Bioluminescent symbionts of flashlight fishes and deep-sea anglerfishes form unique lineages related to the genus Vibrio

BIOLUMINESCENT symbioses range from facultative associations to highly adapted, apparently obligate ones¹. The family Anomalopidae (flashlight fishes) encompasses five genera of tropical reef fishes that have large suborbital light organs². The suborder Ceratioidei (deep-sea anglerfishes) contains 11 families. In nine of these, females have a bioluminescent lure^3,4 that contains bacterial symbionts⁵. In all other fish light-organ symbioses (occuring in 10 families in 5 orders⁶), the symbionts belong to three Photobacterium species ⁷; nonsymbiotic luminous bacteria are Vibrio species⁸. The bacteria are extracellular and tightly packed in tubules that communicate with the exterior⁷, releasing bacteria into the gut of the host or the surrounding sea water. The released bacteria are usually cultivable and can contribute to planktonic populations^9,10. Although anomalopids release bacteria⁹ and ceratioids have pores that would allow release, the fate of these bacteria is unknown and they cannot be cultured by standard isolation techniques. We report here phylogenetic analysis of 16S ribosomal RNA gene sequences from light organs that show that anomalopid and ceratioid symbionts are not known luminous bacteria, but are new groups related to Vibrio spp. They are characterized by host specificity, deep divergence between symbionts from different genera (anomalopids) or families (ceratioids) and, possibly, parallel divergence of hosts and symbionts.