Beginning with the larval stages, marine pufferfish such as Takifugu niphobles contain tetrodotoxin (TTX), an extremely potent neurotoxin. Although highly concentrated TTX has been detected in adults and juveniles of these fish, the source of the toxin has remained unclear. Here we show that TTX in the flatworm Planocera multitentaculata contributes to the toxification of the pufferfish throughout the life cycle of the flatworm. A species-specific PCR method was developed for the flatworm, and the specific DNA fragment was detected in the digesta of wild pufferfish adults. Predation experiments showed that flatworm larvae were eaten by the pufferfish juveniles, and that the two-day postprandial TTX content in these pufferfish was 20–50 μg/g. Predation experiments additionally showed flatworm adults were also eaten by pufferfish young, and after two days of feeding, TTX accumulated in the skin, liver and intestine of the pufferfish.

The pufferfish Takifugu niphobles (at present Takifugu alboplumbeus) possesses highly concentrated tetrodotoxin (TTX), an extremely potent neurotoxin that provides effective protection from predators, at least at the larval stages. However, the source of the toxin has remained unclear. Recently, DNA from the toxic flatworm Planocera multitentaculata was detected in the intestinal contents of juveniles and young of the pufferfish, suggesting that the flatworm contributes to its toxification at various stages of its life. In this study, we describe the behavior of the pufferfish in the intertidal zone that appears to contribute to its toxification before and during its spawning period: pufferfish were found to aggregate and ingest flatworm egg plates by scraping them off the surface of rocks. DNA analysis based on 28S rRNA and cytochrome c oxidase subunit I (COI) genes identified the egg plates as those of P. multitentaculata. Liquid chromatography with tandem mass spectrometry analysis revealed that the egg plates contain highly concentrated TTX. The feeding behavior of the pufferfish on the flatworm egg plates was also observed in the aquarium. These results suggest that pufferfish feed on the flatworm egg plate, which enables them to acquire toxicity themselves while providing their offspring with the protective shield of TTX.

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