I wanted to ramble about the merfolk sign language in my Guppy Diplomacy fic! I had a blast thinking about the linguistics of it and considering shark biology, underwater acoustics, real-world diving culture, and one very specific jewelry-making scene from the game.
Start With the Animal
The argument for merfolk sign language begins and ends with one fact: sharks don't vocalize.
Sharks are one of the only major vertebrate groups that produce essentially no intentional sound. Dolphins have their whistles. Whales sing across ocean basins. Even fish grunt and pop using their swim bladders. Sharks are silent (apart from a few specific species that click their teeth or bark air).
What they do instead is communicate with their bodies. A shark's communicative toolkit is entirely somatic (gah I have to talk about the idea of magic/spellcasting as an evolutionary pressure in this fantasy world someday. I mean, somatic components? hellooo!). The arched back signals aggression, the lowered pectoral fins indicate submission, the exaggerated swimming patterns establish territorial boundaries, the jaw gape serves as a threat display. Their social information travels through posture, movement, and proximity—not sound.
They also receive information through channels humans don't have. The lateral line system detects pressure changes in the water, letting a shark sense movement and current shifts. The ampullae of Lorenzini—the electroreceptive organs—detect the faint bioelectric fields produced by other living things. A shark senses your heartbeat before it sees you.
So the baseline is an animal that talks with its body, listens with its skin, and has never* made a sound on purpose in its entire evolutionary history. That's the shark merfolk biological inheritance. That's what I want the worldbuilding to grow from.
Scale It Up to a Person
Finn isn't a shark. He's a "sexy, sophisticated shark-man." He runs a business, negotiates deals, delivers motivational pep talks, watches reality TV, makes fish puns, and has an existential crisis about his life's purpose. He's a sapient being, not an animal.
A real shark doesn't need a complex language. Its communicative needs—threat, territory, mating interest—are well served by body posture. But Finn needs to argue about contract terms. He needs to express doubt, irony, tenderness, regret. He needs the full bandwidth of a complex language.
When you take a creature whose entire communicative heritage is gestural and somatic, give it human intelligence and the need for abstract expression, and ask what kind of language it develops… the answer isn't acoustic. It's physical. Body language scales up into a full language the same way human vocalizations scaled up from primate calls into syntax and grammar. The medium was already there. The cognitive complexity just gave it somewhere to grow.
What the Game Already Implies
The source material never names a merfolk sign language. But it does something interesting: it consistently characterizes Finn as someone who deploys communication outside of speech.
Finn explains that sharks have electroreception, the ability to pick up information from bioelectric fields. He uses this to read people's emotional states, to sense whether he scares someone "a little… or a lot." This isn't a party trick. It's a fundamental part of how he experiences other people.
At the carnival, the captions describe his body language with unusual precision: the "casual lean" against the stall that's actually blocking the worker's only exit, the way he leans in close and speaks low. Sylvia can't hear what he says. She doesn't need to. His posture tells the whole story. The game is showing us a character who communicates as much through his body as through his words—and in this scene, his body is doing the real work.
In one friend variant of the aquarium hangout, a shark emerges from the gloom and its dark eyes fix on Finn. The dialogue reads: "Finn says nothing. / … / Welp! Ready to head back when you are." That loaded silence—the ellipsis, the abrupt redirect—suggests something passed between them that Sylvia couldn't perceive. It's a moment of recognition or communion that exists on a channel she doesn't have access to. (It's so mysterious! I wanna know what's going on there!)
Then there's the jewelry-making hangout. The captions note that "his large hands are surprisingly gentle as he strings beads and teeth in an alternating pattern." Finn has freaking meathook hands, and the game goes out of its way to tell you they're capable of delicate, precise, dexterous work (on top of the "surprise! scary tough guy shark makes jewelry!" flavor). He threads tiny beads and shark teeth into intricate patterns. Those are hands that can sign.
It's easy to overlook because the scene is about the emotional gesture. He's making an anklet with his grandmother's teeth (hrrk). But the physical observation matters! A gestural language requires fine motor precision, and Finn has it in spades.
The Physics
My core argument for merfolk sign language is biological and cultural: sharks communicate with their bodies, so shark-people would develop a body-based language even though they're clearly capable of (eloquent, menacing) speech. The underwater acoustics don't have to do the heavy lifting. But they do validate the choice, and they're interesting in their own right.
Spoken language underwater faces several physics problems. Sound couples poorly from air to water. When you vocalize, the compression waves in the air pocket around your mouth mostly reflect off the air-water boundary instead of propagating outward. Underwater, bone conduction replaces ear-based hearing, and listening with your skull nixes your ability to localize where a sound is coming from. Water's nonlinear acoustic properties distort frequency content, meaning what you produce and what gets received aren't the same. At depth, the effects compound, warped by pressure.
None of this means a fantasy species couldn't evolve underwater vocalization. Dolphins and whales obviously did. But it's worth noting that their vocalizations are produced through specialized biological mechanisms completely unlike a human larynx. They evolved purpose-built hardware. A shark-person underwater is more likely to work with the hardware the shark provides: body, fins, electroreception.
Meanwhile, real-world divers use hand signals as their primary underwater communication method. And here's the detail that sealed it for me: deaf divers who use full sign language hold conversations underwater, discussing what they're seeing in real time, while hearing divers are limited to a few dozen tactical signals and have to save everything else for the surface. A full gestural language doesn't just work underwater, it's better underwater than speech. The concept isn't speculative.
Designing the Language
Once the premise is established—shark-people communicate with their bodies, the physics supports it, the game already implies it if you squint—the design question becomes: what does the language actually look like? (lotta punctuation stacked in that sentence, don't look too closely, lol)
Human sign languages run on two primary channels: hand shape and facial expression. ASL grammar lives in the eyebrows, the mouth, the tilt of the head. These are elegant, fully expressive languages. But they're built for a human body.
Merfolk have more channels available. They have hands, yes, but they also have a tail, fins, three-dimensional positioning between you and your conversation partner, and the capacity for humming and clicking. A language designed for these people would use all of it. So the merfolk sign language in Guppy Diplomacy works on a layered system: hands carry vocabulary, tail and fins carry grammar and tone, and humming and clicking provide emotional color underneath.
The tail-grammar is the piece I'm most interested in, because it's what makes this a merfolk language rather than a relocated human sign language. Grammar encoded in a body part that humans don't have means this system could only belong to these people.
The practical design details matter too. Signs need to work in murky/dark water, which means they tend toward clear, compact gestures—"flat palm turning outward, fingers together" rather than intricate finger-spelling. Touch-based signing pressed to the body extends the system to zero-visibility environments, which is critical for the deep ocean where light doesn't penetrate. And electroreception solves the attention problem. With diving, you have to physically tap your buddy to say "look at me." A shark can sense that someone is trying to communicate before they even see the signs.
Plenty of Fish in the Sea
Ultimately, this is a shark merfolk language. The body shapes the language, and a different body would shape a different one. Cetacean merfolk would have something acoustically rich that shark merfolk can't reproduce (whale folk telegram service? they'll bellow your message to their hub location and pass it on!). A cephalopod civilization would have—I don't even know. Extremely elaborate, multi-arm signs? Chromatic grammar? Mantis shrimp having a visual dimension that other merfolk literally can't perceive? Eel merfolk communicating via sinuous interpretive dance? lol
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