Flexible Arm Use in Octopuses

Octopuses are widely regarded as animals with relatively high cognitive ability and complex behavioral repertoires, which has long attracted considerable scientific attention. Much of an octopus's body mass is composed of its arms and the numerous suckers distributed along them. These structures participate in nearly every aspect of the animal's life, including locomotion, prey capture, den construction, camouflage, communication, and reproduction.

An octopus arm is a complex structure composed of nerves, four distinct muscle groups, and surrounding connective tissues. These four muscle systems include transverse muscles, longitudinal muscles, oblique muscles, and circular muscles. Working together, they allow the arm to elongate, shorten, bend, and twist, forming the basis for a wide range of more elaborate movements.

Octopus vulgaris（圖片來源：albert kok，採用 CC BY-SA 3.0 授權。）

Through the coordinated action of nerves and muscles, octopus arms achieve remarkable flexibility and an extremely high number of degrees of freedom. This allows octopuses to perform complex movements within only a few seconds. Because of this extraordinary versatility, octopus arms have become an important research subject in fields such as animal behavior, neuroscience, and biomechanics. Although many studies have examined octopus arm function, most observations have been conducted under artificial laboratory conditions. How octopuses use their arms in natural environments has remained far less understood. To address this gap, one study analyzed underwater video recordings of octopus behavior, examining how octopuses use their arms in natural habitats and thereby building a more comprehensive understanding of their behavioral capabilities.

Six different filming locations（圖片來源：Bennice, C. O et al. (2025). ，採用 CC BY-NC-ND 4.0 授權。）

The video material analyzed in the study was recorded between 2007 and 2015. Professional divers filmed octopuses at depths of approximately 1 to 10 meters in sunlit shallow seabed environments. Six filming locations were included: Vigo in Spain, southern Florida, Puerto Rico, the Cayman Islands, and the islands of Saba and Bonaire in the former Netherlands Antilles.

Because octopuses have relatively short lifespans, it is unlikely that the same individuals could be followed continuously across many years. Consequently, researchers attempted to record as many individuals as possible during the study period. The octopuses observed were accustomed to the presence of divers and therefore behaved naturally. The species documented in these recordings included the common octopus (Octopus vulgaris), the American octopus (Octopus americanus), and the Caribbean reef octopus (Octopus insularis), which are broadly similar in body size and general morphology.

Across 3,907 movements recorded from 25 octopuses, researchers identified 15 distinct behaviors and 12 types of arm movements. Most of these movements appeared in multiple behavioral contexts. In many cases, a single arm—or adjacent arms—could perform several movements simultaneously while the animal carried out a particular behavior. Among all recorded movements, five occurred most frequently and together accounted for 78% of the total: extension (19%), lifting (18%), lowering (17%), contraction (14%), and curling (10%). These five movements appeared across the majority of observed behaviors. Other actions, such as umbrella spreading, rolling, grasping, stilt-walking, and tiptoeing, were much less common.

Octopus behaviors and arm movements. (a) Relationships between the 15 behaviors and 12 arm movements. (b) Octopuses performing multiple movements simultaneously with several arms（圖片來源：Bennice, C. O et al. (2025). ，採用 CC BY-NC-ND 4.0 授權。）

When the arms were grouped into left and right sides, the frequency of movements was nearly identical, accounting for 49% and 51% of actions respectively. A Mann–Whitney U test indicated no statistically significant difference between the two sides. However, a clear difference emerged when comparing anterior and posterior arms. The anterior arms accounted for 64% of movements, whereas the posterior arms accounted for 36%. Movements such as extension, lifting, lowering, and curling were used substantially more often by the anterior arms. In contrast, rolling and stilt-walking occurred more frequently in the posterior arms.

As previously mentioned, the movements of octopus arms arise from combinations of four fundamental deformation patterns. All four patterns can occur in any of the eight arms. Among them, bending was the most common, representing about 70% of observed deformations. This was followed by elongation (22%), shortening (6%), and twisting (2%). These deformation types also varied in frequency along different regions of the arm; for example, bending occurred more frequently in distal regions.

The analysis indicates that octopus arms can generate 12 distinct movements through combinations of these four deformation modes, enabling the animals to perform at least 15 different behaviors. Individual arms—or multiple arms simultaneously—can execute several movements at once, reflecting a highly sophisticated system of arm coordination. Earlier studies had found limited evidence for functional specialization among cephalopod appendages. However, the present analysis clearly indicates a division of roles between anterior and posterior arms. The results suggest that the anterior arms are used primarily for exploration and grasping, whereas the posterior arms are more involved in locomotion.

Laboratory experiments had previously suggested that captive octopuses may show lateral preferences when using their arms. In contrast, this study found no such left–right bias in octopuses observed in natural environments. Instead, the arms on both sides tend to operate in coordinated pairs.

Octopuses inhabit a wide range of ecological environments and must therefore possess highly adaptable behavioral abilities. Their prey frequently hides in crevices or concealed locations, making arm flexibility essential for successful foraging and daily survival. The present study highlights the remarkable versatility of octopus arms. Future research will need to explore how different habitat types influence arm use, examine additional octopus species with different body forms, and compare their behavioral strategies. Such work will greatly deepen our understanding of the functional diversity of octopus locomotion and manipulation.

Author: Bai Leng

Reference:

Bennice, C. O., Buresch, K. C., Grossman, J. H., Morano, T. D., Hanlon, R. (2025). Octopus arm flexibility facilitates complex behaviors in diverse natural environments. Scientific Reports.