It is well-known that the rotifer
Brachionus plicatilis cultured for mass seed production of various marine fishes contains bacterial flora which consists principally of
Vibrio,
Pseudomonas, and
Moraxella at about 10
7-10
8 CFU/g. The rotifers, previously treated with penicillin G and streptomycin, were exposed to bacterial cell suspensions of 10 strains of
Vibrio,
Pseudomonas, and
Moraxella isolated from rotifers. The concentration of each test bacteria rapidly reached 10
7-10
8 CFU/g in the rotifers exposed to 1.0×10
8 CFU/m
l. But, by changing the seawater every h, the concentration of
Pseudomonas,
Moraxella, and an unidentified strain decreased from 1.2 ?? 7.1×10
7 CFU/g to 1.7 ?? 5.2×10
6 CFU/g after 4h. In additional experiments, it was shown that this decrease was caused mainly by inactivation of the rotifers. On the other hand, the concentration of
Vibrio never decreased. It became apparent that
Vibrio was different from other bacteria in its affinity or resistance to rotifers. It was considered that the relationship between rotifer and
Vibrio was parasitism or commensalism, while that between rotifer and
Moraxella-Pseudomonas seemed to be a predator-prey interaction.
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