Merfolk

There are several sub-races of merfolk, those of which include the Merrow, Melusine, Finnfolk, Marindaga, Sirena, Siyokoy, and the Marakihau.

When leaving water, merfolk can swap out their tails for legs. Some merfolk, like deep sea merfolk, have more trouble with it than others, as they rarely have reason to do so.

Those in Ariki and Waauru's depth-level are often much more comfortable with it, as there are a good number of merfolk villages built on land for the sake of farming, hunting, and trade. They travel back and forth between the surface towns and submerged city very often.

Merfolk like Ariki and Waauru who live in water where light still permeates are most comfortable in reasonably deep water and on land, but those who have adapted to live in deeper, darker, colder water have difficulty venturing up higher than almost halfway into the warmer regions, and vice versa. If Ariki or Waauru were to spend too much time at an unsafe depth, they’d be in really bad shape.

They won’t die (immediately, but after a while it’ll happen), but they shouldn’t stay out of their preferred water-pressure for longer than a few days. A week is getting risky, and will probably be followed by severe health problems. 6 weeks is unheard of.

The cities merfolk live in are often constructed in trenches, allowing its inhabitants to live at whatever depth they need to without being cut off from the community, and there are pieces of tech that can be worn in order to allow deep sea folk to travel into shallow waters (and shallow water folk into deeper water) indefinitely.

The neck gills on merfolk are highly sensitive. It depends bit on exposure to water how sensitive they are, but the difference between the two states is so small it’s basically negligible. They are a bit more sensitive than normal skin.

(Like the rest of the body, the feeling all intends on the intent of the touch. If you punch a merfolk in the gills it is definitely going to hurt a lot, but touching with the intent to please would be very different.)

Exposure to a Rayet in its impure form can cause serious diseases which dissuades most landlocked races from trying to mine it, but most merfolk have developed an immunity to it after hundreds of years of exposure.

A merfolk could have a child with one of the land-locked species, but it’d take some medical intervention to make it possible. It’s not dangerous, just a bit time consuming. Stocking up on important vitamins that they might not otherwise have, some New Fangled Technology™, etc.