Pearlfish are slender, eel-shaped fish that often live inside various invertebrates including sea cucumbers. Because a sea cucumber breathes by taking in water through its anus, a pearlfish can wait for the cucumber to open for a breath and swim in.

The relationship between pearlfish and sea cucumbers is beneficial to the pearlfish, providing shelter for this slender, scaleless, and often translucent fish. The relationship may be commensal, in which no harm is done to the host sea cucumber, but in some species the pearlfish can function as a parasite, causing harm to its host by eating its gonads and other internal organs! Pearlfish are most common in tropical shallow-water sea cucumbers, but they have been found in hosts in depths of up to 2,000 meters! There may be just one pearlfish to a sea cucumber, but some sea cucumbers may shelter five or more pearlfish. Other animals, including oysters and sea stars, can also play host to pearl fish.