Most of the smallest animals are aquatic, partially because the layer of mud on the ocean floor is an excellent habitat for tiny organisms. The small invertebrates that live in the mud are known as the meiofauna, infauna, or the benthos. Aside from bacteria, they are probably among the most numerous animals on the planet especially nematodes, found at modest densities in almost every square mile of the ocean floor, where they consume the endless organic “snow” that falls from above.

The smallest animals are also among the most numerous here. They coexist with abundant bacteria, which they feast upon. Note that bacteria and other unicellular organisms are not considered the smallest animals because the definition of an animal is an organism that is by definition multicellular.