Gut Epithelium of the Highly Toxic Ribbon Worm Cephalothrix cf. simula (Palaeonemertea, Nemertea) Contains Tetrodotoxin-Positive Bacterial Endosymbionts

Tetrodotoxin (TTX), widely known as pufferfish venom, is a low-molecular-weight guanidinium neurotoxin. It can accumulate to extremely high concentrations in certain animals, including pufferfish, blue-ringed octopuses, flatworms, and nemerteans. However, the origin of TTX and the mechanisms that enable such extreme accumulation in these animals remain poorly understood. In this study, using confocal laser scanning microscopy combined with electron immunocytochemistry and ultrastructural analysis, we demonstrate the presence of TTX-positive bacteria associated with specialized cellular structures-type II phagosomes of gut enterocytes-in the highly toxic nemertean <i>Cephalothrix</i> cf. <i>simula</i>. We hypothesize that TTX production in <i>C</i>. cf. <i>simula</i> results from interactions between the nemertean host and its endosymbionts. These findings clarify the origin and accumulation of the toxin in nemerteans and have broader implications for other TTX-bearing species.