The Collaborative International Dictionary of English v.0.48:

Pearlfish \Pearl"fish`\, n. (Zool.)
   Any fish whose scales yield a pearl-like pigment used in
   manufacturing artificial pearls, as the bleak, and whitebait.
   [1913 Webster] Pearlins

WordNet (r) 3.0 (2006):

pearl-fish
    n 1: found living within the alimentary canals of e.g. sea
         cucumbers or between the shells of pearl oysters in or near
         shallow seagrass beds [syn: pearlfish, pearl-fish]