Tonguefish

Tonguefishes are flatfish in the family Cynoglossidae. They are distinguished by the presence of a long hook on the snout overhanging the mouth, and the absence of pectoral fins. Their eyes are both on the left side of their bodies, which also lack a pelvic fin.^[2] This family has three genera with a total of more than 140 species. The largest reaches a length of 66 cm (26 in), though most species only reach half that size or less.^[3][4][5] They are found in tropical and subtropical oceans, mainly in shallow waters and estuaries, though some species are found in deep sea floors,^[6] and even a few in rivers.

Symphurus thermophilus lives congregating around "ponds" of sulphur at hydrothermal vents on the seafloor. No other flatfish is known from hydrothermal vents.^[7] Scientists are unsure of the mechanism that allows the fish to survive and even thrive in such a hostile environment.^[8]