Origin of tetrodotoxin and toxification mechanism in juvenile puffer fishes

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The toxification of puffer by tetrodotoxin is an important subject. The facts that the toxicity in natural puffer varied regionally and individually, and that the cultured puffer was innocuous may suggest that the toxification was caused by the food chain mechanism. In Okkirai bay, Iwate prefecture, highly toxic juvenile puffer fish appear in summer and autumn. Then, to have actual proof of the food chain mechanism we conducted rearing experiments in Okkirai Bay. Non-toxic cultured juvenile kusafugu Takifugu niphobles were confined in a plastic fish cage (80x56x37cm) submerged in seawater and fed natural living organisms intruding into the cage from outside through narrow gap. The rearing experiments were conducted for two to four weeks at two spots in Okkirai Bay. (Onisawa fishing port and Sodenosawa) and repeated during summer and autumn for three years. After each rearing experiment, the toxicity of specimens used were assayed using mouse to monitor whether toxification had occurred … More or not. At Sodenosawa among total 14 experiments conducted none showed toxification occurred. On the other hand, at Onisawa among 14 experiments 5 experiments conducted from August to October showed toxification occurred. Above result suggest that non-toxic cultured juvenile kusafugu become toxic when reared in a cage submerged in seawater in certain times at certain spots.In the experiments described above, toxification of cultured juvenile puffer was supposed to be occurred by ingesting natural living organisms, but the causative organisms were not identified. Then, in the next experiment, we tried to narrow the range of the causative organisms. Fish rearing tanks containing non-toxic cultured juvenile kusafugu were set up near Onisawa fishing port. Seawater of the port was pumped up and filtered by 2, 0.3 and 0.1 mm mesh filters respectedly and sequentially. Filtered and condensed microorganisms with 0.3 or 0.1 mm mesh filters were provided to the rearing tanks and fed to the kusafugu. In the experiment conducted from August to October showed toxication occurred. This suggests that the causative organisms are very small. Less