Seashell Bra

Mimic: By th' way, nice seashells.
Siren: They're just B-shells, but thanks.

—Rusty and Co.

For some reason, mermaids feel compelled to fashion bras for themselves out of seashells, but they never care about making any other kinds of clothing. Although this provides Fan Service and Fetish Fuel, it was originally created for the reverse reason. Logically (as in, using the same logic that would apply to humans *cough*), a mermaid would be an Innocent Fanservice Girl, but we can't have that in a kids' movie, can we?

Since, by definition, a mermaid is a fish from the waist down, it's only her top half that must be covered, and often Godiva Hair just won't do. Thus, the Seashell Bra was invented to preserve the modesty of female merfolk everywhere. You probably think this originated with Disney's The Little Mermaid, but Disney was using it all the way back in 1953 to depict the mermaids in Peter Pan (although they looked distinctly less bra-like in those days). Expect bras to also be made out of other marine trappings, such as starfish (though glomping a living creature to one's own body doesn't sound like a good idea in hindsight—especially if it ejects its stomach and starts digesting your nipples) or perhaps seaweed or other vague underwater flora, though the shells are often depicted as somehow clinging magically without any accompanying harnesses or straps, seaweed or otherwise. Also, the bra will almost always be made from scallop shells rather than clam, oyster, conch, or nautilus shells (not unless they are hollowed out).

Of course, not every culture shares the notion that the sight of female nipples somehow corrupts children, hence the Japanese animated version of the classic Hans Christian Anderson tale features bare-chested mermaids. It's also possible to avert the whole issue entirely by making the "fish-parts" ascend all the way to the bustline and mimic a one-piece swimsuit or a strapless dress as Disney did with Ursula.

Provides additional proof (if any were necessary) that most writers are either male or sufficiently unimaginative to be unable to envision just how uncomfortable such attire would be. Or both.

A Sub-Trope of Improvised Clothes.

Compare Fur Bikini, Bare Your Midriff.

Examples of Seashell Bra include:

Anime and Manga

Mermaid Melody Pichi Pichi Pitch takes Color Coded for Your Convenience to the extreme, so of course, mermaids always have seashell bras that match their tails. They do, however, wear other kinds of clothing at times, such as skirts, but these are purely ceremonial, and usually a mermaid is depicted in only the shell bra.
Somewhat related example from Love Hina: Having found an oasis after getting lost in the middle of the desert, Naru makes improvised bikinis for herself and Nyamo (and then later, Shinobu, whose clothes had been destroyed by one of Motoko's sword techniques) out of giant leaves. Not explained is why Kitsune put Motoko in the exact same type of outfit when they were still in civilization. This also allows a "trap" that unleashes caterpillars to be somewhat threatening, in a way.
One Piece:
- Sanji has some Imagine Spots of gorgeous sirens wearing Seashell Bras, but it's averted when they meet an actual mermaid, who wears a T-shirt. Also, Princess Shirahoshi often wears a blue blouse or golden bikini top, and while both are rather daring, she avoids the seashell style.
- Gonk mermaid Kokoro, on the other hand, does wear a Seashell Bra. One person asked if they really were seashells -- Oda's initial response: "Does 'I don't want to think about it' count as an answer?"
- Also, a few of the younger mermaids, especially the dancers, do. It's just not normal mermaid clothing.
In Dragon Ball too, when Goku brings a mermaid to Kame-Sennin, she wears a T-shirt (but doesn't look wet). Wouldn't the wet T-shirt potential open a whole new can of Fan Service?
Yu-Gi-Oh Red Archery Girl and Toon Mermaid have these, although the English version of Red Archery Girl gives her an extra bit of cloth to wear under her bra.
Shows up in a dream/hallucinatory sequence in Kaleido Star. Dream Mermaid!Sora gets this to a T (well, the current performance is The Little Mermaid). She also seems a bit more... ample in this sequence.
San the mermaid Yakuza Princess from Seto no Hanayome wears this in her first appearance.
Lampshaded in Kamichama Karin, in which Kazune points out, "How do those stay on?"
Misty from Pokémon wears a pink one whenever she plays the Magical Mermaid.
Seria from Rave Master has an interesting variation: she wears a bikini top... with a shell motif.
Galaxy Angel 3rd (manga) has the girls turned into mermaids at one point, complete with Seashell Bra.
In an episode of the Ranma ½ anime, during an Eyecatch showing Akane as a mermaid, she wears a pink Seashell Bra (can be seen on this Caption Humor site).
Guu from Haré+Guu wears one during the Beach Episode.
There is an episode of Maicching Machiko-sensei in which a male teacher imagines the main character as a mermaid with the obligatory seashell bikini.
In book 10 of the Urusei Yatsura manga, both Lum and Shinobu are disguised as mermaids and wear seashell bras.
Towards the end of the Panty & Stocking with Garterbelt episode "If Angels Wore Swimsuits", Panty's bikini becomes ripped by a Giant Crab Ghost and falls off. Unfortunately for all the onlooking male beach-goers, two Mini-Starfish Ghosts and a Sea Anemone Ghost jump up and latch onto her in just the right places.
Averted in Marine Boy, in which his mermaid friend Neptina uses Godiva Hair.
In K-On! high school, Sawako-sensei offers Nao a seashell bra bikini when they're at the beach house, but she refuses to wear it.

