In any work of fiction with mermaids, there's a good chance that a male character will begin a romantic, or even a sexual, relationship with one.
There's just one problem. There's no... obvious point of entry.
That isn't to say, of course, that mermaids are completely lacking in sexual organs. Real fish, of course, have actual genitals. They just aren't compatible with those of a human, let alone easy to find or obvious to point out. A few biologists have a hard time even calling Piscine reproductive organs "genitals" because they are so very different from what we humans would recognize. Their version of reproduction is external, meaning that conception happens outside of the body. Females deposit their eggs into the water to be fertilized by the males' similarly free-floating sperm.
Of course, most writers simply disregard the biological impossibilities. Many writers simply Hand Wave any difficulties by saying that mermaids have the ability to assume fully human form under special circumstances. Alternatively, a sufficiently bawdy work—which one is likely to be if it brings up the Mermaid Problem in the first place—may simply point out that lower half aside, well... she's still got a mouth, doesn't she?
This trope mainly deals with the variety where they don't gain human legs. It is equally applicable to mermen, since they'd lack a penis for all the same reasons. Sometimes this problem may be lampshaded, but others no explanation is given. Compare Non-Mammal Mammaries. A good amount of series and fiction writers just have the fish/human separation dip a little in the front (in a U-shape) to allow for the genitalia, or use other methods to get the same result.
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Magical Pokaan has Yuuma falling in love with a pretty boy... who turns out to be a merman.
Mermaid Melody Pichi Pichi Pitch does turn its mermaids human on land... but in the manga, once Kaito gets his memories back, guess what's the first thing he and Lucia do. Underwater. Since it's a shoujo series, it's all offscreen, of course, and the problem is never mentioned.
The genders are reversed in Slayers. When instructed to kiss Lina, Nunsa comments that he's considered one of the best catches in his school, then sits down and patiently waits for... something. It turns out that, as a fish, Nunsa is only aware of the concept of "kissing" as being vaguely related to mating, so he's waiting for her to lay the eggs.
In One Piece, we see that when a merperson turns 30, their tail splits into a pair of legs and they can walk on land. The character Kokoro is actually a mermaid past 30, who married and had children with a human.
Later, Brook imagines what mermaid panties look like, until he is told that mermaids don't wear panties.
While the problem is never actually mentioned, Seto no Hanayome avoids it in that mermaids can take human form. Younger mermaids do revert if they get wet, but it's implied that they grow out of that.
This became a big issue for Master Roshi in an early chapter of Dragon Ball when Goku brought him a mermaid girl.
The Kyo Kara Maoh manga has a brief episode of this - Yuuri, Conrart, and Wolfram have a conversation about a past love of Conrart's. After a minute, Yuuri thinks they're talking about a mermaid. Turns out it's a maidmer, a reverse mermaid. Yuuri remarks that Conrart's strike range is wider than he thought...
Classical mermaids were often depicted with two fish tails with presumably more or less human genitals in between, to get over this trope.
In fact, the Mermaid was used as an Unusual Euphemism for a 'Whore' - Mary, Queen of Scots was referred to one by some of her subjects.
The Starbucks Coffee logo features a mermaid of this type◊ (a mermaid was chosen to represent Seattle's origin as a port town). She's essentially giving whoever's looking a free show. Apparently this was too risque, so they zoomed in. Now it looks like a woman's face with two random fin things next to her head. This can lead to confusion to those who don't know the origin.
The original logo itself was merely a less-risque version of an old woodcut◊ in which the mermaid is obviously quite human above the tails. Starbucks, in addition to a few other retouches, covered her with scales from the waist down in order to allay any suspicion that she might actually have a vulva.
Quite a few paintings simply portray a mermaid having her fish tail below the pelvis, allowing her to have a human vulva and butt.
Polish painter Jaroslaw Kukowski has a painting◊ of a mermaid looking in puzzlement at a magazine page showing a vulva (NSFW).
Bette Midler, while on-stage and costumed as a mermaid, once quipped "The question before us is where's her clitoris?"
Demetri Martin had a mermaid in an animated sketch in his comedy special, Demetri Martin. Person., that had an even bigger problem: it was a vertical mermaid, which meant that the left side of it was a fish and the right side was a person.
Nipsey Russell once commented in rhyme on an episode of Hollywood Squares, "I like mermaids/I don't know why./Not enough woman to make love to,/not enough fish to fry!"
The screwed-up scansion of that third line implies it's a Bowdlerised version...
Mocked on NeverMedia. "You writers have no imagination, do you?"
