Carapus acus, also known as Pearlfish are able to live inside sea cucumbers and sea stars respectively. Pearlfish is a general name for a variety of marine fish species in the Carapidae family. Pearlfish have been found in tropical waters of the Atlantic, Indian, and Pacific Oceans at depths up to 2,000 metres along oceanic shelves and slopes.
Pearlfish are slender fish, distinguished by having dorsal fin rays that are shorter than their anal fin rays. They have translucent, scaleless bodies reminiscent of eels. The largest pearlfish are about 50 centimetres in length. They reproduce by laying oval-shaped eggs, about 1 millimeter in length.
Pearlfish are unusual in that the adults of most species live inside various types of invertebrate. Typically, they live inside clams, starfish or sea squirts, and are simply commensal, not harming their hosts. However, some species are known to be parasitic on sea cucumbers, eating their gonads.
Series: bony fishes (Osteichthyes)
Class: ray-finned fishes (Actinopterygii)
Order: fish guts behaved (Ophidiiformes)
Family: fish guts
Species: Carapus acus