Fish
Pearlfish
Carapidae · also called pearlfishes
Slipping tail-first into a sea cucumber’s rear opening, a pearlfish makes its home inside a living host’s body cavity, sometimes as a harmless tenant and sometimes as a quiet parasite feeding on its landlord’s own organs.
Overview
The pearlfish, family Carapidae, is a slender, scaleless fish native to tropical and temperate ocean habitat worldwide, closely associated with sea cucumbers and other invertebrate hosts. Its unusual habit of living inside a host’s body cavity, tail-first entry technique, and range of relationships from harmless to parasitic make it one of the more behaviourally unusual fish found anywhere in the ocean.
A home found inside another animal
Few fish have committed as fully to living inside another creature as the pearlfish, whose entire lifestyle revolves around finding, entering, and surviving within the body cavity of a sea cucumber or similar invertebrate host.
Status
Least Concern, remaining widespread across suitable ocean habitat worldwide, tied closely to the availability of sea cucumber hosts.
Pearlfish: common questions
How does the pearlfish get inside a sea cucumber?
This species typically enters its host tail-first through the sea cucumber's respiratory opening, a method that avoids triggering as strong a defensive reaction from the host as a head-first entry might.
Does the pearlfish harm the sea cucumber it lives inside?
It depends on the species, this family includes species that live as harmless commensals within their host, alongside other species that feed on the host's gonads or organs in a more parasitic relationship.
Why does the pearlfish have a scaleless, eel-like body?
This family has a slender, eel-like body lacking scales, an adaptation suited to slipping in and out of a host's body cavity without injury or excessive friction.
Is the pearlfish only found living inside sea cucumbers?
While sea cucumbers are its most well-known host, some species within this family have also been found sheltering inside sea stars, bivalves, and even pearl oysters, which may be connected to the origin of its common name.