Bipedal locomotion in Octopus vulgaris: A complementary observation and some preliminary considerations

Abstract Lacking an external shell and a rigid endoskeleton, octopuses exhibit a remarkable flexibility in their movements. Bipedal locomotion is perhaps the most iconic example in this regard. Until recently, this peculiar mode of locomotion had been observed only in two species of tropical octopuses: Amphioctopus marginatus and Abdopus aculeatus. Yet, recent evidence indicates that bipedal walking is also part of the behavioral repertoire of the common octopus, Octopus vulgaris. Here we report a further observation of a defense behavior that encompasses both postural and locomotory elements of bipedal locomotion in this cephalopod. By highlighting differences and similarities with the other recently published report, we provide preliminary considerations with regard to bipedal locomotion in the common octopus.


| INTRODUC TI ON
Invertebrate molluscs typically show a very limited flexibility in their movements. A thick external shell dramatically constraints locomotory patterns in most members of this group. In octopuses, however, the molluscan shell is absent. The bodies of these animals lack of any rigid structure, with the exception of a cartilaginous "skull" and a chitinous beak that are located in the head (Wells, 1978). Furthermore, in octopuses the molluscan foot was transformed partly into a set of eight suckered appendages and partly into a mobile funnel that allows the fast ejection of water from the mantle (Shigeno et al., 2008). As result of these adaptations, octopuses exhibit an extraordinary versatility of movements and postures. For instance, each arm can be elongated, shortened, bent, or twisted independently from the others and with virtually infinite degrees of freedom (Sumbre et al., 2001), adopting different postures, maneuvres, and locomotory patterns (review in Borrelli et al., 2006;Mather, 1998). At the same time, the mantle can assume shapes as different as a swelled and vertically oriented sack (i.e., mantle ballooning, review in Borrelli et al., 2006) or a flattened sack oriented parallel to the substrate (i.e., mantle rounded, Packard & Sanders, 1971), to mention some.
Locomotion is equally diverse in octopuses. These molluscs can crawl across the substrate via coordinated pushing and pulling actions performed by arms and suckers, swim forward or backward by expelling expel water jets from the siphon, and even walk bipedally on two arms (Huffard, 2006; for a review, see Hanlon & Messenger, 2018;Levy & Hochner, 2017). The latter is an extremely sophisticated mode of locomotion, from a biomechanical perspective.
Bipedal locomotion in the octopus is not produced by the action of antagonistic muscles against a rigid skeleton as in vertebrates; rather, it is achieved through the concerted action of differently oriented components (Huffard et al., 2005) within a muscular hydrostatic system (Kier & Smith, 1985). The differential contraction of transverse, longitudinal, and oblique bundle of muscles allows the octopus to stiffen and relax different segments of the same arm, thereby supporting bipedal walking (Huffard et al., 2005).
While engaging into bipedal gates (also termed rolling gates), typically through the action of arms IV (Huffard et al., 2005), octopuses may assume deceptive appearances. Abdopus aculeatus often exhibits a highly disruptive Flamboyant-like body pattern (sensu Packard & Sanders, 1969, 1971 with spread and helically coiled arms tips (arms I-III), mottled coloration and raised papillae (Huffard, 2006;Huffard et al., 2005). Note, however, that body patterns expressed by this species during bipedal locomotion are variable and can encompass also arms I-III held close to the body and a striped pattern (dark coloration with pale medial stripes, Huffard, 2006). On the other hand, Amphioctopus marginatus displays a more rounded and homogenous appearance, with arms I-III tucked on the side or below the body, usually exhibiting smooth skin and brownish coloration with dark stripes on the arms (Huffard et al., 2005). The postural, chromatic, and textural features expressed by these octopuses might resemble the appearance of distinctive elements of their environment (e.g., detached algae in A. aculeatus, coconut shell in A. marginatus), such that when the locomotory component of the bipedal walking is also taken into account, octopuses seem to impersonate a loose environmental element dragged around by the current. As a result, predators' ability to form a specific search image for these species may be hindered (Huffard, 2006). Thus, bipedal locomotion in octopus may constitute an anti-predatory strategy laying in between crypis and flight response, respectively, representing, the primary and (one possible) secondary defense tactics (Huffard, 2006).
In addition, the fact that arms I-III are typically not involved in the locomotion, and thus free for other purposes, may provide an added value in terms of defense for the octopus (e.g., "arm-slap," Woods, 1965;"punching," Sampaio et al., 2020).
Until recently, bipedal locomotion in octopus had been observed only in the two aforementioned species. Yet, new evidence indicates that this peculiar form of locomotion may also be part of the behavioral repertoire of one of the most iconic cephalopod, the common octopus (Octopus vulgaris).
Formerly described as a single taxonomic unit with a cosmopolitan distribution (e.g., Norman, 2000), O. vulgaris is now considered a group encompassing multiple cryptic species (Amor et al., 2019;De Luca et al., 2014, 2016, including O. vulgaris sensu stricto (Mediterranean and eastern North Atlantic) and O. vulgaris Type III (South Africa). In addition, some populations that were initially considered part of O. vulgaris species complex are currently treated as distinct species (e.g., Octopus sinensis, Gleadall, 2016).
In the Atlantic waters of Spain, Hernández-Urcera et al. (2020) observed a small-sized O. vulgaris (sensu stricto) performing a defense behavior that has been classified as bipedal locomotion. While keeping contact with the bottom, the octopus engaged in a backward rolling gate mainly through the action of arms IV. However, arms III and even II also appear to be involved in the locomotion (see Video S1 in Hernández-Urcera et al., 2020), such that it is possible that the observation might represent a mixture of bipedal and multi-arm-walking (sensu Huffard, 2006 Note: For definitions see (Borrelli et al., 2006) a Note that the postural components of this pattern are remarkably similar to a posture that has been described in Abdopus aculeatus (cfr. fig. 4a, Huffard, 2006). b The textural and postural components of this appearance closely resemble those expressed by Octopus cyanea in the "Moving Rock," a cryptic body pattern through which an octopus assumes the appearance of a rounded coral/rock and slowly moves on the substrates by using the tips of its arms (Hanlon et al., 1999 Figure 1). For instance, in one case an octopus directed water jets toward the camera while exhibiting a dark and smooth skin, with arms I-III curved and interbrachial web maximally spread ( Figure 1a).
Here, we report a further observation of a defense response in O. vulgaris sensu stricto from Mediterranean Sea that encompasses postural and locomotory elements of bipedal walking (Video S1).