A type C mermaid exists for the sake of erotic humor (insert random Playboy cartoon here if necessary), which has two legs (and naturally the stuff in-between), just with the usual fishtails-instead-of-feet. Cue C saying to A: "You don't know what you're missing!"
Somewhere in Aquaman volume 3, officials in a city of mermaids complain about outsiders "swimming over their eggs".
In Milo Manara's City Hunter (not to be confused with City Hunter), there's a scene where Odysseus and his friends go off to catch some mermaids. He ends up with the traditional one, and his friend gets her ugly friend, with the parts mixed.
Mermaids in borderline porn comic Tarot: Witch of the Black Rose get around the issue by having human-style crotches. Not genitals; crotches. As in, human-like hips and legs that merge together into a fish tail somewhere around the knee. The result can be ... disturbing◊. Especially when they wear panties that they have no earthly way to put on or take off.
Maybe they go for string-bikini-style bottoms, with fasteners over the hip?
A similar issue arises with a lamia (a snake-tailed woman) rather than a mermaid in Ironwood. She reveals that there is a specific split scale that gives access to (presumably) human compatible genitals.
Played with in a Harry Potter fanfic called Jewel of the Nile in which a couple of characters don polymorphing mer-costumes before splashing about and indulging in heavy makeout sessions underwater. The tails prevent them from doing anything below the waist, but as a temporary restriction that can be quite entertaining...
Absit Omen, a Harry Potter roleplay site, uses the explanation of 'merman spawn and witch magic' as the parentage of a mermaid half-breed girl who looks like a permanent Gillyweed user.
Which suggests she's never spent much time with dolphins, else she'd have seen plenty of demonstrations of mammalian sexual practices.
Eric is also rather horrified when he realizes she hadn't known what to expect and he'd mistaken her alarm for enthusiasm.
Here's a longer version with a better ending: Ariel and Eric's Wedding Night. It also explains the dolphin thing: Ariel thought her sister was lying to her about it.
American Pie has two of the girls-obsessed teenage characters mention the issue. "Did you see The Little Mermaid on TV yesterday? Ariel, she's so hot." "She's a mermaid, dude." "Yeah, but not when she's on land, Oz."
Splash used the aforementioned Hand Wave: Madison only had a tail while exposed to water.
Pirates Of The Caribbean: On Stranger Tides solves this by making the mermaids' tails turn to legs when removed from water, much like in Splash.
The mermaids in Jack L. Chalker'sRiver of Dancing Gods series are 100% mammal (more half-dolphin than half-fish), and when a male character gets involved with one it's explicitly mentioned that their bits are human-compatible. Conversely, the mermaid-like Umiau from his Well World series are hermaphroditic, not shy about that fact, and not at all interested in sex outside their species.
In Fred Saberhagen's Lost Swords series, mermaids don't have genitals, and therefore can't mate. The only reason they exist at all is because of a curse on a human family that causes some of their daughters to be born as mermaids. When a wizard looks for a way to cure the condition he only manages to undo it temporarily, just long enough to get one of the mermaids pregnant. When the wizard dies she's left to worry about what will happen when it's time to give birth...
His novel Mer-Cycle has this as a minor plot point (of course). The "mermaid" the main characters encounter is the product of genetic engineering. The protagonist quickly discovers that her 'scaly' fish-tail is actually a series of overlapping plates that can be lifted like a skirt (!?) to reveal her fused legs and human genitalia underneath. How did he find out? He goosed her, causing her to jackknife and give him the mermaid equivalent of a Panty Shot. (Oh that Piers Anthony)
Likewise, his book Mute includes a "mermaid" who is actually a mutant woman with legs that are fused from the knees down, allowing her to squat. And yes, he is explicit about why she'd want to do that.
In Xanth, mermaids and mermen can assume human form, but when they mate with each other, they prefer to do it underwater, in their half-fish shape. When a (human) character asks how it's done, she is told to mind her own business.
The protagonist of Fredric Brown's classic horror short "Fish Story" falls in love with a mermaid. It isn't until after he successfully petitions Triton to turn him into a merman that she informs him merfolk spawn like fish, causing him to regret that he can no longer drown.
Alida Van Gores' 1989 novel Mermaid's Song takes the dolphin-based mermaid route straight into soft-core porn territory.
