Trophic Role of the Tetrodotoxin-Bearing Flatworm Plancera multitentaculata in the Marine Food Web

Predator-prey interactions in marine ecosystems are extremely complex, involving a wide variety of species. In contrast, toxin-mediated predator-prey interactions are constrained by species-specific tolerance to the toxin. Tetrodotoxin (TTX), commonly known as pufferfish toxin, is a common factor in such interactions across diverse taxa. It has been recently suggested that TTX-bearing planocerid flatworms contribute to the toxification of other TTX-bearing organisms. However, their contribution to the transfer of TTX in the food web remains unclear. In this study, we investigated the trophic position of the TTX-bearing flatworm Planocera multitentaculata in this transfer using carbon and nitrogen stable isotope analysis. We compared isotope values from wild P. multitentaculata, its putative prey (mollusks such as gastropods and polyplacophorans), and a known predator, the pufferfish Takifugu alboplumbeus. We also reared P. multitentaculata exclusively on the gastropod Monodonta confusa and estimated the trophic enrichment factors for stable isotopes of carbon and nitrogen to be 0.85‰ and 3.28‰, respectively. Based on these findings, the inferred prey of the wild flatworm had stable isotope values similar to those of the gastropod M. confusa and two polyplacophoran species. In contrast, the inferred prey of T. alboplumbeus had significantly different isotope values from P. multitentaculata, supporting the role of flatworms as an intermediate TTX source in marine food webs. These results highlight the importance of planocerid flatworms in TTX transfer and their contribution to toxin accumulation at higher trophic levels.