text: Of all light emitters in the ocean, bio-luminescent bacteria is the most abundant and diverse. However, the distribution of bio-luminescent bacteria is uneven, which suggests evolutionary adaptations. The bacterial species in terrestrial genera such as Photorhabdus are bio-luminescent. On the other hand, marine genera with bio-luminescent species such as Vibrio and Shewanella oneidensis have different closely related species that are not light emitters. Nevertheless, all bio-luminescent bacteria share a common gene sequence: the enzymatic oxidation of Aldehyde and reduced Flavin mononucleotide by luciferase which are contained in the lux operon. Bacteria from distinct ecological niches contain this gene sequence; therefore, the identical gene sequence evidently suggests that bio-luminescence bacteria result from evolutionary adaptations.