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.

[3][4] Carapus acus is native to the Mediterranean Sea and eastern Atlantic Ocean;[1] it is common in the western half and around the coasts of Spain, France, Italy, Sardinia and the Balearic Islands.

[4] Carapus acus usually lives inside the respiratory tree or the body cavity of sea cucumbers.

The function of these sounds is unclear, various hypotheses have been proposed including finding a mate, indicating the size of the fish while it is concealed in its host, and establishing a territory.

[4] Juvenile fish may nibble the reproductive organs or the gills of their host; this causes little damage because sea cucumbers have great regenerative powers.