Pearlfish

Pearlfish Carapus acus
The adult pearlfish has a most unusual home-it lives inside the body cavity of sea cucumbers. To allow it to slip in and out of its host easily, it has an eel-like body, no pelvic fins, and no scales.

Carapus mourlani, Star Pearlfish
Carcharhinus wheeleri, Blacktail Reef Shark
Centrophorus granulosus, Gulper Shark
Centropomus undecimalis, Common Snook
Centropomus ensiferus, Swordspine Snook
Centropomus pectinatus, Tarpon Snook ...

OPHIDIIFORMES
cuskeels, pearlfishes, pyramodontines, livebearing brotulas, aphyonids, false brotulas
BATRACHOIDIFORMES
toadfishes ...

Cardinalfishes and pearlfishes live inside large gastropods and mollusks , respectively (inquilism-sheltering inside living invertebrates).

The parasitism of Sea cucumbers by members of the Pearlfish family (Carapidae) is also well documented. The small thin fish enter the Sea cucumber's body via the anus, and then penetrates the body cavity and feeds on the host's gonads.

See also: Shark, Coral, Sea Cucumber, Diver, Herring