Wittily averted in John Ringo's Emerald Sea: "Blood Lord" Herzer Herrick tries to use the Mermaid Problem as an excuse for his lack of romantic adventures among the merfolk (Herzer eventually gets over his shyness) Later Elf-babe Bast comments on their obvious genital slits. Interestingly, it's the post-partum aspects of mermaid reproduction that form the major MacGuffin of the story. note The mer are warm blooded and their babies don't have nearly enough body mass to avoid succumbing to hypothermia if they spend too much time in the water. Since the adults are about as helpless as seals on land (an explicit comparison is made to baby fur seals) mer babies have to be raised in protected nurseries if they're going to have any hope of survival. So anyone who seizes the nursery caves can hold the entire species hostage.
The classic L. Sprague deCamp short story Nothing in the Rules features a dolphin-based mermaid who was sufficiently compatible (and sufficiently drunk) to share a "romantic interlude' with the protagonist in the back of a limousine.
In a possibly-not-true-inside-the-overall-story story in deCamp's Reluctant King Trilogy a mermaid and a human attempt to have sex. Since the mermaid is dolphin-based, finding the opening isn't a problem. However, almost drowning is.
The sea people in Vonda McIntyre's The Moon and the Sun are aquatic humanoid mammals (and, apart from their aquatic adaptations, have a lot of anatomical similarity to humans), not hybrid creatures. They have two "tails" (actually hind limbs adapted for swimming) and human-like genitalia. (In fact, Sherzad shocks a 17th-century human crowd by flashing them at one point.)
Similarly, McIntyre's genetically-engineered "divers" are more like humans with a few seal or otter traits (fur, claws, webbed hands) than traditional merfolk.
In Francesca Lia Block's short story "Mer" from the story collection Nymph, the title character, Mer, and the protagonist, Tom, solve this problem elegantly by exploring the obvious alternative.
In Kit Whitfield's novel In Great Waters, the problem is averted by having merfolk be entirely mammal, closer to sea lions than fish.
Tanith Lee's short story "Mermaid" portrays a young man who discovers a mermaid and decides that he must have sex with her. The physical part isn't difficult; she has a "flowerlike opening" on the upper part of her tail. The problem is that she's all fish: lipless mouth, lidless eyes, fine tentacles for hair, and cold to the touch.
A comparable problem not involving any merfolk confronts a young man in love with a Remade woman in China Miéville's The Scar, as her legs are permanently embedded inside a steam engine. Lucky for them, her actual genitalia are still human, and both of them are pretty flexible.
In The Golden Globe a theatrical director in the far future complains about the difficulty of finding actresses willing to give up sex in order to play mermaids; it seems far future labor laws have replaced CGI with Magic Plastic Surgery.
The section on mermaids in The Encyclopedia Of Fantasy: People of the Light has a solution for the mermaid's point of view, should she seek pleasure: it's all in the hair. Probably this counts as Fantastic Arousal, although for that you would expect the secret to be in the tail.
Ismael Merindol reveals that mermaids spend so much time combing their hair because it that is their erogenous zone, and a source of incomparable pleasure. He writes: "In my youth I had a mermaid for a lover, but I was unable to give her pleasure in the usual way. However, if I scratched her scalp in a certain way she would very quickly swoon away. For what other women have between their legs, mermaids have in their hair."
This means that all those mermaids you see innocently combing their long tresses are in fact masturbating. Who needs privacy, anyway?
Discussed and averted in Mercedes Lackey's Fortune's Fool, where the mer-princess heroine falls for a human and has to endure some ribald joking from her father about how it's fortunate her ancestors were sirens and not mermaids.
Averted in One Hundred Years Of Solitude, which mentions that "The great swamp in the west mingled with a boundless extension of water where there were soft-skinned cetaceans that had the head and torso of a woman, causing the ruination of sailors with the charm of their extraordinary breasts." Seemingly these beings didn't have intercourse with their victims.
In the second book of The Princess Series, Lirea and Gustan prove that it's possible for the two species to have sex, although Lirea, being a royal, has two tails instead of the usual single one. At one point, Snow starts to wonder out loud about how merpeople have sex, but is cut off by Talia.
A Little Sacrifice short story from The Witcher series (published in Sword of Destiny) deconstructs The Little Mermaid. The merfolk reproduce like fishes and their ways of life are completely alien to land dwellers. In addition the prince and the mermaid don't speak each other's languages and need Geralt as a translator. The solution is simple, but drastic: the mermaid becomes human, sacrificing her familiar way of life.
In an Imagine Spot from Scrubs, JD imagines falling through a portal into a fantasyland where a mermaid is waiting to have sex with him. But he can't figure out how...
And then Satyr!Turk points to a gill and starts to get freaky.
Parodied in Red Dwarf. When the crew were in a VR machine, Cat waved to his new girlfriend, who was a mermaid. She had a fish head and human legs. When asked, "Shouldn't it be the other way around?", Cat responded, "No! That's the stupid way around."
