What is the smallest animal on Earth?

Tiny animals are, by their very nature, often overlooked. Most animal enthusiasts know that blue whales (Balaenoptera musculus) are the largest animals, but few are aware of our planet's most miniature inhabitants. So, what is the smallest animal?

There's always the potential for undiscovered species to emerge, but it's hard to imagine Animals getting any smaller than myxozoans, a group of microscopic invertebrates related to jellyfish.

Beth Okamura, a professor at the Natural History Museum in London, believes myxozoans are the smallest Animals on Earth because many myxozoan species are just a single cell as adults.

"All of us at some point in our life cycle have tiny stages, but when we're mature, we're complex multicellular animals," Okamura said. "That's not the case for Myxozoa. When they mature, they're still single cells."

Related: What is the most genetically diverse species?

Myxozoans can be as small as 0.02 millimeter (20 micrometers) across as adults. For scale, that's up to 100 times smaller than a grain of sand, according to the University of Hawaii. 

The Myxozoa subphylum has evolved a simple form that suits its parasitic life cycle. Most myxozoan species produce infectious water-borne spores. Cells from these spores invade and develop inside both vertebrate hosts, such as fish, and invertebrate hosts, like annelid worms, Okamura explained.