Tetrodotoxin is a potent neurotoxin found in certain marine animals, most notably in pufferfish. It blocks sodium channels in nerve cells, preventing the transmission of nerve impulses. This can lead to paralysis and, in severe cases, death. The toxin is highly concentrated in the organs of some species, making them dangerous to consume without proper preparation. Despite its toxicity, pufferfish is considered a delicacy in some cultures, particularly in Japan, where chefs undergo extensive training to serve it safely.