The Collaborative International Dictionary of English v.0.48:

Fiddler \Fid"dler\, n. [AS. fi[eth]elere.]
   1. One who plays on a fiddle or violin.
      [1913 Webster]

   2. (Zool.) A burrowing crab of the genus Gelasimus, of many
      species. The male has one claw very much enlarged, and
      often holds it in a position similar to that in which a
      musician holds a fiddle, hence the name; -- called also
      fiddler crab, calling crab, soldier crab, and
      fighting crab.
      [1913 Webster]

   3. (Zool.) The common European sandpiper (Tringoides
      hypoleucus); -- so called because it continually
      oscillates its body.
      [1913 Webster]

   Fiddler crab. (Zool.) See Fiddler, n., 2.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fiddler crab
    n 1: burrowing crab of American coastal regions having one claw
         much enlarged in the male