Myxobolus

Myxobolus is a genus of myxozoa that includes important parasites of fish like Myxobolus cerebralis. The genus is polyphyletic, with members scattered throughout the myxozoa. Some stages of Myxobolus species were previously thought to be different organisms entirely, but are now united in this group. Some fish species, such as the thicklip grey mullet, can harbour a dozen Myxobolus species.

One of the smallest species (Myxobolus shekel) is no more than 8.5 μm (0.0085 mm) when fully grown, making it the smallest known animal.

Source: World Register of Marine Species