TIL… Thank you, kind stranger, for making me curious enough to look further

en.m.wikipedia.org/wiki/Fugu

Researchers have determined that a fugu’s tetrodotoxin comes from eating other animals infested with tetrodotoxin-laden bacteria, to which the fish develops insensitivity over time.[10] Whether tetrodotoxin is sequestered from or produced by symbiotic bacteria is still debated.[11] As such, efforts have been made in research and aquaculture to allow farmers to produce safe fugu. Farmers now produce poison-free fugu by keeping the fish away from the bacteria – see § Aquaculture below.[10]