''Film/TheIncredibleMrLimpet'': 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.
* ''Film/TheThirteenthYear'': 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.
* ''Film/XMenFirstClass'' has the character of Armando Muñoz / Darwin, whose mutant ability lets him adapt to any environment. To demonstrate his powers, he sticks his head in a fish tank and immediately grows gills and fins. [[spoiler:He also dies at Sebastian Shaw's hands.]]
[[/folder]]
[[folder:Literature]]
* ''Literature/AndThenITurnedIntoAMermaid'' A series of children's drama books focusing on Molly Seabrook a young girl who discovers she's half mermaid and has to navigate the difficulties of secrets, school, friendships, and a crazy family.
* ''Literature/{{Animorphs}}'': The titular leads are five young humans who gain powers from an alien that allows them to turn into any animal they can think of. Naturally, some of these include water-based animals.
* ''Literature/TheDeep2019'' is about a society of merfolk descended from pregnant women thrown off of slave ships.
* ''Literature/EmilyWindsnap'': 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 [[HalfHumanHybrid long-lost father was a merman]] and her mother was enchanted to forget him.
* ''Literature/HarryPotter'': Gillyweed allows a human to grow gills as well as webbing between their fingers and toes. Harry uses this in ''Literature/HarryPotterAndTheGobletOfFire'' to compete in the second task of the Triwizard Tournament. Viktor Krum uses a spell to give him a shark's head, while Cedric and Fleur instead use a bubble charm to help them breathe.
* ''Literature/LostVoices'' 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.
* ''Literature/TheMer'': 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.
* In the Creator/PaulJennings story ''Nails'', Lehman appears to have [[BodyHorror fingernails growing all over his body]], so his father goes to FindTheCure for him. The "nails" are later revealed to be fishy scales, as Lehman's [[MissingMom absent mother]] was actually a mermaid, meaning that he was [[HalfHumanHybrid 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.
* Creator/HPLovecraft gives us "''[[Literature/TheShadowOverInnsmouth The Shadow Over Innsmouth]]''", where, after running from the eponymous town 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 HalfHumanHybrids. 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.
* In ''Literature/ATaleOf'', the story focusing on Ursula, ''Poor Unfortunate Soul: A Tale of the Sea Witch'', revealed Ursula was an orphan raised by humans until her powers kicked in and revealed herself to be a Sea Witch. The resulting trauma from people find out ain't pretty, and kickstarts her down the path towards becoming evil.
* In ''Literature/TheWaterBabies'', abused and neglected children are transformed after death into tiny people who have both lungs and gills and can live in rivers or the ocean.
[[/folder]]
[[folder:Live-Action TV]]
* ''Series/BuffyTheVampireSlayer'' has the episode "[[Recap/BuffyTheVampireSlayerS2E20GoFish Go Fish]]" where a [[FishPeople humanoid fish creature]] is praying on the school swim team. However it turns out [[spoiler:not only is it multiple fish people, they are the former members of the school swim team who are not being killed but permanently transformed into [[DeathOfPersonality mindless]] "Gill Monsters" thanks to the side effects of a concoction of Fish DNA and steroids their coach was feeding them.]]
* ''Series/Charmed1998'': "[[Recap/CharmedS5E1AWitchsTailPart1 A Witch's]] [[Recap/CharmedS5E2AWitchsTailPart2 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.
* ''Series/H2OJustAddWater'' is about three women 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 [[WaterTriggeredChange getting splashed with water turns them into their aquatic forms]].
*''Series/MakoMermaidsAnH2OAdventure'', a sequel to ''[=H2O=]'', deals with a ''merman'' when a human boy named Zack falls into a similar pool and turns into one. As punishment for letting him fall in, the mermaids guarding it have to watch over him.
* ''Series/RoundTheTwist'' episode "Nails" features a NewTransferStudent and TemporaryLoveInterest for Linda Twist, Andrew, who is handsome, polite, charming and a fantastic actor in the school play who has an affinity for the sea and who suspiciously always wears gloves. It turns out this is because [[spoiler:he is [[HalfHumanHybrid half-merfolk on his mother's side]] a fact his father was trying to hide from him]] and the gloves are because [[spoiler: 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.]]
[[/folder]]
[[folder:Tabletop Games]]
* ''TabletopGame/ArkhamHorror'': Both Amanda Sharp and Silas Marsh have the infamous Innsmouth Look originating from ''Literature/TheShadowOverInnsmouth''. Fail their personal stories, and they will become Deep Ones.
[[/folder]]
[[folder:Video Games]]
* ''VideoGame/CuteKnightKingdom'': One of the more magical endings involves the protagonist turning into a mermaid, to [[spoiler:make a romance work]].
* ''VideoGame/EnchantedPortals'': 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.
* ''Franchise/KingdomHearts'': In order to survive in [[WesternAnimation/TheLittleMermaid1989 Atlantica]], Donald Duck uses his magic to turn Sora's party into merfolk. Sora becomes a blue-tailed merboy dolphin, 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 [[AlienNonInterferenceClause World Order]] that prevents them from telling her they're from another world).
