Hear me out!!!
Merman AU | Deep-sea merman! Bucky x Marine Ecologist! Reader where Bucky gets caught in your research net.
WC 4.6k
TW mentions of captivity and unethical experimentation, sedation/restraints, institutional dehumanisation, permanent transformation, body horror, mating-bond references, fem!reader
You are a marine biologist aboard a research vessel studying an unmapped trench, expecting your sampling net to come back with cold-water coral, maybe some translucent ocean creature nightmare with too many teeth and no eyes.
Instead, the winch screams like it is being dragged to hell.
The whole deck shudders as the floodlights swing wildly overhead. The crew starts shouting over each other, someone yelling that you must’ve caught a shark, someone else swears it’s simply too heavy to be anything alive. You push through the bodies gathering near the rail, already pulling your gloves on, already thinking like a scientist because that’s what you are trained to do: observe first and panic later.
Then the net breaks the surface…. And there is a man inside it.
Or maybe not exactly a man.
He’s enormous, tangled in the torn mesh, his long dark hair plastered across his face, his chest heaving against the ropes. Eventually, you notice the black scales begin on his ribs and spread down into a tail that thrashes hard enough to dent the deck. You see fins flare along his sides, torn in places like old battle flags. Gills open and close violently beneath his jaw, dragging at air that cannot help him.
A merman. The first recorded one seen in decades. For all you know, he might be the only one left.
…. And his left arm is metal.
It isn’t a prosthetic like ones you have ever seen in medical journals. It’s too intricate, too seamless, as dark plates overlap from shoulder to fingertips, each segment fitted directly into scarred flesh. Thin lines of blue light pulse beneath the metal, matching the frantic glow beneath the scales along his throat.
There are markings etched into the arm: serial numbers and old surgical codes. You finally see a star by a faded symbol you recognise from declassified reports you were never supposed to read.
HYDRA.
Your stomach drops. You’ve heard of them before.
Before their products ever reached the weapons market, HYDRA was infamous for testing them on humanoid and hybrid creatures, from werewolves, to nymphs, anything close enough to human to provide useful data and different enough that the law rarely bothered protecting them.
They had been blacklisted years ago for ethical violations so extensive that most institutions refused even to cite their research.
You had always assumed their subjects were dead. You had never considered that one of them might still be alive.
When the merman wakes, the deck explodes into chaos.
The creature tears through the net with his flesh hand, snarling so low that the sound vibrates through your bones. His metal fingers clamp around the nearest railing to crush it inward. A crewman stumbles too close, and the merman whips around with his teeth bared, not attacking so much as trying to make space, trying to get away from the floodlights.
Someone screams, “Sedate it!”
That’s when Bucky—the name stamped beneath the designation on his metal arm—becomes truly violent.
His pupils contract, and the blue light under his skin flares wild. He throws himself against the deck with such force that two people fall backward. His metal arm snaps out and catches the tranquilliser rifle before the dart can fire, crushing the barrel into useless scrap.
You see the recognition in his eyes now.
He has been on a deck like this before. He must’ve been under lights like these, surrounded by people wearing gloves like yours, holding tools like yours, speaking in calm voices while they decided which part of him to cut into next.
“Wait,” you say, but nobody hears you.
The captain is yelling for the cage while the medic calls for restraints. Your assistant is already asking whether the specimen has lungs or gills, because scientists do that sometimes. They turn the horrors they were committing into knowledge so they don’t have to feel responsible for it.
Specimen.
The merman hears that too, and you knew he’s heard that before.
He makes a broken, furious sound, and you step forward. “Stop calling him that.”
Nobody listens, but you shove the second tranquilliser rifle away.
“I said stop!”
The merman’s head turns toward you.
You expected his face to be blank and animal. Instead, you find blue eyes watching you through the wet strands of his hair.
Eventually, you realise he’s breathing too fast. He’s suffocating.
His gills are collapsing in the open air.
You grab the seawater hose and drag it across the deck, ignoring the captain shouting your name. The merman flinches when you kneel, but metal arm lifts immediately, shaking with the effort of holding himself upright.
You freeze with both hands visible.
“I’m not going to hurt you,” you say, though the words feel pathetic. People like you have absolutely hurt him. People with research grants, with latex gloves, who write reports about how long he survived before his body tried to shut down.
His eyes drop to the knife at your belt.
You unclip it and place it on the deck between you.
Then, just as slowly, you pick it up by the blade and offer him the handle.
His stare flickers, but he takes the knife.
