Tantulocarida

Tantulocarida is a highly specialised group of parasitic crustaceans that consists of about 33 species, treated as a subclass of the class Maxillopoda (copepods and barnacles). They are typically ectoparasites that infest copepods, isopods, tanaids, amphipods and ostracods. Members of this subclass are minute – less than 0.3 millimetres (0.012 in) in length and have a dramatic reduction in body form compared to other crustaceans, with an unsegmented sac-like thorax and a much reduced abdomen. One tantulocarid species, Tantulacus dieteri, is the world's smallest arthropod, with a total body length of only 85 micrometres (0.0033 in).

The tantulocarid life cycle is unique. It does not follow the usual moulting cycle of all crustaceans; instead, the mature adult actually develops within an attached parasitic tantulus larva.