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Do the (Bipedal) Locomotion

Two-“legged” walking in octopods.

By Sydney L. O’Brien and C. E. O’Brien, The School for Field Studies Center for Marine Resource Studies, South Caicos

Have you ever seen a walking coconut? If you have, chances are it was actually an octopod, running on two arms across the sand. This may sound like an odd piece of science fiction or a Saturday morning cartoon, but keep reading and you will see just how versatile octopod locomotion can be.

Octopods have the ability to “walk” or “run” on two arms across the ocean floor.

What is bipedal locomotion?

Walking and running on two feet, known as “bipedal locomotion,” is often thought of as a characteristically human or great ape behavior, since most amphibians, reptiles, and mammals walk quadrupedally (using four limbs), and many invertebrates use multiple limbs or none at all to locomote. However, bipedalism is occasionally observed in these other groups. For instance, bears will sometimes rear up on their hind limbs for a variety of reasons, including during fights or attacks, in order to prepare to climb a tree or to get a better view of the landscape. 

Bipedalism also occurs in animals you might not expect. The lowly cockroach, which normally scuttles around on six legs, switches to running bipedally when it needs to move at maximum speed, since the extra four limbs tend to get in the way and slow it down. Similar to the cockroach, many lizards can run at much higher speeds on two legs than four. The Common Basilisk Lizard (also known as the Jesus Christ Lizard) can even run across the surface of water on its two hind legs with the aid of air bubbles trapped under flaps between its toes.

Bipedal locomotion has also been documented in octopods, those wacky, eight-armed molluscs known for their color-changing abilities, regenerating limbs, and surprising smarts. Bipedal “walking” or “running” has been recorded in four species of octopods so far: Abdopus aculeatus, the algae octopus of Australia; Amphioctopus marginatus, the coconut octopus of Indonesia; Callistoctopus furvus, the sand octopus of the Tropical Western Atlantic; and Octopus vulgaris, the common octopus of the Mediterranean and eastern Atlantic.  

A tour of the armory: Octopus anatomy 101

Although often referred to as “tentacles” in conversation and popular media, octopods do not have tentacles at all! The eight limbs of an octopus are actually referred to as “arms,” which is a more multi-purpose appendage than a “tentacle,” which specializes in food capture. Octopus relatives—squids and cuttlefish—also have eight arms, as well as two tentacles that can usually be ejected forward rapidly to capture prey. In addition to a difference in function, the anatomy of arms and tentacles vary: arms usually have two or occasionally one row of suckers along their entire length, while tentacles are smooth along most of their length, but terminate in a paddle shape with several suckers or hooks to capture prey. 

This cuttlefish tentacle ends in a paddle-shaped tip.

The eight arms of an octopus are used in a wide variety of tasks, including for locomotion, investigating their surroundings, and to aid in camouflage or mimicry, as well as for catching prey and passing it to the mouth. Intriguingly, the arms do not just have the ability to feel an octopod’s surroundings—they can also taste and smell, thanks to dozens to thousands of flexible suckers lined with receptors like taste buds. These sensory abilities help octopods navigate in their environment, identify mates, and flush out hidden prey. Individual octopods often favor particular arms for certain tasks, much the same way humans are usually right- or left-handed for writing and throwing. To help differentiate between them, researchers refer to the four arms on the left side of the octopus as LI, LII, LIII, and LIV from front to back and RI, RII, RIII, and RIV on the right. 

. . . And I would walk 500 (nautical) miles: Why an octopus walks bipedally 

When they move around, many species of octopods change their color and texture in order to blend into their surroundings or to disrupt their telltale outline as they pass over different types of sea floor, such as sand, coral, or rock. To prevent their motion from giving their presence away, they will also often move at a measured pace, taking their time and avoiding sudden movements that might attract a predator’s attention. Octopus cyanea, the day octopus, for example, spreads its webbed arms, creeps slowly, and changes its body pattern in order to appear to be a moving rock in order to traverse an open stretch of territory. But sometimes octopods need something quicker. For instance, to escape an attack, an octopus may “jet” away by forcefully blowing water out of the siphon, propelling its body rapidly in the opposite direction, often expelling ink as it flees. 

There are also times when they need to move quickly but still want to remain disguised. This is where mimicry and masquerade, in which octopods pretend to be something they are not in order to avoid being noticed or recognized by a predator, can come into play. The aptly named Mimic Octopus is the octopod poster child of this ability, as it can impersonate several other species, ranging from deadly sea snakes to the prickly and venomous lionfish. This tactic, known as Batesian mimicry, allows the mimic octopus to appear dangerous or inedible to potential predators, while not having to go to the trouble of manufacturing sea snake or lionfish venom itself.

This Image shows how the octopod can masquerade as an organism that is relatively uninteresting to predators.

But octopods don’t always impersonate something dangerous. Instead, they sometimes opt to masquerade as an organism that is relatively uninteresting to predators, such as a plant or a rock. By changing their shape, posture, and color pattern, octopods can take on forms that to an untrained eye appear to be a rock, seaweed (algae), or even a coconut. This is often when bipedal locomotion occurs: using only two of their arms as “legs” to “walk” or “run” allows them to use their remaining arms to craft the perfect disguise, contorting into what appears to be a tumbleweed of algae, or tucking them away to imitate the round shape of a coconut. These postures are often accompanied by color changes that enhance the effect, usually by turning a darker color to match the color of the object that they are imitating.