Everyone behind you panics and yet you don’t move
The merman grips the knife in his flesh hand, but he doesn’t use it on you. He only watches as you reach for the hose and turn it toward his gills. Saltwater pours over his throat, and his entire body looks relieved.
“There,” you whisper. “That’s better, isn’t it?”
His metal hand, still buried in the deck, flexes once.
You see the old injuries more clearly now: circular scars along his ribs where monitoring discs must have been attached, thin marks on his throat where restraints rubbed raw, a deep seam at his left shoulder where the metal arm has been fused.
You have spent your entire career loving the ocean. Bucky has spent decades surviving the people who claimed they wanted to understand it.
The net is still wrapped around his tail, tightening each time he thrashes. One of the cords has cut beneath the scales, and dark blood leaks into the water spreading across the deck.
You reach for the rope.
He snarls immediately.
“Okay,” you say. “Okay. I know. I know.”
You do not know, not really, but you can imagine enough to hate yourself for the gloves on your hands.
When you peel them off, he watches every movement.
“I need to get this off you,” you tell him. “That’s all. Then you can go.”
Behind you, your captain says your name in warning.
You don’t look back.
“He goes back in the water,” you say.
“He could’ve killed us!”
You look at them. “He could have already.”
The merman’s eyes stay fixed on you, so you start cutting.
When the final cord snaps loose, he doesn’t flee even when he has every reason to.
Instead, he stares at a cut across your palm where the rope burned you.
“I’m okay,” you say.
He looks at the blood again.
Then, very carefully, he takes your wrist in his metal hand. He lowers his head and presses his mouth to the cut.
A… magical seal, healing it instantly.
The crew goes silent.
Then Bucky releases you and lets you guide him back to the rail.
He drops into the water without another sound. For a moment, the floodlights catch the black fan of his tail beneath the surface. Then he sinks, blue lights fading into the dark.
You think that will be the end of it, but of course it’s not.
The first gift appears at dawn: tuna the size of a child lies on the lower deck, arranged carefully outside the lab door. Nobody knows how it got there. The second gift is a cluster of shells, placed in a neat spiral beneath your window. The third is a jagged piece of deep-sea glass, set directly on top of your notebook.
Your assistant asks if this is some kind of threat.
You look at the offerings and feel something ache inside your chest.“No,” you say. “I think he’s saying thank you.”
Bucky begins following the ship.
At first, he’s only a shadow on the sonar. Then you start seeing him during sample collection, surfacing just far enough away to watch you work. If another crew member approaches the rail, he disappears. If you come alone, he stays.
He never speaks.
You’re not sure he can.
But he understands more than the crew gives him credit for.
When you say his name, he looks up.
You learn it from the old HYDRA file buried in the university research archive. An experimental deep-sea weapons program with the designation: Bucky, The Winter Soldier.
Program abandoned when the labs lost contact with its primary subject in arctic waters.
Primary subject.
You sit in the lab at three in the morning, reading what they did to him with one hand over your mouth.
They replaced the arm he lost with abyssal alloy and pressure-responsive machinery. They studied whether pain, cold and isolation could produce obedience where hypnosis had failed.
The notes describe his distress responses as “aggression.” They describe his attempts to escape as “territorial agitation.” They describe his refusal to eat as “subject resistance.”
Not once does anyone write that he was afraid.
After that, you stop sleeping much.
You find him outside the glass observation window beneath the deck every night, suspended in the black water, one metal palm pressed to the glass. The blue lights beneath his scales pulse whenever he sees you.
The first time you press your hand to the other side, he goes perfectly still.
The metal fingers spread against yours through the glass.
You teach him to sign through the observation window. Simple things first, like food, safe, stay, friend, no hurt.
Bucky learns slowly, not because he is unintelligent, but because nobody has taught him anything with patience in a very long time. He expects corrections to be violent, to the point where the first time he signs something wrong, he jerks back from the glass.
Your face falls.
“No,” you say quickly, pressing both hands up. “No, Bucky. It’s okay. You’re okay.”
He watches your mouth form the words as you sign, Okay.
His throat glows faintly as he signs it back. Okay.
After that, he begins appearing with less fear.
He brings you fish you cannot possibly eat and looks offended when you try to explain refrigeration. He leaves coral samples from depths your equipment cannot reach, then hovers proudly outside the window while you label them. He scares off a shark during one of your dives and then spends the next hour sulking because you scold him for ramming the cage.
“You cannot solve every problem with violence,” you tell him through the speaker.
