The Collaborative International Dictionary of English v.0.48:

Fierasfer \Fi`e*ras"fer\, n. [NL.] (Zool.)
   A genus of small, slender fishes, remarkable for their habit
   of living as commensals in other animals. One species
   inhabits the gill cavity of the pearl oyster near Panama;
   another lives within an East Indian holothurian.
   [1913 Webster]