Their bodies are greatly elongated, though more tapered than in true eels.

The caudal fin is small or nonexistent, while the anal fin is lengthy, as long as half of the total body length.

[3] Although not true eels, these fish do have a similar leptocephalus larval form.

However, while the larvae of true eels are about 5–10% of the length of the adult, those of deep-sea spiny eels can grow considerably larger than the adult, and shrink when they develop into their final form.

[3] Notacanthidae includes the following extant genera, although Tilurus is known only from leptocephalus larvae:[4]