Comic Books

Issues drawn be Adam Hughes (and some other artists about the same time) of the Legion of Super-Heroes and Legionnaires comics have had seashell bras (and bottoms) indicating nudity (simply assume they aren't there, then the characters' reactions make sense).
In the comic book Boneyard, one of the local Universal-like monsters is a female version of the Gill Man from Creature from the Black Lagoon. Nessie, as she's called, covers her monstrous Non-Mammal Mammaries with tiny seashell pasties. She also wears actual fishnet stockings.
The Blue, a fictional underwater race from the Fathom comics, make all clothing up to full body armor using bits of underwater flora and fauna. Despite them also having far more advanced technology than surface dwellers.
The mermaid encountered by Superboy (a sort of next-gen Lori Lemaris) in Adventure Comics 80 Page Giant. Post-Crisis Lori herself sometimes wears a normal bikini top, but more often has Godiva Hair. Pre Crisis Lori wore a blouse at all times.
When Namorita, Distaff Counterpart of the Sub-Mariner, turned blue and changed her name to Kymera, she added a net skirt and one seashell "cup" to her bikinilike outfit.

Film

In Madagascar, when Gloria washes ashore, she has two starfish on her chest (beside the fact that there is nothing there to cover) and a crab on her crotch (make of that what you will).
Interesting variation in Disney's Fantasia, with female centaurs wearing tops and necklaces made out of flowers that appear to be motivated more by the Theiss Titillation Theory than actual modesty, even though the centauresses otherwise exhibit Barbie Doll Anatomy...
The Little Mermaid of course, though Ariel is unusual in that her top has a strap. Ursula has octopus flesh go over her chest instead, though. Some people think the overweight mermaid illusion seen in Ursula's song has her chest covered by scales, but looking closely reveals she too wears a Seashell Bra.
Before The Little Mermaid there was Peter Pan, whose mermaids sported a motley mix of Godiva Hair, seashell bras, and starfish pasties.
The title character of Paprika wears one while briefly in mermaid form.
Averted in the movie Splash where Madison's obvious nudity is (mostly) disguised by Godiva Hair, careful framing and artful camera angles.
Also averted in Pirates of the Caribbean: On Stranger Tides, when Godiva Hair is used by the dangerous man eater mermaids.

Literature

Averted in John Ringo's Council Wars book Emerald Sea: The Merfolk are naked solely because the ocean doesn't provide anything they can use to make clothing and it's explicitly mentioned that this forces them to burn extra calories to stay warm. One mermaid even complains about Male Gaze and expresses wistful envy for the heroine's bikini top (the bottom wouldn't do her any good.) The heroes promptly promote fabrics to the top of their list of proposed trade goods.
Not a mermaid, but in I was a Teenage T. rex, one of the leads becomes a Leptoceratops, and despite not having anything to cover she didn't feel comfortable going naked. She tries sticking leaves on with sap—they fall off. She ends up getting the quetzalcoatl to make a top-and-bottom out of seashells with holes poked in them and strung together by vines; these last a little longer.
Averted in Donna Jo Napoli's novel Sirena: Mermaids don't wear any clothes. Oddly, the humanoid Sea Nymphs do, and not just bikinis but elegant, flowing gowns. The mermaid heroine and her sisters don't understand this strange concept they've heard of among gods and humans called "modesty".
In an early scene from Robert A. Heinlein's I Will Fear No Evil, Eunice and her husband are discussing the mermaid body paint she'll be wearing to work the next day. (Office dress codes having gotten really casual in the future.) They make a bet over whether a literally painted-on Seashell Bra will fool her employer into thinking it's real. (It does.)

Live-Action TV

Faerie Tale Theatre's adaptation of The Little Mermaid used an interesting scheme: if a mermaid was out of the water (like sitting on the beach for instance), she had Godiva Hair, whereas while she was underwater (and thus her hair would be floating around and unable to do its Godiva Hair function), she had a Seashell Bra, or more specifially, two scallop shells stuck to her chest with no straps whatever, not even one connecting the two shells.