Bucky’s lights flicker as he signs, Worked.
You try very hard not to laugh.
He’s sweet, even though there’s nothing harmless about the creature who once dragged an entire illegal HYDRA vessel into the trench and left no survivors. But around you, Bucky becomes almost shy. He ducks his head when you praise him. He flinches away from sudden movement, then pretends he didn’t.
The crew still calls him dangerous, but you call him misunderstood.
Then he proves both things can be true.
A private salvage vessel starts following your research ship. At first, the captain says it is probably nothing. Then you catch encrypted transmissions on the radio and realise they are not interested in your samples.
They are interested in him.
Bucky knows before you do.
The night the other ship comes too close, you are in the observation room when he appears outside the glass. His lights are dimmed to near-black. His metal hand is flexing slowly, each plate sliding into place like a weapon remembering itself.
“Bucky?” you whisper.
He looks at you, but frowns before he goes.
The salvage ship loses power three minutes later.
By dawn, it is drifting empty, its hull ripped open below the waterline, its weapons systems crushed. Nobody dies because you once signed no kill to Bucky until he understood you.
But every single person aboard is found locked in the emergency raft, refusing to speak.
Your crew quits within the week. They call him a monster.
You stand at the rail while the replacement vessel takes them back to shore, watching Bucky circle beneath the water in restless patterns. He hasn’t come close since the salvage ship. You think he believes he has frightened you.
Eventually, sick of not seeing him, you climb into the dive cage before anyone can stop you.
Bucky appears immediately, slamming his metal hand around the bars as if he can physically drag you back to safety.
Not scared, you signed.
He bares his teeth.
Liar, his expression says.
You were scared for him. There was a difference.
You reach through the bars, slowly, slowly, until your bare hand rests against his cheek.
He closes his eyes.
After that, everyone knows there is no separating you from him.
When the expedition ends, he whines because he cannot come with you.
The closer your ship gets to land, the worse he becomes. He follows too high in the water for too many days, ignoring the pressure sickness that makes his movements unsteady. Blood leaks from his gills in thin dark threads as his lights flicker unevenly. Every time you beg him to dive, he signs the same thing with increasing desperation.
You leave.
“I have to go back,” you say from the deck.
No.
“Bucky—”
You leave. They take.
You understand then, that he doesn’t think you are returning to your life.
He thinks the surface is another lab, another cage that takes things and doesn’t give them back.
You try to explain leases and universities and funding boards and family, but none of those words mean anything to him, not the way he has started sleeping under the hull because it is the closest he can get to sharing a room with you, even though being so close to the surface for so long makes him feel sick.
On the last night, he surfaces beside the ship in a storm.
He’s shaking so violently that he could barely keep his metal hand curled around the ladder. The waves kept dragging him away from the hull and throwing him back against it, but he refused to let go.
You climb down before anyone could stop you, rain soaking through your shirt and stinging your eyes. By the time you reached the lowest rung, the boat was pitching so violently that you could no longer keep your footing.
“Bucky, you’re killing yourself.”
Another wave struck the hull. Your hands slipped from the ladder, and Bucky caught you instantly, one arm locking around your back as you wrapped your legs around his waist to keep from being swept away.
You gasped.
You had touched him through the observation glass. You had rested your hands against his face while wearing a diving suit and thick protective gloves. But this was the first time there was nothing between you.
Your soaked shirt had ridden up around your waist, leaving your bare thighs pressed against the cold scales covering his hips. Your arms were looped around his shoulders, your chest flush against his. His skin was freezing beneath your palms, but the strange blue light beneath his ribs pulsed warmly against your body.
Bucky went completely still.
Even half-suffocated and trembling from the pressure sickness, he stared at the place where your skin touched his as though he had never imagined you could feel like this. His metal hand spread carefully across your back, while his flesh hand hung on to the ladder, now both of you half-submerged.
Then the boat rocked again, forcing you tighter around him.
His eyes lifted to yours.
“You can’t stay this close to the surface,” you whispered, tightening your arms around his neck. “Please. You have to go back down.”
His forehead dropped against yours.
His eyes are too bright in the dark as he carefully lifts one hand, encouraging you to hold on tighter to him. Stay.
“I can’t live down there.”
For a moment, he looks away.
Then he brings your hand to the centre of his chest, where the blue light glows beneath the scales over his heart.
You have seen that glow before in fragments of the HYDRA file. The researchers called it an adaptive organ. They believed it was the source of his survival in the trench, it was a biological engine of pressure, salt and light.
