Asymmetry in the frequency and proportion of arm truncation in three sympatric California Octopus species.

Octopuses have eight radially symmetrical arms that surround the base of a bilaterally symmetrical body. These numerous appendages, which explore the environment, handle food, and defend the animal against predators, are highly susceptible to truncation or loss. Here, we used scaling relationships specific to the arms of three sympatric octopus species of the genus Octopus, to calculate the proportion of arm truncation. We then compared the frequency and proportion of arm losses between different body locations. Truncated arms were found in 59.8 % of specimens examined, with individuals bearing one to as many as seven injured arms. We found a significant left side bias for greater proportion of arm truncation for all species and sexes except in O. bimaculatus males. We also found that sister species O. bimaculatus and O. bimaculoides had a greater proportion of their anterior arms (pairs 1 and 2) truncated, while in O. rubescens, posterior arms (pairs 3 and 4) were more truncated. The mean percent of arm that was truncated was 28.1 % overall but varied between species and by sex and was highest in O. rubescens females (56 %). The arms of O. rubescens also exhibited the steepest scaling patterns, and showed a positive correlation between body size and number of truncated arms. Overall, we show that arm injuries in our sampling of three intertidal species are frequent and asymmetrical, and that when injured, octopus on average lose a considerable proportion of their arm. Through quantifying the variation in arm truncation, this study provides a new foundation to explore behavioral compensation for arm loss in cephalopods.