A monotypic species is a species that has only one population. There are very few such species on earth. They are usually island species, endemics or relict species.

Characterises a taxonomic unit (genus, family) containing a single species. Monotypic genera are common, while monotypic families are rare.

Monotypic crustacean families: Clavofabellinae, Amphionides reynaudii, Arcturides, Mictyris, Pseudidothea, Tecticeps, Xenarcturus spinulosus.