Craig Charles' little-known sitcom Captain Butler also parodied this. He asks the mermaid girl-of-his-dreams to have sex with him — so she lays a pile of eggs and leaves him to inseminate them. Not exactly what he had in mind...
On How I Met Your Mother, Marshall asks Barney if he'd make out with a classic mermaid or an inverted mermaid. Barney answers, "That depends. Is she fat?" Marshall responds that since she's part fish, it's the good kind of fat.
Ricky Gervais parodies the hell out of this trope in animals.
The Night Gallery episode "Lindemann's Catch" explores this with tragic results.
Rich Eisen: I don't really mind the fish smell...Oh shit, I just said that out loud, didn't I?
The infamous Poxy Boggards sing about this in the rather explicit song, "Nelly the Mermaid"
...Her hips were a swayin', she was a work of art, the sad irony, her legs wouldn't part! (and later...) ...They asked us to follow, so we turned and headed south, they had no lower half... but they still had a mouth!
Shel Silverstein's song "The Mermaid", recently covered by GreatBigSea, is about a sailor falling in love with a type A mermaid, breaking up with her (not, however, for the reason you're all thinking), and then falling in love with a type B.
"But I don't give a damn 'bout the upper part / 'cause that's how I gets me tail..."
Brenda Sinclair Sutton's "End of the Tail" is narrated by the fiancee of a sailor who falls in love with a type A mermaid. She tells him of course he can break their engagement and marry the mermaid.
Apparently not a problem in the classic sea chantey "Eddystone Light"
My father was the keeper of the Eddystone light And he slept with a mermaid one fine night Out of this union there came three A porpoise and a porgy and the other was me! * The video for You and I by Lady Gaga averts this by... ignoring the problem completely, apparently.
In an interview on the topic, Lady Gaga talks about that scene as referencing this trope specifically as metaphor for a relationship where you can't make it work.
In Changeling The Dreaming, mermaids grow legs on land, averting the Problem. They also have a reproductive cycle that involves kissing, making them understandably shy about it while willing to have sex at the drop of a hat. (They still need to have sex to have babies. It's just that kissing "stimulates" the women in ways that sex alone doesn't, making pregnancy possible.)
Castles And Crusades resolves the problem by giving mermaids mostly human legs and butts, except, with scales and fishy-fins starting on the upper thighs.
The Problem must've somehow been averted in the Mystara Dungeons & Dragons setting, in which the queen of Aquas is a Half-Human Hybrid of human and merfolk. Shapechanging magic by one or the other parent is implied to have been involved.
Rune Factory 3 averts this with Pia. Like other half-monsters in this series, she can shapeshift between fully human and monster form. You usually only see her as a mermaid when it's raining.
The Distaff Counterpart of DS, Cute, averts this if you 'best friend' Leia. Since two girls can't biologically have children, the Harvest King makes one of you pregnant (or was it the baby just being born?). You can choose to be the pregnant one, thus averting this.
In Kingdom Hearts, Sora turns into half dolphin, which actually averts this, since cetaceans have mammal genitalia. So any porn fanfics involving Sora as a merdolphin and a human are actually correct.
Sidesteped in Yggdra Union. The mermaid-esque Undine race are a One Gender Race that reproduce via some kind of reincarnation fueled via a MacGuffin. Needless to say when this gets stolen, all hell breaks loose.
In the third game of The Spellcasting Series by Steve Meretzky, the player character is required at one point to turn himself into a merman, and the opportunity for sex with a mermaid arises. The game states that the PC is interested to learn how mer-people do 'it' with no apparent genitals, but will reveal only two facts about the act to the player; Firstly, that it would be impossible without the buoyancy effects of water, and secondly, that it ranks above human sex on the pleasure scale to the same extent that human sex ranks above brushing your teeth.
In Tales of Monkey Island, Guybrush learns that Winslow and an ambiguously gendered merperson are in a relationship, and wonders aloud "how that works... logistically."
In Neopets, the Water and Fountain Faeries have their tail extending higher to cover their breasts, making it look more like a dress than a tail.
Not directly mentioned, but figures in the symbolism in Rule of Rose chapter "Mermaid Princess". The titular 'mermaid' is Clara who is all but confirmed to be a victim of sexual abuse, and takes a mermaid appearance by having her legs tied together with coils of rope, invoking either her sexual subservience, or attempt to protect herself, depending on whom you ask.
