pearlfish

from The Century Dictionary.

• noun A common name of Fierasfer affinis, a fish found in the Bay of Panama, in the shells of the pearl-oyster; any fish of the family Fierasferidæ, which lives in the shells of mollusks and in large Holothuria.
• noun A minnow, Alburnus lucidus, and other allied species, ranging over Europe and western Asia. The pearly matter from their scales is used in the manufacture of artificial pearls.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) Any fish whose scales yield a pearl-like pigment used in manufacturing artificial pearls, as the bleak, and whitebait.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of various slender marine fish of the family Carapidae, resembling eels.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun found living within the alimentary canals of e.g. sea cucumbers or between the shells of pearl oysters in or near shallow seagrass beds
• noun found living within the alimentary canals of e.g. sea cucumbers or between the shells of pearl oysters in or near shallow seagrass beds