We recently published a scientific report of octopuses living together in unusual numbers at a site on the south coast of New South Wales.
Then things got a little out of hand.
The gloomy octopus, named for large eyes that can give the animal a doleful appearance, is the most common local octopus in NSW waters. Octopus tetricus, to use its scientific name, has usually been thought of as a solitary animal, and that has been the stereotype associated with most octopus species for many years.
The recent discovery of a site in Jervis Bay, Australia where these octopuses gather in quite high numbers is challenging that perception, and revealing some striking behaviours.
The site consists of three rocky outcrops, around which octopuses have built up an extensive bed of discarded scallop shells, mixed with some human debris.
We think there is a process of “positive feedback” operating at the site. As scallops are brought back to the site to eat, the discarded shells provide material for additional octopuses to dig burrows. The shells line and stabilise the shaft-like dens. When the site was discovered in 2016, a total of 15 octopuses were present, along with several unoccupied dens.
This is the second site of its kind discovered. The first, reported in 2012, seems to have been formed around a discarded object, now very encrusted, of human origin.
The second site, which is entirely “natural,” shows that the same gathering of octopuses can occur without a “seeding” of the process by a human artefact.
At both sites, octopuses engage in quite complicated interactions – they produce displays, probe each other with their arms, and often try to evict other octopuses from their dens.
Other individuals of this species probably do live more solitary lives – when observed around Sydney, for example, they are almost always alone. This suggests that the octopuses have an ability to individually adapt their behaviour according to their circumstances.
In September 2017, our scientific report of the second site was published, written with our colleagues David Scheel, Stephanie Chancellor, Stefan Linquist, and Matt Lawrence.
This paper received a good deal of media attention, with initial stories fairly accurate. But they seem to have started a self-sustaining process of their own, especially as a couple of early reports used the term “city” in their title. For example: “Scientists discover an underwater city full of gloomy octopuses.”
This was probably influenced by the nickname chosen for the site, “Octlantis,” though our article did not talk about “cities” or anything similar. Soon the authors were fielding interview requests from around the world, wanting more details of the hidden octopus city and the lives of its denizens.
Octlantis is not a city, and no artworks, fences, or buildings have been made. In an era of rapid and unconstrained circulation of information around the internet, often with important political ramifications, the buzz around Octlantis is a reminder of how quickly rumours can arise and feed off each other, generating a literature that becomes less and less accurate at each step.
Accident versus intent
The Octlantis site does raise interesting questions about what the octopuses intend to do, and which effects of their actions are entirely inadvertent. Questions of “intent” are very difficult in work on animal behaviour, but we think some distinctions can be made – provisionally at least – in these terms.
Octopuses collect scallops for use as food. This requires them to make excursions from their den and find their way home. They bring the scallops home to eat, we assume, because it is safer than eating in the open. They also dig dens in the shell bed, and sometimes arrange shells and other objects around the edge of their den.
It seems quite likely to us that the collection of scallops and the building and maintenance of dens are all intentional behaviours (in a low-key sense of that term).
Dens are sometimes maintained with some care, and octopuses will expel debris either by carrying it away, or with use of their jet propulsion mechanism, the “siphon.” But this does not imply that octopuses have any inkling that when they bring scallops back to the site, they are improving the den-building possibilities for themselves or others. Those effects may be entirely inadvertent.
Work is continuing on these animals and their unusual homes. One interesting question is whether other octopus species behave like this in some circumstances.
Another is why we observe groups of gloomy octopuses at these particular sites, and not in other areas where a solid object has been placed on the sea floor in what looks like similar circumstances.
How many octopuses’ gardens are out there, waiting to be discovered?
For wild predators, catching, killing and eating prey can sometimes be a risky business. We can see this on the African savannah, where a well-aimed kick from a zebra can spell trouble for a hungry lion.
But the same can also be true in the ocean, where some prey types are far from helpless seafood.
In particular, a large octopus can be a risky prey for predators to tackle. This is especially so for marine mammals, such as dolphins, which don’t have hands to help them keep control of this clingy, eight-armed prey.
Our new research highlights the development of complex behaviours that allow dolphins to eat octopus, thereby improving their ability to survive and reproduce.
It’s another example of a strategy that helps to drive the success of dolphins in coastal environments around Australia.
Dangers of eating octopus
In 2015 an adult male bottlenose dolphin was found dead on a beach in Bunbury, southwest Australia.
Wild dolphins face many threats in today’s oceans, yet it was a gruesome surprise when we found octopus arms hanging out of the stranded dolphin’s mouth.
An examination by a veterinary pathologist revealed that this otherwise healthy dolphin, known as “Gilligan” to the research team, had suffocated to death while trying to eat an octopus.
As strange as it sounds, this is not the first recorded case of a dolphin choking to death on an octopus in southwest Australia. There have also been several observations from around the world of dolphins facing difficulties while tackling octopus.
So what is it that makes octopus so hard to handle?
Octopus can grow quite large, with some species bearing muscular arms reaching more than a metre long. Each of their eight arms have powerful suction cup-like suckers on the underside, which are normally used to help octopus capture their own prey while crawling along the seafloor.
But when attacked by a dolphin, these suckered arms also help octopuses to defend themselves by latching onto the dolphin’s smooth skin.
When this happens, dolphins have been observed leaping rapidly out of the water before crashing onto the surface in an attempt to dislodge an octopus.
The real problem is that these arms stay active even after an octopus has been mortally wounded. So even while a dead octopus is being processed, the suckers may still be able to find something to stick onto.
Australia’s octopus-eating dolphins
But we’ve observed some wild bottlenose dolphins that have found a way to handle and feed on octopus, with the findings published today in Marine Mammal Science.
These observations were made between March 2007 and August 2013, while we were conducting boat surveys to study the dolphins living off Bunbury’s coast.
Over this time, we observed 45 octopus handling events by dolphins. Most were performed by adults (male and female), although we also saw four juveniles and two calves performing this behaviour.
During these events, dolphins were observed shaking and tossing octopus around at the water’s surface. In some instances, the prey was gripped in the teeth before being slapped down onto the water.
This likely helped both to kill the octopus and to tear it into smaller, more digestible pieces. In other instances, the octopus was tossed across the surface of the water before being recaptured and tossed again.
By tossing the octopus across the water, dolphins avoid letting the octopus latch onto their bodies. This behaviour also likely assists in wearing out the octopus’s reflex responses that make the suckered arms so dangerous to swallow.
Once the prey has been battered and tenderised enough that the arms are unresponsive, it is then safe for the dolphins to proceed with swallowing their catch.
It’s quite a process the dolphins have developed to deal with the octopus. They have a short, fused neck which means they have to arch their whole body to toss their prey out of the water.
Given the danger, why eat octopus at all?
When we looked closely at when these observations were made, we found that the dolphins were targeting octopus more frequently over winter and spring. These cooler times of year are also the octopus’s breeding time.
Octopus are semelparous, which means they slowly become weaker and then die in the weeks after they finish breeding. It is possible that as they become weaker, they also become easier to catch, making them a relatively easy meal for any opportunistic dolphins swimming by.
At the end of the day, octopus are just part of the broad diet eaten by wild bottlenose dolphins.
Dolphins have also been found to use several other highly specialised feeding behaviours, including processing cuttlefish by popping out the cuttlebone, stranding themselves while hunting fish, and using a marine sponge as a tool to probe the seafloor while searching for buried fish hiding in the sediment.
Octopus shaking and tossing is yet another example that illustrates how intelligent and adaptable these charismatic marine predators are.