The Collaborative International Dictionary of English v.0.48:

Swimming \Swim"ming\, a.
   1. That swims; capable of swimming; adapted to, or used in,
      swimming; as, a swimming bird; a swimming motion.
      [1913 Webster]

   2. Suffused with moisture; as, swimming eyes.
      [1913 Webster]

   Swimming bell (Zool.), a nectocalyx. See Illust. under
      Siphonophora.

   Swimming crab (Zool.), any one of numerous species of
      marine crabs, as those of the family Protunidae, which
      have some of the joints of one or more pairs of legs
      flattened so as to serve as fins.
      [1913 Webster]

WordNet (r) 3.0 (2006):

swimming crab
    n 1: marine crab with some legs flattened and fringed for
         swimming