As an interesting application of these prefixes, I came upon a classification scheme for deep-sea fauna :
- megafauna: fish, crabs, lobster, starfish, urchins, sea cucumbers, sponges and corals
- macrofauna: small polychaete worms, crustaceans and mollusks
- meiofauna: forams, copepods and nematodes
- microfauna: bacteria
There is an inverse relationship between animal size and depth. Craig McClain refers to the findings of Hjalmar Thiel of the University of Hamburg in northern Germany, who observed that the deep sea is a habitat mostly populated by small organisms :
Thiel's specific findings were that megafauna and macrofauna decrease more rapidly with depth than do meiofauna or bacteria. In fact, with increased depth meiofauna and bacteria become increasingly more dominant. Thus, at depth greater than 4 kilometers on the vast abyssal plains where food is extremely limited, there is a shift toward diminutive size.The term meiofauna is specifically used for aquatic organisms. Worms and nematodes living in the soil are grouped as mesofauna. The prefix meso, meaning middle or intermediate, defines size in relative terms like meio—somewhere between micro and macro.
 Craig McClain: An Empire Lacking Food. American Scientist November-December 2010, 98 (6), pp.470-477.