Music

In the official Alestorm video for the song Shit Boat (No Fans), an animated Christopher Bowes is, for a short time, naked except gloves and having a sea shell over his genitals. However considering what is covering his junk changes without explanation, there is a solid chance the sea shell is a Gag Censor.

Recorded and Stand Up Comedy

Old joke: Why did the mermaid wear seashells? 'Cause she outgrew her B-shells.
Featured in a joke by comedian Ron White:

Ron White: I didn't know this. If you find a girl wearing a seashell bra, and you pick her up, and put her to your ear, you can hear her scream. I thought I'd hear the ocean, but not over that bitch.

Tabletop Games

Averted in Old World of Darkness game Changeling: The Dreaming. Female Merfolk (outlined in Blood-Dimmed Tides) are just as topless as the men most of the time.
Averted in Dungeons and Dragons (3.5 ed.). The mermaid in the Monster Manual has Godiva Hair. The 2nd edition Monstrous Manual, however, depicts an aquatic elf in a Seashell Bra and a mermaid in something a little scalier.
In Yu-Gi-Oh!, Red Archery Girl had this in the OCG, although the TCG made it look like a halter top with shells on it.

Video Games

The Neireid queen in Soul Nomad and The World Eaters.
The Pokémon Gorebyss appears to have one even though it isn't remotely humanoid.
Averted in Neopets. Water faeries (mermaids) have scaly coverings that extend to cover the upper body rather than seashell lingerie
Averted in The Goonies II, where Annie has no covering (or definition) at all. See here
Averted in Harvest Moon DS. Leia wears a blue halter top, although she does wear seashells in her hair.
Used in King's Quest II; Graham has to charm a merwoman that's sunning herself on Kolyma's shore.
Helena's Godfather bikini (and its pallette swaps) from Dead or Alive Xtreme 2 is this... though it's so small it's more like Seashell pasties...

Web Animation

Subverted in somewhat disturbing fashion by guro manga artist Shintaro Kago. Watch at your own risk.
Homestar Runner: In one Strong Bad E-mail, SB nicknames a cardboard cutout of a mermaid Princess Shellbra.

Web Comics

The Mermaids in the Monster-only world of Monsterful stick to this trope as well, featuring Starfish and Shellfish bras.
Parodied in a recent^[when?] Questionable Content strip. In a panel clearly labeled as shameless fanservice, Faye is clutching a pair of seashells to her breasts, claiming that her bikini top was swept away.
The mermaids in Kate Beaton's Hark! A Vagrant wear these. Aye, they be a sexy omen.
The page quote is from this strip of Rusty and Co. The "Monster Guide" page on the same site also mentions this about the siren:

Don't ask what keeps the seashells on.

The comic Tsubomi Blue Dreams has the protagonist wearing starfish as a bra. Say it with me now: "OOOOOOOOOOOOOOWWWW!!"
In Yet Another Fantasy Gamer Comic, Mi, the gen familiar of Sha'ir Waleed (likely a maridan from the Elemental Plane of Water) is wearing a seashell bra. And bottom. And headband ornament.

Web Original

Asked about here. Considers the theory if females are born with it, and if not, how they measure cup/clam size. Also deconstructs it by asking why Mermaids wearing dead clams as their bras if they are friends with fish.

Western Animation

AndrAIa from ReBoot is a bit more modest than the typical mermaid (at least at first), wearing one gigantic scallop shell that covers her entire torso. After the Time Skip she gets a more Stripperific outfit with a bra that appears to be made out of some kind of fish skin.
The Family Guy episode "Lois Kills Stewie" has a seashell speedo on a reverse mer-man: he's a fish on the top, and a man on the bottom.
Subverted during both episodes of The Backyardigans that have mermaids, "Viking Voyage" and "Into the Deep". During those episodes, Tasha and Uniqua are mermaids (well, Uniqua just in the latter), but their tails cover the entire torso, so they don't need seashell bras.
Fan Disservice/Parody: SpongeBob SquarePants' superhero parody Mermaid Man has a pair of shells over his completely round torso, despite the Aquaman-esque orange scale shirt he also wears.
- Averted in The Movie. Mindy wears a normal blouse.
On Jimmy Two-Shoes, Heloise wears one during a Dream Sequence where she's a mermaid.
In Regular Show, during Mordecai's dream sequence in the episode "Meat Your Maker", he envisions Margaret wearing one that just barely covers her lady pecs.
Angel in Fish Police.
In an episode of Futurama, Fry's mermaid fiancée wears a stereotypical clam bra (even though her father and other citizens of Atlanta wear full clothes). She strips it off and it floats away right as Fry learns about the Mermaid Problem.

Real Life

Many companies that make swimwear have marketed actual bikinis with tops like this. Such swimwear tends to be very expensive, and usually isn't the type you should actually swim in.