They occur in a wide range of depths, from the littoral zone to the deep sea.

[1] The juvenile form is known as a 'praniza', and it is a temporary parasite of marine fish.

Mancae of the Gnathiidae closely resemble the adult form, however they lack the final pair of pereiopods.

Taxonomy in the family relies on male characters, such that females and juveniles cannot be reliably identified.

[1] The family contains 182 species,[3] divided among the following genera:[4] The extinct Jurassic genus Urda is thought to represent a stem-group to the family, and is suggested to have had a similar parasitic lifestyle.