Dictionary of Ichthyology  

fila = a thread-like structure, a filament. Eleotrioides helsdingeni is provided with two caudal fila - caudal rays prolonged beyond the rest of the fin while Sturisoma aureum has one.

Sturisoma aureum from D. S. Jordan's "A Guide to the Study of Fishes", 1905. Photograph by Brian W. Coad.

Canadian Museum of Nature
www.briancoad.com