Carapus acus emerging from sea cucumber (after developing within)

English: Pearl-fish, Fierasfer acus (Linnaeus). Issuing from a Holothurian, Coast of Italy
Subject: Carapidae, Fierasferidae
Tag: Fish
Date 1907
Author David Starr Jordan (1851–1931)
Source/Photographer Jordan, David Starr (1907) Fishes, New York City, NY: Henry Holt and Company
Source: https://commons.wikimedia.org/wiki/File%3AFMIB_51567_Pearl-fish%2C_Fierasfer_acus_%28Linnaeus%29_Issuing_from_a_Holothurian%2C_Coast_of_Italy.jpeg

The pearl fish or pearlfish (Carapus acus) is a species of bony fish in the family Carapidae, the pearlfishes, and is native to the Mediterranean Sea and the Atlantic Ocean. It lives as a commensal in association with a sea cucumber, spending the day inside its host and emerging at night to feed. Carapus acus usually lives inside the respiratory tree or the body cavity of sea cucumbers. Neither the species nor the size of the host seem important in host choice, and the fish makes use of whatever sea cucumber it happens to encounter; the most common hosts are Parastichopus regalis and Holothuria tubulosa.

Order: Ophidiiformes
Family: Carapidae
Genus: Carapus
Species: Carapus acus (Brünnich, 1768)
Synonyms

Carapus birpex Arnold, 1956
Carapus cuspis Arnold, 1956
Carapus imberbis (Cuvier, 1815)
Diaphasia acus (Brünnich, 1768)
Encheliophis tenuis Putnam, 1874
Fierasfer acus (Brünnich, 1768)
Fierasfer fontanesii (Risso, 1810)
Fierasfer imberbis (Cuvier, 1815)
Fierasfer maculata Swainson, 1839
Gymnotus acus Brünnich, 1768
Notopterus fontanesii Risso, 1810
Ophidium fierasfer Risso, 1827
Ophidium imberbe Cuvier, 1815
Porobronchus linearis Kaup, 1860
Vexillifer dephilippii Gasco, 1870