Sixteen-arm Tussle

Coconut Octopus in bivalve shell.
The amazing Coconut Octopus, Amphioctopus marginatus

Note: Portions of this article were originally published in Asian Diver magazine 2006 and in Scuba Diving magazine 2008.

While exploring the pumice plain of Lembeh Strait in northern Sulawesi, Indonesia, I watched a coconut roll down the steep sandy slope of Teluk Kembahu Bay. Even though much of the Strait’s mountainous terrain is fringed with copra plantations, and huskless shell halves commonly litter the seafloor, I had never seen an intact coconut underwater, much less one tumbling along the bottom.

Coconut Octopus peering out of its coconut shell home
Yep, there was an octopus in there.

I caught up with the wayward nut as it came to rest at the edge of a sponge bed, picked it up and inspected the brown orb in the palm of my outstretched hand. The overlapping edges of the mismatched halves offered a clue: A Coconut Octopus must live inside! I tested my theory with a tug, and sure enough the two sections, gripped heroically from inside by eight sucker-lined arms, wouldn’t budge. So I set the nut down, backed away and settled in for a wait. I passed the time contemplating the rather fanciful notion that the cephalopod inside just might have intentionally instigated its unorthodox roll down the slope. If so, it would represent an unexpectedly ingenious adaptation for an invertebrate. 

Five minutes later the halves separated slightly and an eye peered out. Eventually the shells opened wide and the occupant appeared in all its Coconut Octopus glory. Then, even more amazing than the maverick coconut, the octopus wrapped six arms around its body and walked away on its two remaining appendages. Although impressed, I naively chalked up the curious behavior to a chance happening and put it out of my mind.

Coconut Octopus walking
Walking away on two legs.

Not a week after returning to the States, National Public Radio broadcast an interview with Dr. Crissy Huffard, then a graduate student at University of California, Berkeley who had recently documented the first bipedal walking behavior in octopuses in the March 2005 issue of Science. By coincidence, her paper described the same behavior by the same species, Amphioctopus marginatus, that we observed in Indonesia. 

Making the news item even more interesting, her colleagues at the university investigating applications for soft robotics took a keen interest in how the octopus was able to pull off this two-legged feat. Basing her study on a short video captured by National Geographic filmmaker Bob Cranston, Ms. Huffard offered her finding: The octopus used the outer halves of its two back arms like tank treads, alternately laying down a sucker edge and rolling it along the bottom, while tightly wrapping the remaining six arms around their bodies. She speculated at the time that the animals employ the behavior to mimic a coconut while they make their escape from predators, or, in my case, a pesky diver.

Coconut Octopus in coconut shells
If a bivalve shell isn’t available, use coconuts!
Coconut Octopus in bivalve shells
The perfect home.

Unlike most of their relatives that live and hunt around reefs with plenty of hiding holes, Coconut and other sand-dwelling octopuses bury beneath the sand to avoid predators. As added protection, these eight-armed sand dwellers historically utilize abandoned bivalve shells to encapsulate their bodies while beneath the surface. More than likely, at some point after copra plantations proliferated in the region, octopuses began taking advantage of the unusual abundance of processed shell halves.

Coconut Octopus walking away with its home.
A portable home.

During times when bivalve shells and coconut halves are in short supply the animals, especially younger individuals, resort to natural and artificial litter, such as shell fragments, glass shards, shoe soles, plastic containers and even toothbrush handles, to cover their bodies. Occasionally when our observations became too intrusive, an octopus would clasp its coconut or bivalve halves to its sides and stride away on extended arms, in a behavior dubbed stilt-walking. The animal’s utilization of random materials and transporting them for later use has been asserted by a group of Australian scientists to be the first documented utilization of tools by an invertebrate.

A tiny Coconut Octopus sheltering in an abandoned bottle.
A tiny Coconut Octopus finds shelter inside a discarded bottle.

On our very first dive in Lembeh the following summer, we chanced upon a coconut octopus no larger than a golf ball, squeezed inside the neck of a beer bottle. Like a beckoned genie, the little fellow popped out of the opening and instantly changed from a golden hue into deep reddish brown to better match the dark bottom. Fearing that it was about to bolt, Anna and I eased back. The retreat worked; the octopus slipped Houdinilike back into the bottle and assumed its original complexion. Finding shelter inside discarded bottles and plastic containers is common practice for small recently settled Coconut Octopus. As they mature the youngsters begin cobbling together starter homes from an assortment bottom trash. It’s only later with size that they graduate to coconut halves and large bivalve shells.

Coconut Octopus with its shelter
A little green glass to brighten up the home.

Our run of octopus luck continued with the discovery of two large individuals that, although seeming to be at perpetual odds, chose to hang around together. When we first approached, the larger of the pair jumped out of its big, impressive matching bivalve shells, accented with a large piece of green glass and charged its partner forcing it to abandon its modest collection of shell fragments, a toothbrush handle, and a flattened tin can. The assailant rifled through the material and at length commandeered the flattened can. Perturbed into action, the smaller octopus approached the culprit. Undaunted, the can thief stretched its body to an unimaginable height, and with a flash of chromatophores turned the left side of its dark body white. The message was sufficient to send the aggrieved party slinking back to what remained of its burgled home. 

The following morning, we found the thief still in possession of the can. Its companion had replaced the purloined piece with a flip-flop strap. When Anna and I returned in the afternoon, both homes were empty. Following a brief reconnaissance, the twosome was found foraging a short distance away. The pair seemed so intent on the hunt that they paid us little attention.

Coconut Octopus foraging
Coconut Octopus foraging.

After trailing them for a bit, their primary hunting strategy became clear: On approaching a small algae or sponge thicket outstretched arms searched the clump. If a tasty morsel, such as a crab or shrimp, was detected a tent of tentacles was thrown over the bush as probing arm tips chased the crustacean into the waiting umbrella of arms.

Coconut Octopus burying in the sand
Suddenly, the smaller of the two buried itself in the sand.
A pair of Coconut Octopuses
The larger octopus attacked.
Coconut Octopus trying to steal food from the other.
Sixteen-arm tussle.

Almost an hour into our dive, the smaller octopus abruptly buried itself up to its eyes. Once it emerged, its partner galloped over and pounced, enshrouding its sidekick’s body, except for a lone eye that stared out from the entangling arms. As quick as the scuffle began it ended with the antagonist’s departure. The smaller octopus, as if triumphant, raised its body high on a pedestal of arms and remained as still as a statue. With no idea what was happening, I waited and watched for 10 minutes before the encircling arms were drawn back like curtains on a stage, revealing empty bivalve shells—all that remained of a hard-won dinner

Coconut Octopus opens bivalve shell DeLoach
The smaller octopus finished its hard-won meal.

Here’s a video from our BlennyWatcher YouTube channel that was originally created for the magazine article in 2006:

References: Underwater Bipedal Locomotion by Octopuses in Disguise Christine L. Huffard, Farnis Boneka and Robert J. Full Science 307 (5717), 1927 

“Defensive tool use in a coconut-carrying octopus.” By Julian K. Finn, Tom Tregenza and Mark D. Norman. Current Biology, Vol. 19 No. 23, December 15, 2009