Mermanity Ensues

"The Fish Man of Steel" just doesn't have the same ring to it.

"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, Star Trek: The Animated Series, "The Ambergris Element"

A subtrope of Our Mermaids Are Different and Magical Species Transformation, a character finds themselves in a situation where, somehow, they discover they have aquatic traits and become an Apparently Human Merfolk 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:

• Voluntary Shapeshifting: 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 Travel Transformation, or sometimes they just do it for fun.
• Forced Transformation: 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.
• Hollywood Evolution: Strange circumstances force the characters underwater, and even stranger circumstances give them the means to survive.
• Fisher Kingdom: 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.
• Mad Scientist: Sometimes that one unlucky soul gets kidnapped and used as part of some twisted experiment to forcibly evolve them to live underwater.
• Half-Human Hybrid: 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.
• Traumatic Superpower Awakening: The character suffers an incident where their powers kick in, leading them to the astonishing discovery.
• Transformation Trinket: The character possesses a device that lets them turn into an aquatic form.
• Involuntary Shapeshifting: 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 Reveling in the New Form 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. A Rotten Time to Revert 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 Mermaid Problem 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 Super Not-Drowning Skills, though Making a Splash 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 Humanity Ensues. If the character is pretending to be a mermaid without actually transforming into one, that's Mermaiding Swimsuit. See Animorphism for other examples of humans being turned into animals.

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Examples:

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.
• 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.
• 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:
  • The titular character is a Half-Human Hybrid, 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 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 Apparently Human Merfolk, 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).
  • Aquaman (1962) had the villainous Dr. Starbuck experiment on himself and his animal troupe to breathe underwater, later using them to take over Atlantis.
  • The Sub-Diego storyline in Aquaman (2003) deals with an entire portion of San Diego, California sinking 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 as a result of extreme distress.
• Brightest Day: 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.
• 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 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).
• 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.
• Sub-Mariner has a similar version of Atlantis to DC, and it's titular lead is likewise a Half-Human Hybrid, 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.
• 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": Kara gets temporarily turned into a mermaid by a Red Kryptonite asteroid, and lives several submarine adventures until the effect wears off.
• Teen Titans:
  • 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.

Fan Works

• Dungeon Keeper Ami: "Deadline" has the magic of "partial amphibian transformations" on humanoids, for water-breathing purposes.
• Just Add Water (Akela_Victoire): An Encanto fanfic where Mirabel spontaneously gains a gift that turns her into a mermaid when she touches water.
• Pokémon Reset Bloodlines: 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 Broken Bird until Ash came along.
• 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 it's 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.
• In The Sword in the Stone, Merlin turns himself and Wart into fish for a lesson on swimming like a fish.

Film — Live-Action

• The Adventures Of Shark Boy And Lava Girl: The first half of the titular duo, Sharkboy, was an ordinary human who was Raised By Sharks, and gained shark characteristics...somehow.
• Aquaman (2018): 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.
• Black Panther: Wakanda Forever: 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. The plant itself was infected with Vibranium from a meteor that fell to Earth millions of years ago.
• 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.
• X-Men: First Class 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. He also dies at Sebastian Shaw's hands.

Literature

• And Then I Turned Into a Mermaid 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.
• The Deep (2019) is about a society of merfolk descended 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.
• Harry Potter: Gillyweed allows a human to grow gills as well as webbing between their fingers and toes. Harry uses this in Harry Potter and the Goblet of Fire 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.
• 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.
• 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.
• In 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.
• 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.
• H. P. Lovecraft gives us "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 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.
• In A Tale of..., 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 The Water-Babies, 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.

Live-Action TV

• Buffy the Vampire Slayer has the episode "Go Fish" where a humanoid fish creature is praying on the school swim team. However it turns out 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 mindless "Gill Monsters" thanks to the side effects of a concoction of Fish DNA and steroids their coach was feeding them.
• 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.
• 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.

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, 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.

Video Games

• 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.
• 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.
• 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.
  • 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.
• 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.
• 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

• 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 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.
• Futurama: The entire city of Atlanta, Georgia sunk underwater 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. Of course, that just raises further questions! It can be seen in "The Deep South".
• G.I. Joe: A Real American Hero: 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.
• 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 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.
• 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.
• 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.
• 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.
• The Star Trek: The Animated Series episode "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.
• 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: In the episode "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 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 human 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.
• Winx Club: Fairies are able to gain a mermaid form when transformed by a magic spell used by the mermaid queen.
• Xiaolin Showdown: The episode "Scream of the Siren" introduced the Gills of Hamachi, a Shen Gong Wu that grants its user gills, fins and scales so they can breath 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.
  • Delphis was an Indian teenager trafficked to an abandoned factory in Cuba where Klarion 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.
  • 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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