ABOBO DEFIES TROPE! ABOBO NO EAT MERMAID, HE MATE WITH MERMAID AND HAVE SONS! SONS PROTECT ABOBO FROM ENEMIES!
Chrono Cross never really explains just how the human Fargo and the mermaid demi-human Zelbess not only had a relationship, but managed to have two kids!
Subverted in the Visual Novel Nocturnal Illusion, the main character finds a representation of Hans Christian Andersen's version of the The Little Mermaid in a well. When the H-scene occurs, the entry is right where it should be on a human woman, although considering the mermaid's body, it requires a rather odd position for penetration to happen.
Lampshaded here at the where it simply goes "WHERE ARE THEIR REPRODUCTIVE ORGANS?!!!". The art is even CALLED "The Mermaid Problem".
Invoked in, of course, Accidental Centaurs. The heroes are turned into merfolk in order to cross the sea and when Alex says he's looking forward to weeks of kinky mermaid sex Sam asks him if he remembers exactly how fish mate. Cue the disappointed Alex.
Yuan-tis in Goblins mate similarly to snakes, but Dellyn somehow found a way to rape one. It's still uncertain how this works, but it's since become clear that they can also engage in consensual sex with humans (although the only couple portrayed thus far was from an alternate universe.)
It's definitely worth noting that unlike the vast majority of fish, all snakes engage in internal fertilization. Male snakes have a twin organ, and though the internal biology is somewhat different, they still engage in "insert tab A in slot B" sex.
The short-lived comic WCI High had one female student who was an inverted Mermaid; she had a fish head, tentacles for arms, and was human below the waist. As a member of the "Student Organization of Superhumans" (S.O.S.), her code name was "Maidmer".
The above quote from Futurama, of course. The mermaid in question explains that her people do it fish-style.
"I'm not your first time am I? ...I lay my eggs, and then I leave, and you release your fertilizer!"
Fry really makes a break for it after hearing that.
Family Guy has a similar scenario where Lois is rescued by a reverse merman. She declines his invitation to have sex with him, as his upper half is too repulsive. When she mentions she'd rather him be a regular merman, he becomes indignant and invokes this trope. When she can't answer how she would have sex with a merman without a penis, he forces himself on her. She pushes him over and runs away while he flops around like a normal fish.
In an episode of The Adventures of Super Mario Bros. 3, Mario was rescued by a mermaid named Holly Mackerel, who fell in love with him. Complicating matters was the fact that she was a reverse mermaid, and that she saw him in his frog suit which made her think he was a frog.
In the Batman The Animated Series episode "The Laughing Fish", the Joker seems to be flirting with Harley Quinn by asking her to "be [his] little mermaid." He then puts a giant fake fish head over her head, to which Harley responds, "You're really sick, you know that, boss?" Funny this got past the censors.
In the French cartoon Zig And Sharko, Marina's parents are revealed to be a mermaid and a human sailor. Subverted in that this trope isn't called attention to, partly because every episode is a No Dialogue Episode.
Somewhat disturbingly, the Mermaid's real-life counterpart inverts this problem, depending on how you look at it. Female Manatees (aka "Sea Cows"), which many historians believe inspired the Mermaid legend, happen to have a set of reproductive organs that would be very familiar to us humans (especially the Dugong). Simply put, "Tab A" would fit into "Slot B" very comfortably (aside from the obvious Squick factor), moreso in fact than with any other non-human creature on earth. The implications of a bunch of lonely sailors discovering this phenomenon (and subsequently mythologizing it) will have you running for the Brain Bleach, if you weren't already. Of course, it doesn't involve the problem from the male manatees' point of view.
There is a type of fish known as the "fisherman's friend" for broadly similar reasons. It's up to you to decide whether this is better or worse that that activities ascribed to lonely shepherds and the Welsh. Or the New Zealanders, for that matter.
For children born with the Real Life developmental defect of sirenomelia — "mermaid syndrome" — the real "Mermaid Problem" isn't having sex, but surviving longer than a few days without a workable urinary tract or lower intestine. Drastic surgery has prolonged life in milder cases, but reconstructing a sirenomeliac's genitalia isn't much of a priority when planning such life-saving operations.
This apparently wasn't considered a problem in 16th century Scotland. Mary, Queen of Scots, was infamously caricatured as a mermaid, with 'mermaid' being a slangy way to call her a whore.
There is a fringe science hypothesis alternative to the manatee hypothesis above. It describes mermaids as "sea primates" (extinct or near-extinct), possibly cetacean, with vaguely humanoid arms, a dolphin's tail and probably long head hair and pale skin. They are mammals, thus avoiding Mermaid Problem.