The toxins present in poisonous and venomous animals have both offensive and defensive functions. The blue-lined octopus Hapalochlaena fasciata has tetrodotoxin (TTX) in various organs, including the salivary glands, muscles and skin, and TTX is assumed to be used in both offence and defence. In this study, we explore the evolution of toxicity in H. fasciata by measuring the levels and distribution of TTX in H. fasciata specimens collected in Japan and by investigating the TTX allocation within the body. TTX was detected in all 13 adults and 3 paralarvae studied. The concentration of TTX was highest in the anterior and posterior salivary glands, and the total amount of TTX was highest in the muscles and skin. Larger individuals had significantly higher amounts of TTX, but the TTX concentrations in the muscles and skin were not correlated with body weight. There were large individual differences in TTX concentration and some individuals showed extremely low TTX levels. These individual differences may be related to regional, seasonal and sexual differences, although the origin of TTX in this species is still unclear. Furthermore, we found a strong positive correlation between TTX concentrations in the posterior salivary gland and TTX concentrations in muscles and skin, with the former being c. 20 times greater than the latter. This suggests that the allocation ratio may not result from a plastic allocation, reflecting the TTX needs of each organ or the amount of available TTX, but rather may, at least partly, be evolutionarily fixed.

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The xanhid crab Atergatis floridus and the blue-lined octopus Hapalochlaena cf. fasciata have long been known as TTX-bearing organisms. It has been speculated that the TTX possessed by both organisms is exogenously toxic through the food chain, since they are reported to have geographic and individual differences. The source and supply chain of TTX for both of these organisms, however, remain unclear. On the other hand, since crabs are one of the preferred prey of octopuses, we focused our attention on the relationship between the two species living in the same site. The aim of this study was to determine TTX concentrations and TTX profiles of A. floridus and H. cf. fasciata, collected simultaneously in the same site, and examine the relationship between them. Although there were individual differences in the TTX concentration in both A. floridus and H. cf. fasciata, the toxin components commonly contained 11-norTTX-6(S)-ol in addition to TTX as the major components, with 4-epiTTX, 11-deoxyTTX, and 4,9-anhydroTTX as the minor components. The results suggest that octopuses and crabs in this site acquire TTX from common prey, including TTX-producing bacteria and/or may have a predator–prey relationship.

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The Japanese pygmy octopus Octopus parvus is a small octopus that occurs commonly along the coast of southern Japan, and is caught using traditional fishing methods. To date, however, there have been no studies on the ecology of this species. In this study, we investigated the annual seasonal occurrence of O. parvus in the intertidal zone of Fukue Island, Nagasaki, Japan, and examined the growth, maturation, and feeding habits of this octopus. We accordingly found that the octopus inhabits the intertidal zone of the study site from August to January, during which time it appears to move from the high to low intertidal zone, and subsequently migrates to the subtidal zone. During low tide at night in the intertidal zone, we observed that the octopuses actively captured and fed on prey, such as fish, crabs, hermit crabs, shrimp, snails, and bivalves. Both males and females were found to be characterized by similar body sizes and growth, although gonadal development occurs relatively earlier in males. However, any mating or brooding behaviors were not observed during the course of the study. Our observations thus indicate that O. parvus uses intertidal zones as feeding grounds for rapid growth and maturation, and may thereafter move to the subtidal zone for spawning.