The viperfish's anterior teeth are transparent, needle-thin and disproportionate to the skull that carries them. When the jaws close, the longest lower fangs pass outside the upper jaw and lie alongside the snout. The head can also be tilted back and the jaws swung wide, giving the small fish an unusually large gape for impaling prey it strikes from below.
The body is lined with photophores, and a specialised light organ sits near the eye. A long first dorsal fin ray extends over the head, and the fish is widely described as dangling it as a lure. Like most deep-sea fish, the viperfish is a vertical migrator, rising towards the surface layers after dark where prey is denser.
Size is where popular accounts most often go wrong. Photographs of gaping specimens invite an impression of a large predator, but Sloane's viperfish typically reaches only around 20 to 30 centimetres in length.