Photobacterium leiognathi

Photobacterium leiognathi demonstrates that the Vibrio fischeri-squid symbiosis has a parallel in fish. This bacterium colonizes the light organs of ponyfish (Leiognathidae) and other luminous fish species, providing bioluminescence for communication, camouflage, and predator avoidance. The core story is similar to V. fischeri: quorum sensing-controlled light production in a specialized host organ. But fish and squid diverged over 500 million years ago, meaning this symbiotic strategy evolved independently twice.

The convergent evolution reveals which features are essential for bioluminescent symbiosis. Both fish and squid provide bacteria with protected, nutrient-rich environments. Both require quorum sensing-controlled light production that can be modulated for host behavioral needs. Both expel bacteria periodically, maintaining selection for symbiont fitness. The deep similarities suggest these features represent optimal solutions to the problem of maintaining beneficial microbial partners.

P. leiognathi's fish hosts use bioluminescence differently than squid. Ponyfish have evolved elaborate light-directing structures—shutters, reflectors, and lenses—that can project bacterial light in specific directions. Some species use light for communication, flashing patterns to potential mates or rivals. The bacteria provide the light source; the fish provides the signal processing. This division of labor—bacteria generate energy-expensive light, hosts direct it for specific purposes—represents efficient resource allocation between partners with different capabilities.