Source : Webster's Revised Unabridged Dictionary (1913)

Threadfin \Thread"fin`\, n. (Zo["o]l.)
   Any one of several species of fishes belonging to {Polynemus}
   and allied genera. They have numerous long pectoral
   filaments.

Source : WordNet®

threadfin
     n : mullet-like tropical marine fishes having pectoral fins with
         long threadlike rays