They also believed that his kind can also share it in a magical ritual, when they mate with non-merpeople.
Bucky signs slowly, each movement careful. Can change.
You stare at him.
He signs again. Depths. Like me.
Your breath catches.
He doesn’t look triumphant. He looks miserable, because this isn’t a romantic offer to him, at least not entirely. It is a wound he is willing to reopen for you, the same biology HYDRA violated.
You, a human.
He seemed to want you to know that The Depths can make a mate, by sharing the living light under the heart, by letting it enter another body and teach it how to survive the abyss.
He can give you eyes that can see without sun and blood that does not rupture under pressure.
But it’s permanent.
Bucky makes sure you understand that.
He signs it again and again, increasingly frantic. No force. Your choice. No hurt if no. I wait. I wait.
The fact that he thinks he has to promise not to force you breaks your heart.
You press your forehead to his. “Oh, Bucky.”
You kiss his salted nose goodbye and tell him you have to go back to land.
And when you do, you try to be normal.
You teach lectures about deep-sea ecosystems while thinking about the man beneath the water. You analyse the coral samples he brought you and find structures no human instrument has ever recorded.
The university asks you to present your findings, and military contractors attend the lecture.
One of them asks whether the organism displayed signs of trainability.
You stop speaking.
The organism?
You look at the room full of people like you. Humans, who are now all beginning to look like monsters.
And finally, you understand why Bucky looked at your gloves and saw a weapon.
You leave before they can ask more questions.
Two weeks later, you return to the trench alone.
Bucky is waiting.
The moment your boat reaches the coordinates, the water beside you lights blue. He surfaces slowly. His hair is longer, and there are new scars across his metal arm. He looks more tired, but so relieved. You climb down the ladder.
He catches you carefully, metal hand braced beneath your back, tail keeping you both steady in the dark water.
“I couldn’t stay up there,” you tell him.
His eyes search your face. You place your hand over the light beneath his heart.
“I want to come home with you.”
His grip tightens for half a second, then loosens immediately, terrified of holding too hard.
He signs with shaking hands. Sure?
“Yes.”
Forever?
“Forever.”
Bucky can only smile.
You kiss his forehead, because he deserves gentleness in every language you have.
The transformation hurts, but not the way HYDRA hurt him.
Bucky doesn’t strap you down or cut you open. He holds you against his chest in the black water and presses his glowing heart to yours. As he sinks with you and begins the mating ritual, light enters you warmly.
Your lungs burn first.
You panic when water rushes into your mouth, when your body tries to reject it, when your legs twist with pain and your bones feel briefly, horribly, breakable.
Bucky holds on, anchoring you.
His metal hand cradles the back of your head, stroking again and again, as the ocean opens inside you.
Eventually, after a long and awful process of two days and two nights, you breathe water.
It fills you, and your body doesn’t die. It changes.
When you gather your thoughts, you’re floating in Bucky’s arms beneath the surface. Your skin glows with the same blue light as his as black scales shimmer along your hips and ribs. Your legs are gone, remade into a tail smaller than his but strong enough to move with the current. When you lift your hands, you find webbing between your fingers and tiny claws where your nails used to be.
Bucky hovers in front of you, looking wrecked as his dark blue lights flicker in a ramble.
For one horrifying moment, you think something has gone wrong. Then you realise he’s waiting for you to hate him for changing you.
You swim to him clumsily.
Bucky catches you before you can drift sideways. Even now, in the middle of his own terror, his first instinct is to keep you from falling in a world where falling is no longer possible.
You take his face in both hands.
Your lights flicker uncertainly beneath your skin. You don’t know what language you’re speaking yet, but you know this much.
Safe. you sign. Home.
His whole body lights up, every marking beneath his scales flares bright enough to turn the water around you blue.
He touches his forehead to yours.
For the first time since you met him, Bucky looks loved.
Then he takes you down into the Depths, past the last reach of moonlight, past cliffs of glowing coral and forests of black anemones, into a trench-city built from volcanic stone and old shipwreck bones. Its towers are shaped from old shipwrecks and the bones of things too large to imagine alive. Windows glow from within the rock, ropes of pearls and polished glass hang between structures, moving gently in the current, and gardens of luminous coral spread across the rooftops.
You barely have time to take it in before the merpeople begin emerging.
You realise, amazed, that he was never some lone monster wandering beneath your ship.
He was one of theirs.
