The highly extensile mouth is lined with sharp, slightly recurved and depressible teeth and it extends well past the eyes.

The body lacks scales, but is covered in easily abraded, silvery guanine, which imparts a greenish to purplish iridescence in life.

[2] Telescopefish are presumed to be solitary, active predators, frequenting the mesopelagic to bathypelagic zones of the water column, from 500 to 3,000 metres (1,640 to 9,840 ft).

Their eyes may also help telescopefish to better judge distance of prey; these visual adaptations are typical of deep-sea fish (barrel-eye, tube-eye).

Owing to the telescopefishes' highly extensile jaws and distensible stomachs, they are able to swallow prey larger than themselves; this is also a common adaptation to life in the lean depths (sabertooth fish, black seadevil).