While the effect appears comical to our eyes, all of these contortions and color changes enhance octopod crypsis, which is a fancy word for being sneaky and avoiding detection—an octopod’s primary form of defense. Walking bipedally helps octopods fool many fish predators into thinking that they are inedible algae, allowing them to move stealthily across the seafloor without becoming dinner. Pretty clever for a sea creature!

While the coconut octopus attempts to resemble a round coconut when moving bipedally, the sand octopus, the algae octopus, and the common octopus utilize the “flamboyant display” during their bipedal locomotion. In this display, the octopus raises its front arm pair and twists them into a corkscrew shape, extends its mantle bumps, and holds its other arms close to or under its body. These contortions and texture changes, along with color changes, cause the octopod to resemble a piece of floating Sargassum or another seaweed. The flamboyant display is also seen in many other cephalopods, including cuttlefish and squid, where it can play a role in crypsis or in communication. Although it does not function exactly the same way in every species, the existence of this display in such distantly-related groups suggests that it is evolutionarily-conserved, meaning that it plays an important role in cephalopod survival. 

Walk this way: How do octopuses walk bipedally?

Octopods are molluscs, a group characterized by a soft body and a hard outer shell. However, over the course of their evolution, cephalopods lost this protective covering in favor of more sophisticated “squishy” defenses, such as crypsis, enhanced predator detection abilities (sight and “smell”), and a complex nervous system. Cephalopods do not have the rigid system of either internal or external hard structures (bones or exoskeletons) that we and many other animals use to get around. In human bodies, our bones work together with our cartilage, muscles, ligaments, and tendons to produce movement. The muscles are attached to the rigid skeleton which provides anchor points and support against which muscles can push and pull. Octopods, by contrast, lack this support, and instead utilize the pressure created by the fluid-filled tissues inside their bodies to provide support for limb movement. 

C.E. O’Brien photographed this Octopus insularis off South Caicos at the moment when it inks and begins to jet away from danger.

Instead of bones or an exoskeleton, octopods are composed almost entirely of soft muscle and tissue. The muscles of octopod arms are organized into two groups which perform opposing but complementary actions to produce movement: While one group of muscles contracts to provide force, the other group relaxes, causing it to elongate and stretch, thus causing limb extension. The general lack of hard parts in their bodies allows octopods a wider range of motion than other species, since they are not limited by the range of motion of a joint, but can bend a limb almost anywhere along its length. Moreover, the lack of hard parts in the octopod body (except the beak) gives them the ability to squeeze through any gap or hole in the substrate that is wider than that beak. 

So, if they don’t have any rigid structures, how do octopods use their limbs to walk or run? Rather than utilizing bendable limbs with a joint like vertebrates and arthropods do, octopods use either a smooth continuous rolling motion along the length of two arms, or alternate between a stiffened LIV and RIV. In the algae octopus, coconut octopus, and sand octopus, bipedal locomotion is achieved by the octopod rolling backwards along the rearmost pair of arms (IV), while the common octopus “hops” backwards on two arms of the same side, such as RIII and RIV or LII and LIII. 

Stepping into the light: Discovering bipedalism in other octopods

Bipedal locomotion has now been documented in four genera of octopods living in three ecologically-distinct regions: the Indo Pacific, the Mediterranean, and the tropical western Atlantic. One of these was discovered recently by students and scientists at the School for Field Studies Center for Marine Resource Studies (CMRS) on South Caicos in the Turks & Caicos Islands in the Fall of 2021.

The research team (nicknamed “NoctoSquad”) “stumbled” upon this fascinating behavior while filming Callistoctopus furvus for a directed research project on octopus foraging and skin patterning. In these video sequences, three C. furvus can be seen “walking” bipedally using mainly arms LIV and RIV on fifteen separate occasions from anywhere between one and a dozen steps. While doing so, the octopuses turn brown and engage in the flamboyant display, causing them to resemble strands of brown algae floating nearby. Recognizing the importance of the first sightings of this behavior in this genus and species of octopus, faculty and staff of the CMRS published their observations in the Journal of Molluscan Studies. Their observations were also notable in that the individuals that engaged in this behavior were distinctly larger than was thought possible for bipedalism to occur in octopuses.

Bipedalism is likely even more widespread among octopods than currently recognized. More observations of octopod behavior in the wild are needed, especially as new species of octopods are discovered or reclassified every year. (Currently there are around 300 species.) Formal research is critical to this effort, but so too is “community-” or “citizen-” science. Anyone living in proximity to an ocean can grab their mask, fins, and camera and non-invasively (no touching!) document cephalopods or other marine animals in their native habitats, no credentials needed. So, get out there and explore!

This article was originally published on OctoNation.com, https://octonation.com/bipedal-locomotion-two-legged-walking-in-octopods/?fbclid=IwAR2LHjf5Sf9tsHy5p_9HI04-zZlTFvvcsbN-2-XZbzgtGKfTFY9JrEGHwFc.

For detailed article references or more information about The School for Field Studies, contact Director Heidi Hertler on South Caicos at hhertler@fieldstudies.org or visit www.fieldstudies.org.



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