The Collaborative International Dictionary of English v.0.48:

Polyneme \Pol"y*neme\, n. [Poly- + Gr. ? thread.] (Zool.)
   Any one of numerous species of tropical food fishes of the
   family Polynemid[ae]. They have several slender filaments,
   often very long, below the pectoral fin. Some of them yield
   isinglass of good quality. Called also threadfish.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Threadfish \Thread"fish`\, n. (Zool.)
   (a) The cutlass fish.
   (b) A carangoid fish (Caranx gallus, or Caranx crinitus)
       having the anterior rays of the soft dorsal and anal fins
       prolonged in the form of long threads.
       [1913 Webster]

WordNet (r) 3.0 (2006):

threadfish
    n 1: fish having greatly elongated front rays on dorsal and anal
         fins [syn: threadfish, thread-fish, Alectis ciliaris]