Siphamia tubifer

During the day, the sea urchin cardinalfish hides among the spines of sea urchins, and it emerges to feed at night.^[4] 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.^[5][6] 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.^[5]

S. tubifer is thought to acquire the bacteria through ingestion of seawater after its light organ has mostly developed.^[7] 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.^[8] One study showed that at twilight, S. tubifer left its urchin and luminesced to attract and feed on zooplankton near the ocean bottom.^[8] Once completely dark again, it stopped emitting light and returned.