Synbranchidae - Swamp eels

The swamp eels (also written swamp-eels) are a family (Synbranchidae) of freshwater eel-like fishes of the tropics and subtropics. Most species are able to breathe air and typically live in marshes, ponds and damp places, sometimes burying themselves in the mud if the water source dries up. They have various adaptations to suit this lifestyle; they are long and slender, they lack pectoral and pelvic fins, and their dorsal and anal fins are vestigial, making them limbless vertebrates. They lack scales and a swimbladder, and their gills open on the throat in a slit or pore. Oxygen can be absorbed through the lining of the mouth and pharynx, which is rich in blood vessels and acts as a lung.

Although adult swamp eels have virtually no fins, the larvae have large pectoral fins which they use to fan water over their bodies, thus ensuring gas exchange before their adult breathing apparatus develops. When about a fortnight old they shed these fins and assume the adult form. Most species of swamp eel are hermaphrodite, starting life as females and later changing to males, though some individuals start life as males and do not change sex.