The Selkies usually have the cetacean/seal traits such as horizontal tails and breathe air. Nereids have more fish/aquatic animal tendencies such as gills (which uses the existing nose structures with modifications for the shallow water ones and gill slits in the deeper water ones) and vertical tails and an abundance of fins (though this one only has a fin on the head).
I varied from my usual sketch colouring (which is fill circle for flats and soft mix brush for shading and sometimes big texture stamp brush for background) and used the wet bristle brush for everything. Will definitely be using it for sketches that don't involve intricate patterns or tattoos as unlike the mix brush (which should be used on the same layer but you can get away with using it on a different layer) the wet bristle brush really needs to be used on the same layer. And me and things on the same layer don't always work out so good.