Unmarked spoilers are ahead, per Spoilers Off rule.
Eating mermaid's flesh to attain immortality has quite an allure to it. But it's not worth it.
- The game of russian roulette when ingesting the flesh of a mermaid.
- Promptly die after eating the flesh, considering it's extremely poisonous.
- Get turned into a grotesque, deformed monster that lusts for bloodshed and may or may not retain a conscience.
- Fail to obtain immortality, but gain other advantages — like the outward appearance not aging, yet the internal organs continuing to age like that of a regular human; becoming a not-decomposing corpse, though there are implications the person might have been alive, but completely paralyzed the entire time.
- End up being one of the lucky ones that actually gained immortality. But now they cannot live a peaceful life anywhere because the lack of aging will raise questions; are not immune to pain and can die, although they will revive after twelve hours; and will outlive anyone they ever come to like and love.
- Unlike most million-to-one chances, achieving immortality really is a rare occurence. Yuuta didn't meet another person that survived ingesting mermaid's flesh for 500 years.
- Mermaids that choose to live on land need to feed on a mermaid living in the water, so they can give birth.
- Mermaids Do Not Smile portrays well how creepy the entire settlement was. They kept Mana locked up from a young age, with her ankles being stuck in a device to keep her from walking; have several women all sharing the same face; and their first action upon an outsider like Yuuta arriving is to stab him with three spears.
- Then there’s the simple fact that the mermaid village had been stealing girl babies for a very long time, and raising the girls with the express purpose of eating them. Most of the girls succumbed to mermaid’s flesh, and those that turned into Lost Souls were discarded. They clearly had no love or affection for these poor girls, and only saw them as meat. Who knows how many girls were killed trying to preserve their youth.
- Mermaid's Forest has a mermaid impaled on a wooden stake inside a cave. She was alive the entire time.
- Mermaid's Promise also has the ash of a mermaid and its surrounding legend. The ash will resurrect a person, but they are implied to be soulless and bloodthirsty. As shown with a resurrected Nae, who is generally a sweet and kind woman, but triggering the memories of her death — usually involving loud noises, like alarms or the cawing of crows — results in her causing harm to anyone around her. Except for Yuuta. Then there’s the fact that Nae's fiancé beat her to death because she loved Yuuta and not him.
- Masato from Mermaid's Scar is nightmare fuel incarnate. The biggest thing their appearance entailed was getting Yuuta the closest to actually dying, by almost chopping his head off with an axe. He also kept feeding mermaid's flesh to women he has chosen to become his 'mother' for a while. And one of his victims was Yukie, a woman that was giving up her job as his caretaker to get married.
- Adding to the Nightmare Fuel of Masato is that his death via car accident is ambiguous because his body wasn’t found in the wreckage. Meaning there’s a high probability that Masato is still alive and is still preying on unsuspecting people.
- The Bone Princess has the hangon ritual. This ritual involves placing the bones of a dead person and organs like the liver between two mats, basically resurrecting the person. Except they end up requiring to eat the organs of other living things to continue to walk around.
- The old man wanting to cut out Yuuta's liver while he's still conscious because he'll just regrow another liver as an immortal.
- Mermaid's Gaze having Shingo not only stealing his sister's eye to replace his missing one, but he keeps getting haunted by the sight of himself taking it. When he realizes that those images will keep appearing even after having killed his sister, he chooses to commit suicide.
- Mermaid's Mask has Nanao's mother. She was obsessed with her son and force-fed him mermaid's flesh in an attempted murder-suicide, though both survived. Nanao continued to age and eventually had a son of his own, whom his mother kidnapped and raised as a replacement. She feeds her grandson ground mermaid's flesh as medicine and plans to repeat the inital incident, in hopes of having him become as immortal as she is.
- Nanao's mother also sliced off a dead woman's face and placed it over her own. When she goes to meet the adult Nanao, she goes into the storehouse and slices the 'new' face off and replaces it with her 'old one. There's even a panel showing, just barely beyond a silhouette, all the flesh and muscle when the skin is removed.
