Seasonal and Sex Differences in Arm Loss Frequency in Two Small Coastal Octopus Species (Octopus parvus and Hapalochlaena cf. fasciata)

Cephalopods, as prey for predators of various taxa, frequently experience arm loss. Arm loss in octopods may arise not only from predation pressure but also species-specific reproductive ecology. Therefore, investigating the occurrence of arm loss may provide insights into various aspects of octopods' reproductive ecology. In this study, we explored the seasonal and sexual differences in arm loss occurrences in two small octopus species, the Japanese pygmy octopus, <i>Octopus parvus</i>, and the Japanese blue-lined octopus, <i>Hapalochlaena</i> cf. <i>fasciata</i>. Notably, <i>O. parvus</i> exhibited a higher arm loss rate compared with previously studied species, potentially attributable to its relatively small size and sublethal predation by large crabs sharing the species' intertidal habitat. Regarding <i>H.</i> cf. <i>fasciata</i>, arm loss in males and females tended to increase toward the spawning season, with males experiencing a more rapid increase. The smaller size of males and arm loss due to male-male conflict related to reproduction and sexual cannibalism during copulation are likely contributing factors. Additionally, the impact of arm loss on egg retention and protective measures may prompt females to adopt behaviors minimizing the risk of arm loss.