Chorion-localized tetrodotoxin in Japanese blue-lined octopus (Hapalochlaena cf. fasciata) eggs mediates predator-dependent deterrence.

The Japanese blue-lined octopus, Hapalochlaena cf. fasciata, is known to harbor the potent neurotoxin tetrodotoxin (TTX) throughout its body. In the present study, we detected TTX and its analog 5,6,11-trideoxyTTX (TDT) in the maternal tissues and eggs of H. cf. fasciata, the latter showing individual variation in toxin profiles. Immunohistochemical analysis of H. cf. fasciata eggs using an anti-TTX polyclonal antibody revealed that TTX is primarily localized in the chorion, the outermost egg layer, implying its role in chemical defense against predators. In egg predation experiments, the goby Bathygobius fuscus took the eggs into its mouth but immediately spat them out, whereas Favonigobius gymnauchen frequently ingested the eggs. These results suggest that the predation deterrence by TTX in H. cf. fasciata eggs is predator-dependent. In addition, VGSC (Nav1) sequences revealed that H. cf. fasciata shares three amino acid substitutions at putative TTX-binding sites with other Hapalochlaena species, consistent with TTX resistance.