fairy shrimp

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various small branchiopod crustaceans of the order Anostraca that lack a carapace and characteristically swim upside-down, found in freshwater habitats and saline lakes.

from The Century Dictionary.

• noun The popular name of a small British fresh-water phyllopodous crustacean, Branchipus (or Chirocephalus) diaphanus.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A small freshwater branchiopod having a transparent body with many appendages and swimming on its back.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun small freshwater branchiopod having a transparent body with many appendages; swims on its back