Smallest invertebrate

The smallest invertebrates (and indeed smallest animals) are the myxozoans, some of which, belonging to the myxosporean class, measure 10–25 micrometres (µm) in their adult state, with one species, Myxobolus shekel, measuring a mere 8.5 µm when fully grown. Myxozoans are highly specialized marine cnidarians (relatives of jellyfish, sea anemones and coral), which have become obligate endoparasites, featuring both an invertebrate (usually an annelid worm or a bryozoan) as their definitive host and a vertebrate (usually a fish) as their intermediate host within their complex parasitic life cycles. During these cycles, they mostly exist as various types of spore (some cyst-like, others triradiate), though a few, such as Buddenbrockia plumatellae, are worm-like in form.

Despite being microscopic in most cases, myxozoans are multicellular organisms, as opposed to being single-celled organisms like bacteria, amoebae and other protozoans.

Almost rivalling them in minute size as the smallest invertebrates are another microscopic but multicellular animal group, the rotifers or wheel animalcules, of which the freshwater bdelloid rotifers sometimes do not exceed 50 µm when fully grown.