Among the deepwater species, Symphurus thermophilus lives in congregating around "ponds" of sulphur at hydrothermal vents on the seafloor. No other flatfish is known from hydrothermal vents.[7] Many species will enter brackish or fresh water, and a smaller number of soles (families Achiridae and Soleidae) and tonguefish (Cynoglossidae) are entirely restricted to fresh water