A NEW NICHE FOR VIBRIO-LOGEI, THE PREDOMINANT LIGHT ORGAN SYMBIONT OFSQUIDS IN THE GENUS SEPIOLA

Two genera of sepiolid squids-Euprymna, found primarily in shallow, co astal waters of Hawaii and the Western Pacific, and Sepiola, the deepe r-, colder-water-dwelling Mediterranean and Atlantic squids-are known to recruit luminous bacteria into light organ symbioses, The light org an symbiont of Euprymna spp, is Vibrio fischeri, but until now, the li ght organ symbionts of Sepiola spp, have remained inadequately identif ied, We used a combination of molecular and physiological characterist ics to reveal that the light organs of Sepiola affinis and Sepiola rob usta contain a mixed population of Vibrio logei and V. fischeri, with V. logei comprising between 63 and 100% of the bacteria in the light o rgans that we analyzed, V. logei had not previously been known to exis t in such symbioses, In addition, this is the first report of two diff erent species of luminous bacteria co-occurring within a single light organ, The luminescence of these symbiotic V. logei strains, as well a s that of other isolates of V. logei tested, is reduced when they are grown at temperatures above 20 degrees C, partly due to a limitation i n the synthesis of aliphatic aldehyde, a substrate of the luminescence reaction. In contrast, the luminescence of the V. fischeri symbionts is optimal above 24 degrees C and is not enhanced by aldehyde addition , Also, V. fischeri strains were markedly more successful than V. loge i at colonizing the light organs of juvenile Euprymna scolopes, especi ally at 26 degrees C. These findings have important implications for o ur understanding of the ecological dynamics and evolution of cooperati ve, and perhaps pathogenic, associations of Vibrio spp, with their ani mal hosts.