Size-assortative mating and arm loss in the wild shallow-water octopus Abdopus sp. (Cephalopoda: Octopodidae)

The ability to autotomise and regenerate body parts as a means of escaping predation or renewing damaged tissues has evolved in several animal groups. Some octopuses belonging to the genus Abdopus are known for a high frequency of arm loss and their capacity to regenerate arms. I investigated the mating behaviour and the incidence of arm loss in a shallow-water Abdopus sp. in the wild. Mating occurred at night and at low tide. The male octopus tracked and copulated by repeatedly stretching the hectocotylised arm into the female’s mantle cavity during mating bouts, mostly while the mated female actively moved to forage at the same time. Relatively smaller males adopted sneaking tactics in which they stayed around a mating pair and sometimes gained opportunistic copulations with the female. A significant positive relationship between the body weights of the male and female in each mating pair was detected. Moreover, the proportion of sneaker males that had lost at least one arm prior to mating and their number of lost arms per individual were significantly higher than those of paired males. These results suggest that male arm loss in Abdopus sp. may directly or indirectly influence male competition over females and mating success, resulting in a positive size association between members of a mating pair. This finding would be important when considering the life-history strategies of octopuses.