They live off of the chemicals coming out of the sea floor. Deep sea fish have adapted to survive in low oxygen environments. Mexican cave fish, for example, have larger red blood cells that produce higher concentrations of hemoglobin, the protein that carries oxygen throughout the body. More fish dwell that way near the surface, according to a 2020 study. All of these adaptations allow creatures to thrive in the dark abyss of the deep ocean in one of the harshest environments on our planet