* ''Franchise/{{Pokemon}}'':
** 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.
** Castform changes into a Water type when the weather gets rainy.
** Azurill starts out as a Normal/Fairy type, but as it evolves, it gains Water typing.
** Arceus, being the god of all Pokémon, has one of 18 plates that lets it alter its typing from Normal, Water being one of them.
* ''VideoGame/{{Shantae}}'': Three of Shantae's many forms she can take on is a mermaid, a crab and a frog, which involves her belly-dancing to change into it.
* ''VideoGame/TheSims'': 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.
[[/folder]]
[[folder:Western Animation]]
* ''WesternAnimation/DoraTheExplorer'': 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.
* ''WesternAnimation/{{Futurama}}'': The entire city of Atlanta, Georgia sunk underwater before the series began because they overbuilt, and the populous eventually became merfolk. How? [[HollywoodEvolution The nearby Coca-Cola plant leaked out and turned everyone into mermaids]]. Of course, [[VoodooShark that just raises further questions!]] It can be seen in "[[Recap/FuturamaS2E12TheDeepSouth The Deep South]]".
* ''WesternAnimation/GIJoeARealAmericanHero'': In "Memories of Mara", the Joes learn that Cobra had kidnapped an innocent woman and genetically modified her to make her only able to survive underwater. She's rescued by the Joes, and winds up falling for Shipwreck.
* ''WesternAnimation/HighGuardianSpice'': In "[[Recap/HighGuardianSpiceTheSchypith The Scypith]]", episode has the main four girls turning into mermaids while trying to complete a mission.
** ''WesternAnimation/JusticeLeague'': During the ''Unlimited'' saga, Project Cadmus made a group of [[{{Expy}} expies]] of the characters from ''WesternAnimation/SuperFriends'', including the Wonder Twins in the forms of Shifter and Downpour. Like their counterparts, they could transform into animals (including aquatic ones) and water-based forms, though they could do so voluntarily instead of having to make contact with each other. The latter was dumb enough to try and take on ''Aquaman'' in his natural element.
-->'''Aquaman:''' King of the seas, remember?
* ''WesternAnimation/KimPossible'':
** In "[[Recap/KimPossibleS1E2SinkOrSwim 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 [[NeverMyFault 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 "[[Recap/KimPossibleS2E19ReturnToWannaweep Return to Camp Wannaweep]]", a seemingly cured and rehabilitated Gil [[spoiler: reveals he actually misses being a mutant, and willingly re-mutates himself with the concentrated Wannaweep muck in the grotto, 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.]]
* ''WesternAnimation/MyLittlePonyFriendshipIsMagic'': In "[[Recap/MyLittlePonyFriendshipIsMagicS8E6SurfAndOrTurf 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.
* ''WesternAnimation/SabrinaTheAnimatedSeries'': 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.
* ''WesternAnimation/SofiaTheFirst'': In "[[Recap/SofiaTheFirstS1E22TheFloatingPalace 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.
* The ''WesternAnimation/StarTrekTheAnimatedSeries'' episode "[[Recap/StarTrekTheAnimatedSeriesS1E13TheAmbergrisElement The Ambergris Element]]" has an ''Enterprise'' shuttlecraft visit the watery planet Argo. A sursnake attacks the craft, and pulls Captain Kirk and First Officer Spock into Argo's depths. Five days later, a rescue party finds the chief officers alive, but changed into water-breathing mermen, including webbed digits. Determined to reverse this process (he can't command a starship from an aquarium after all!), Kirk and Spock return to Argo, and discover that its populace also transformed into aquatic people, to survive after their land masses sank into the seas.
* ''WesternAnimation/SuperFriends'': 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.
* ''WesternAnimation/TheTransformers'': In the episode "[[Recap/TransformersG1SeaChange Sea Change]]", the Autobots learn an entire race had fallen under attack to the Decepticons and immediately move to render aid. They learn these aliens have avoided capture by using a special tree 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 heartless machines). Seaspray falls in love with one, and uses the tree to make himself human to prove he's not a soulless machine, and temporarily takes on a merman form to aid his lover. [[spoiler: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.]]
* ''WesternAnimation/WinxClub'': Fairies are able to gain a mermaid form when transformed by a magic spell used by the mermaid queen.
* ''WesternAnimation/XiaolinShowdown: Recap/XiaolinShowdownScreamsOfTheSiren'' introduced the Gills of Hamachi a Sheng Gong Wu that grants its user gills, fins and scales so they can breath and swim underwater.
* ''WesternAnimation/YoungJustice'':
** Miss Martian is a VoluntaryShapeshifter, 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.
** Delphis was an Indian teenager trafficked to an abandoned factory in Cuba where Klarion [[ForcedTransformation activated her meta-gene, resulting in her growing gills and webbing on her hands]], along with blue body marks and eye and hair color changes. No longer able to breathe on land, once she was rescued, Kaldur arranged for her to stay in Atlantis, where she chose her new name.
** Calvin Durham was a human experimented on by Black Manta to be able to live underwater in order to infiltrate Atlantis, but he fell in love with Sha'lain'a and gave up his mission, raising her son Kaldur'ahm with her.
[[/folder]]
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