But what if mermaids only appeared humanoid at the surface?
What if, the deeper they dove, the more fish-like they became?
The light doesn’t penetrate as deeply as they swim lower in the ocean, so they adapt; eyes growing smaller, gills larger, scales along their backs emerge and spread out along their limbs, glowing softly at first, then more radiant as the darkness becomes more encompassing.
Mermaids at the bottom of the sea are hardly maids at all, but more like the creatures in dark fantasies that haunt children’s nightmares, with numerous limbs and sharp teeth. Pale and translucent skin, or else pitch black to blend in with their surroundings.
Maybe when they die at those depths, they rise to the surface and don’t change back to their human forms.
They wash up on shore as massive beasts, and humans cower in fear, afraid of the unknown things that lurk in the waters.
Stories of giant squids and colossal whales spread throughout mankind, and soon mermaids are more of a myth than anything, since the surface is a dangerous place now, and the hidden depths of the ocean are safer.
Over time their forms became less intimidating, as they still need the light to survive, but are shy of the ever present human gaze. Many take lesser forms; species of fish that can enjoy the warmth of the sun without becoming conspicuous.
And as time goes on, mermaids that once took humanoid forms no longer exist, but the variety of their species remains rich and diverse.
Some remain at the deepest of depths, introverted and peaceful with the silence of nothing. Others prefer the surface, where the light is constant and the colours vibrant, but grow smaller to avoid predation.
And then some, still fond of humans and more connected with their past selves, find they need air to survive, and breach surface frequently, living in relative peace along side us.
Maybe dolphins are these such creatures, sharing in human intelligence and mannerisms but no longer able to transform; a skill long lost to the ages.
The ocean is a mysterious world we know very little about, so why not?