1) Fish 2) Pearl-fish 3) Percoid 4) Percoid fish 5) Percoidean 
• (n.) Any fish whose scales yield a pearl-like pigment used in manufacturing artificial pearls, as the bleak, and whitebait.
any of about 27 species of slim, eel-shaped marine fishes of the family Carapidae noted for living in the bodies of sea cucumbers, pearl oysters, ... [2 related articles]
Pearl'fish` noun (Zoology) Any fish whose scales yield a pearl-like pigment used in manufacturing artificial pearls, as the bleak, and whitebait. 
The Pearlfish is an elongate and very slender fish of the family Carapidae, widespread in tropical and warm temperate seas where it lives inside sea cucumbers, sea urchins and other marine invertebrates. It grows to about 30 centimetres long and has a pointed tail and no pelvics.
[n] - found living within the alimentary canals of e.g. sea cucumbers or between the shells of pearl oysters in or near shallow seagrass beds
pearl-fish 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