Functional Specialization and Arm Length in Octopus bimaculoides

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Although the functional specialization of octopus arms may seem unlikely due to the appearance of radial symmetry, there is significant evidence for preferential arm use in various species of octopus. For example, in O. maya, O. digueti, and O. vulgaris, the anterior arms are utilized more frequently for grasping prey and exploring (Lee, 1992; Voight, 1992; Byrne et al., 2006), while posterior arms are more frequently utilized for crawling in O. vulgaris (Levy et al., 2015). Additionally, O. aculeatus and O. marginatus preferentially use two of their posterior-most arms during camouflage walking (Huffard et al., 2005). Mazzolai and colleagues (2013) established a link between arm morphology and specialization, demonstrating that the longest arm was preferentially used in reaching movements in O. vulgaris. Given these observed functional differences and the potential for morphology to provide further insight into octopus arm specialization, the goal of this study was to determine if morphological differences exist between different arm identities (L1-L4, R1-R4) in O. bimaculoides. To test this hypothesis, the relationship between arm length and other factors including: arm identity, body mass, sex, anterior versus posterior location, and left versus right side was analyzed statistically. The data set included 111 intact arms from 22 wild-caught specimens of O. bimaculoides (11 males and 11 females). Simple linear regressions were performed to test for a relationship between these morphological features and arm length and an analysis of covariance was performed to test for differences in arm length by arm identity. Mass demonstrated a significant linear relationship with arm length and a one-way ANOVA demonstrated that arm identity is significantly correlated with arm length. Moreover, an analysis of covariance found that independent of mass, arm identity has a significant linear relationship with arm length. Thus, despite an overall appearance of radial symmetry, arms of different identities do not have statistically equivalent lengths in O. bimaculoides. Furthermore, differences in arm length do not appear to be related to sex, anterior versus posterior location, or left or right side. These results call into question the existing practice of treating all octopus arms as equivalent by either using a single arm measurement as representative of all eight, or calculating an average length, and suggest that morphological analyses of specific arm identities may be more informative. Further investigation into the functional specialization and morphology of octopus arms is warranted.