| Study animal and site
On 10 May 2010, in the morning, we were conducting a SCUBA diving survey at Capri (Italy), to collect data for a study on octopus camouflage abilities (Josef et al., 2012). . We started to observe the octopus and video-recorded its behavioral response (Video S1).

| Analysis
To describe the observed behavioral response, we conducted a frame-by-frame analysis of the video on Avidemux (ver. 2.7.6; http:// fixou net.free.fr/avide mux/). The chromatic, textural, postural, and locomotory elements exhibited by the animal were categorized based on definitions by Packard and Sanders (1971) and those reviewed in Borrelli et al. (2006).

| RE SULTS AND D ISCUSS I ON
The animal reacted to our presence by remaining still on the bottom and exhibiting a disruptive body pattern (Figure 2a). The following chromatic (c), textural (t), and postural (p) elements were expressed, c: frontal white spots, mantle white spots, white papillae, arm white spots, arm bars; t: long mantle papillae, long head papillae, papillae on side, back fin; p: arms loose, mantle ogive (Borrelli et al., 2006;Packard & Sanders, 1971). At this stage, water flushes from the siphon were also directed toward us (i.e., funnel directed toward external stimulus, Packard & Sanders, 1971).
Next, the animal gradually raised its arms from the substrate and  (Figure 2j-l) but also during hopping (Figure 2d-g), similarly to what has been observed in A. aculeatus by Huffard (2006).
It should be noted that nonjet-propelled hopping in the octopus might bear some similarities with underwater punting (Chellapurath et al., 2020;Martinez et al., 1998), a type of locomotion described in crabs. In both cases, a thrust-generated by the limb(s) acting against the substrate-allows the body to displace by gliding away in the water. Considering that octopuses can generate the thrust force not only through the muscular action of the arms but also through jet-propulsion, it would be intriguing to characterize the kinematics of hopping in these animals, perhaps in comparison with underwater punting by crabs (e.g., Chellapurath et al., 2020) and/or bipedal locomotion in octopus (Huffard et al., 2005). This may be a particularly interesting comparison given that octopuses are only slightly negatively buoyant.
The differences in locomotory patterns between the observation reported here and the one made by Hernández-Urcera et al. (2020) F I G U R E 2 Still images describing the behavioral sequence. See main text and Video S1 for details are intriguing given that the two behaviors were defensive responses triggered by the same stimuli (i.e., SCUBA divers). It is possible that the lack of a part of arm III L in our octopus might have to some extent limited the locomotory ability of the animal, thereby favoring jet-propelled hopping and swimming over continuous rolling gates.
Alternatively, it is also possible that specific features of the substrates might have played a role. The observation by Hernández-Urcera et al. (2020), as well as the reports of bipedal locomotion in other species (Huffard, 2006;Huffard et al., 2005) Note that this is a fair, although crude, categorization given that this cephalopod can exceed more than five kilograms of body weight (Jereb et al., 2014). Further, the appearances assumed by the animals during locomotion encompassed chromatic, textural, and postural components of the Flamboyant body pattern (e.g., frontal white spots, rough skin, arms I twisted; Table 1).
According to Packard and Sanders (1971) Finally, considering that cephalopods are known for adjusting their anti-predatory according to the hunting strategies of predators (Langridge et al., 2007;Staudinger et al., 2011; for a review, see Amodio et al., 2020), it would be particularly interesting to investigate whether bipedal locomotion is flexibly exhibited depending on the kind of threat, or ecological context, and whether octopuses are more likely to rely on this locomotory strategy to achieve crypsis while moving (Borrelli et al., 2006;Hanlon et al., 1999;Van Heukelem, 1983), in response to a visual predator relatively to a chemosensory predator.

ACK N OWLED G M ENTS
We are grateful to Marino Amodio for producing the sketches of

CO N FLI C T O F I NTE R E S T
Authors declare that they have no conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
Video of the defense response by Octopus vulgaris in Capri, Italy: Supporting Information (Video S1).