The Collaborative International Dictionary of English v.0.48:

Jonah \Jo"nah\, n.
   The Hebrew prophet, who was cast overboard as one who
   endangered the ship; hence, any person whose presence is
   unpropitious.
   [1913 Webster]

   Jonah crab (Zool.), a large crab (Cancer borealis) of the
      eastern coast of the United States, sometimes found
      between tides, but usually in deep water.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Jonah crab
    n 1: large red deep-water crab of the eastern coast of North
         America [syn: Jonah crab, Cancer borealis]