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I must admit this legendary or mythical creature is really fascinating and I am talking about the half fish half woman.
I know most fish have the ability to control buoyancy as they can easily move around in the water without having to tread water; they've got something called a swim bladder.
I am wondering if the presence of a swim bladder could affect the agility of mermaids underwater? Or did their ancestor already abandon buoyancy control over waste processing systems, much like us?
You can really do anything you want. Mermaid stories come in many many many variants. However, I would expect mermaids to have more in common with dolphins, which do not have swim bladders (they manage the air in their lungs instead). Swim bladders are great for ultra-low energy movement. You just adjust the gas balance in the bladder, and it takes no effort to stay still. If you're a fish, with a small caloric intake, this is life or death. For a dolphin or mermaid, they're already stuck paying the caloric cost of their huge brain. Even if they were perfectly trimmed, they'd still be sucking down calories powering all of those neurons. Accordingly, the dolphins tend to be high enough energy swimmers that the benefits from a swim bladder are weakened.
If your mermaids have lungs, they might use them as impromptu swim bladders.
