Very special thanks for his wonderful photo of Astrapogon stellatus in its conch to Serge Aucoin.
The cardinalfish Astrapogon stellatus hides himself during the daytime in its host conch, Strombus gigas, Atrina rigida or Pinna carnea.
This relationsship is called "Commensalism", the cardinalfish gains the benefit of protection agains predatory fishes and the conch is not harmed.
During the night, the mouthbrooding fish leaves the conch to hunt small crustaceans like copepods and amphipods.
What a wonderful biological interaction between two species!