The toxins animals possess are used for prey capture and predator defense. However, toxins are also utilized in intraspecific competition related to reproduction in some species. Octopuses of the genus Hapalochlaena possess the potent neurotoxin tetrodotoxin (TTX) in their posterior salivary glands, and their venom, which includes TTX, is considered to function in foraging and predator defense. However, Chung et al. (2025) reported that male H. fasciata bite females during copulatory mounting to immobilize them and reduce their resistance. Their study suggests the possibility that mounting males inject TTX into females because males have larger posterior salivary glands than females, and females exhibit a decreased breathing rate, a typical symptom of TTX poisoning. In contrast, our study measured the amount of toxin in the posterior salivary glands of the Japanese blue-lined octopus (H. cf. fasciata). It showed no significant correlation between gland size and the amount of TTX therein, resulting in no sex differences in the amount of TTX in the glands. Moreover, we found that the amount of biogenic amines (i.e., histamine and histidine), which are present in many cephalopods and cause pain, increased with the size of the posterior salivary glands and was greater in males than in females. These results suggest that enlargement of the male posterior salivary glands is an adaptation to increase the amount of other toxins, such as biogenic amines, rather than TTX.
