Siphamia tubifer

Siphamia tubifer, also known as the sea urchin cardinalfish, is a small (~7 cm) coral reef fish in the family Apogonidae. Its geographic range extends from East Africa to the French Polynesian Islands.

Details
During the day, the sea urchin cardinalfish hides among the spines of sea urchins, and it emerges to feed at night. Male S. tubifer are mouthbrooders, holding their fertilized clutches of eggs in their mouths and releasing the larvae when they are in the preflexion stage. This fish has a facultative symbiotic relationship with a bacterium, Photobacterium mandapamensis, which provides bioluminescence for the fish in a specialized light organ in its abdomen.

Bioluminescence
S. tubifer is thought to acquire the bacteria through ingestion of seawater after its light organ has mostly developed. The luminescence system primarily consists of a ventral light organ that holds the bacteria and a shutter lens in the abdomen. The fish can open and close this shutter at will, controlling the light that it emits. One study showed that at twilight, S. tubifer left its urchin and luminesced to attract and feed on zooplankton near the ocean bottom. Once completely dark again, it stopped emitting light and returned.