Octlantis

Photo credit: Kylie Brown

Below the clear blue waters of Jervis Bay, a gloomy octopus creeps along the sand foraging on the abundant scallops and hunting crabs when she can locate them. Her soft body is vulnerable to all sorts of predators, but she is a master of disguise, and can jet away in an inky instant as a last resort if she needs to. She is a long way from the safety of her den, but these are good hunting grounds and the risk is usually worth the reward. Today however, she spies a large shadow cruising along in the distance. Her skin morphs to resemble the colour and texture of seaweed as she attempts to creep away. The shadow approaches. A wobbegong shark – homing in on the minute electrical signals given off by the involuntary beating of the octopuses’ hearts (she has 3). No matter how good her camouflage is, the shark knows she is here somewhere and circles closer. Too close for comfort now, the octopus flees using her siphon to jet away. The shark, reacting to the sudden movement, gives chase. The octopus knows that she can’t outrun the shark. She must find cover. She ejects a cloud of ink into the water to cover her next dash. This buys her a second but the shark, highly alert now, knows she is here. She inks again and makes another panicked dash. She sees an irregularity in the otherwise ubiquitously sandy bottom and jets toward it. A small rocky outcrop with some kelp growing on it. Shelter. As she dives for cover under the kelp fronds, she sees a crack between the two boulders that make up this tiny oasis. The crack is only a few centimetres wide, but her body is soft and malleable and she knows that the shark will not be able to get to her in there. The shark soars over the rocks. It knows there is food down there, but it can’t get to it right now. Better to continue its hunting elsewhere, it cruises off into the distance. After a while, sensing that it is safe to emerge, the octopus crawls out of the crevice. Having fled in such a hurry, she has no idea where her old den is and having expended so much energy, she needs to feed. The rocky outcrop is surrounded by scallops and mud arks. Hardly even moving from the safety of the crevice, she reaches out a tentacle and plucks a large scallop from the sand. She deftly pries the shell apart and feasts on the meat within. She drops the shell and reaches for another scallop…

Why is Octlantis special?

Photo credit: Kylie Brown

The gloomy octopus, Octopus tetricus, is usually thought of as a solitary animal. Two sites have now been discovered in Jervis Bay, Australia, where O. tetricus gathers in groups of up to 15 animals which are challenging that perception and revealing some interesting and previously unobserved behaviours.

The second site, dubbed ‘Octlantis’ was discovered last year and consists of three rocky outcrops around which the octopuses have built up an extensive bed of discarded shells and human garbage. The discarded shells have provided material for more octopuses to dig their burrows in the sand which they then stabilise using the shells.

Doughboy scallops, a favourite food for the resident octopuses. Photo credit: Kylie Brown

This second site is centred around natural objects whereas the first site (known as ‘Octopolis’) reported in 2012, was formed around an unknown man-made object. The discovery of the second ‘natural’ site shows that the nature of the object around which the octopuses can make their dens is not as important as other factors (e.g. access to an unlimited food source) for the formation of a large group of octopuses. Social interactions (e.g. mating, attempted mating, reaching, eviction, displays and signals) observed at both sites also points to the ability of these octopuses to change their social behaviour from solitary to group living when conditions allow.

Misinterpretation of the report

We published a site description and some behavioural observations earlier this year. Since publishing this article, both sites have been the subject of a number of news reports. The early reports were accurate accounts of our report and welcome coverage of these unique sites. However many later news articles have exaggerated what we reported with the word ‘city’ appearing in a number of headlines (a word not used anywhere in our report). We did not find (nor did we report) octopus cities. The journalistic flare no doubt stems from the nick-names given to the sites (Octopolis and Octlantis), but we did not report these names. The use of the term ‘ecosystem engineering’ from an earlier paper has also likely led to the claim of ‘engineering underwater cities’, though the authors (of the scientific report) used the term in the sense that the piles of discarded shells created by the resident octopuses had significantly altered the habitat.

Ecosystem engineering and ‘intent’

Ecosystem engineering benefited the octopuses, but has also increased the abundance of some fish species, compared to the nearby sandy areas. Photo credit: Kylie Brown

While the octopuses have modified their environment by ‘constructing’ the extensive shell-bed in which other octopuses have dug their dens, it is not clear whether this ‘construction’ is intentional or not. A city on the other hand is constructed cooperatively with the intent to provide a centre for living, business and entertainment, among other things. It is very likely that the ‘construction’ of the octopuses’ shell-bed is simply the result of individual octopuses bringing food back to their dens (a safe place to eat) and discarding the shells just outside. That is, it is the shell bed is likely the unintended result of individual behaviour. This idea of intent will be explored further in a new paper currently in press.

What’s next?

As previously mentioned, the complex social behaviour observed at both sites is unusual for this species, at least as far as we know. The costs and benefits (e.g. energy expenditure vs access to food) of these interactions is yet to be investigated and may help us to understand why the octopuses at these two sites choose to tolerate each other, rather than conforming to the solitary strategy employed by the majority of individuals of the species.

The hard substrate in the middle of the shell-bed, whether it be of man-made or natural origin, appears to have provided a nucleus for the initial settlement of octopuses who have then created an extended shell-bed. Why then do we not observe large groups of O. tetricus in other areas where a solid object has been placed in seemingly ecologically similar areas?

These questions and many others are of great interest to us and are currently being investigated. Watch this space.