They come from cave mouths, from archways carved into the stone, from the dark spaces between wrecked ships. Some resemble Bucky, with long tails covered in black or silver scales and fins running along their spines. Others have translucent skin through which their glowing organs are visible. Some have delicate fins like veils, while others bear teeth larger than Bucky’s and claws designed for splitting shells.
Many of them carry evidence of the surface world, with old hook scars split their mouths and shoulders, pieces of fishing line remain braided around wrists or woven deliberately into jewellery, transforming from instruments of capture into trophies of survival. One older merwoman has the rusted curve of a fishing hook hanging from her necklace, and another has a section of net wrapped around the handle of her spear.
Your stomach knots.
Of course they know what humans have done to the ocean. Of course they know what scientists did to Bucky. You expect hatred when they see your face, some instinctive awareness that you were born among the people who hunted them, so you press closer to Bucky.
He notices immediately. His arm tightens around you as the lights beneath his scales begin to darken in warning.
Then one of the older merwomen swims forward.
Bucky bares his teeth.
She ignores him completely.
Instead, she takes your face between her hands and studies you with large, black eyes. Her thumbs brush across the new gills beneath your jaw. You brace yourself for anger.
She presses her forehead warmly against yours.
The light beneath her skin blooms gold as if to say, welcome home.
You don’t know what it means yet. By now, you understand that they communicate in colours, but you do not understand any of the colours rippling across her body, or the waves of blue, silver, violet, and green that answer from the merpeople gathered around you. Their language passes, each body responding to the next until the entire trench glows.
For one frightened moment, you wonder whether they are warning one another about you.
Then the elder strokes her thumbs beneath your new gills.
Another fastens a string of polished shells around your neck, and a younger one touches your tail with careful fascination. The smaller merpeople begin circling you excitedly, darting between Bucky’s fins and fleeing playfully whenever he turns toward them.
Beside you, brilliant blue light bursts beneath every one of his scales, illuminating the metal plates of his arm and turning the water around him bright enough for you to see the pride on his face.
The younger merpeople flash rapid patterns in his direction. Whatever they are saying makes Bucky’s fins flatten with embarrassment before he sends them scattering into the coral with one sweep of his tail.
They return almost immediately, glowing with what you are beginning to suspect is laughter.
You realise then that they already know you, because Bucky has been telling them about you for months.
You cannot understand the exact stories moving across their bodies, but you recognise yourself in their gestures.
—
One year later, the language of the Depths moves beneath your skin as naturally as English.
It no longer looks like random colours or disconnected flashes, and each brightness carries tone, each rhythm creates a word.
Sometimes you wake with Bucky curled around you, his tail tangled securely with yours and his metal palm resting over the light beneath your ribs. Dark blue moves across his chest before you have even opened your eyes.
You are awake. Did you sleep well?
You nuzzle into his neck, allowing your baby blue light to answer against his skin. I always sleep well when you are holding me.
Bucky’s arms tighten around you. I missed you while you were asleep.
Your lights flicker with affection.
One year in, you continue your research, technically.
You have filled your cave with stone tablets as journals, labelled samples, pieces of shed scales, and specimens collected from regions no human vessel could ever reach. The Depths have become your field site, an entire ecosystem you’re slowly learning without harming anything inside it.
Bucky appoints himself your research assistant.
Unfortunately, he has little understanding of reasonable sample sizes.
One morning, he proudly deposits an enormous bioluminescent eel across your stone worktable. It is nearly twice your length, still glowing irritably as it coils around your equipment.
You stare at it.
You needed one scale.
Hopeful blue light pulses beneath Bucky’s throat. You asked me to find a specimen with silver markings, so I brought you the best one.
Your lights shimmer with exhausted affection. I only needed a single scale.
His head tilts. This one has thousands.
You smile.
He hovers beside his offering, watching you with those wide eyes. A smaller pulse of uncertain blue moves beneath his ribs. Did I do well?
You look at him, the creature humans had called violent, untrainable, and monstrous. You swim forward and take his face between your hands.
Light blue spreads from your palms and pours across his skin.
You did very well. You are the finest research assistant in the entire ocean.
Bucky glows so brightly that the eel startles.
You kiss the corner of his mouth, then nuzzle your face against his. He ducks his head shyly, but there is nowhere for him to hide the brilliant blue racing through every marking on his body.
Above you, the surface world continues searching for its missing scientist and its missing specimen, never understanding that neither of you is lost.
You are in the Depths, surrounded by people who call him family and who welcomed you as one of their own.
You are home.
—
this is a queued post! I am currently either getting married or on my honeymoon! enjoy! 💕
