Size-defining prefixes: mega, macro, meio and micro

Size-defining prefixes are in common use. Words like megabyte and microchip come immediately to mind. The prefix meio is somewhat different. It means less and makes only sense in relation with other prefixes denoting superior categories of size.

As an interesting application of these prefixes, I came upon a classification scheme for deep-sea fauna [1]:

• 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
  

In fact, the term microfauna is not used in the scheme—just bacteria. But since bacteria are subjects of microbiology, which typically require a microscope to be seen, this term should be appropriate and nicely fits into the line of this terminology.

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 [1]:

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.

Deep-sea reference:
[1] Craig McClain: An Empire Lacking Food. American Scientist November-December 2010, 98 (6), pp.470-477.
http://www.americanscientist.org/issues/feature/2010/6/an-empire-lacking-food/1.