Pearlfish

Pearlfish are slender marine fishes in the family Carapidae, many of which shelter inside sea cucumbers and other invertebrates.

Sea cucumber: A soft-bodied marine echinoderm with a flexible body and a ring of feeding tentacles. Many pearlfish enter its body cavity through the anus and use it as shelter.
Carapidae: A family of marine fishes that includes pearlfish and their close relatives. Pearlfish belong to this family, whose species often have elongated bodies.
Synaptula: A genus of small sea cucumbers that includes hosts of some pearlfish species. Pearlfish have been recorded living inside species of this sea-cucumber genus.
Carapus acus: A pearlfish species found in the Mediterranean Sea and eastern Atlantic. It is a well-studied example associated with sea cucumbers.
Commensalism: An ecological interaction in which one organism benefits while the other is neither helped nor harmed. Some pearlfish associations with sea cucumbers are described as commensal.
Ophidiiformes: An order of ray-finned fishes that includes cusk-eels and pearlfishes. Carapidae is one of the families within this broader fish order.
Holothuria: A large genus of sea cucumbers found in tropical and subtropical seas. Some pearlfish use Holothuria species as sheltering hosts.
Encheliophis vermicularis: A pearlfish species known for living within sea cucumbers. Its host use illustrates the close associations found in Carapidae.
Parasitism: An interaction in which one organism benefits by living on or in another organism and harms it. Certain pearlfish consume their hosts’ internal tissues, unlike harmless sheltering associates.
Ray-finned fish: A vertebrate fish whose fins are supported by bony or cartilaginous rays. Pearlfish are members of this vast group of fishes.

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Cusk-eelRelated: They share an order with cusk-eels but have distinctive lifestyles and body forms.