These tiny animals (less than 0.5mm) are parasites, usually of deep water crustaceans. They have lost many of the complex crustacean morphology and some have very simple, sac-like bodies (Tudge, 2000). They have been known to infest copepods, isopods, tanaids, amphipods and ostracods (Boxshall & Lincoln, 1983).

One species, Tantulacus dieteri, at 85 microns in length, could be the smallest living arthropod (Mohrbeck et al, 2010).