EFFECT OF THE SQUID HOST ON THE ABUNDANCE AND DISTRIBUTION OF SYMBIOTIC VIBRIO-FISCHERI IN NATURE

Euprymna scolopes, a Hawaiian species of bioluminescent squid, harbors Vibrio fischeri as its specific light organ symbiont. The population of symbionts grew inside the adult light organ with an average doublin g time of about 5 h, which produced an excess of cells that were expel led into the surrounding seawater on a diurnal basis at the beginning of each period of daylight. These symbionts, when expelled into the am bient seawater, maintain or slightly increase their numbers for at lea st 24 h. Hence, locations inhabited by their hosts periodically receiv e a daily input of symbiotic V. fischeri cells and, as a result, becom e significantly enriched with these bacteria. As estimated by hybridiz ation with a species-specific luxA gene probe, the typical number of V . fischeri CFU, both in the water column and in the sediments of E. sc olopes habitats, was as much as 24 to 30 times that in similar locatio ns where squids were not observed. in addition, the number of symbioti c V. fischeri CFU in seawater samples that were collected along a tran sect through Kaneohe Bay, Hawaii, decreased as a function of the dista nce from a location inhabited by E. scolopes. These findings constitut e evidence for the first recognized instance of the abundance and dist ribution of a marine bacterium being driven primarily by its symbiotic association with an animal host.