Optimana
Merfolk share a signed native language, Optimana, the linguistic precursor to telluran Sign.
In contrast to Sign, Optimana lends more focus to handshape than movement, as the density of the water they must sign in makes fast, efficient sign language more difficult if motion is the focus. The motion in Optimana is larger, slower, and less emphasized. Instead, the shape of both hands, their position relative to the body, and the placement of the caudal fin are used to distinguish words, phrases, and grammar.
In contrast, Sign lends more emphasis on quick, short motion, a single hand's shape, and movement of a single hand relative to the non-dominant hand's position.
Merfolk do usually know Sign, and often know Solan as well, but Nix, being mute, cannot speak it. Most telluran settlements in coastal areas have large populations of people who know Sign.