This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Perhaps it's time we stopped feeling quite so bad about eating octopus. Octopuses dine on other octopuses, too. And for the first time, that behavior has been caught on video in the common octopus in the wild—three times.
Cannibalistic behavior in the lab setting is well known. This is one of the reasons octopuses can be so challenging to keep and study in captivity, as they demand individual tanks. And—after finding pieces of octopus in the stomachs of other octopuses—scientists have known that it was happening at least occasionally in the wild, too.
The researchers, whose lab I was able to visit in Vigo, Spain while researching my book, were able to capture three different common octopuses (Octopus vulgaris) making a supper of their fellow O. vulgaris during a two-year study of octopus distribution in their area. All of the octo dinners were observed between 12 meters and 18 meters off the coast of Vigo in Galicia, Spain. The incredible finds are described in a paper published online earlier this month in the Journal of Comparative Psychology.
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In the first instance, a diver removed stones from the opening of a den, inside of which found a male octopus weighing about 2 kilograms feasting on an already-dead octopus of less than half a kilogram. This male, startled mid meal, pulled its arms overhead, exposing all of its suckers to the diver. Despite this defensive display, it kept hold of its dinner until sneaking off to camouflage on rocks close by with a raised-texture guise.
In a second video, a diver found an even larger male—about 2.2 kilograms—slowly carrying another octopus (which weighed roughly 0.54 kg) in its arms, beneath its web. But this victim was not yet dead. "The diver realized that the prey was still alive because it poked and moved one of its arms between [a] pair of arms of the predator," the researchers describe in their paper. Upon meeting the diver, the preying octopus "opened its arms and allowed the smaller octopus to escape," they note.
The third film shows a female of about 1.8 kg hiding in her den, which she had blocked off with rocks. The female had been eating the arms of her prey, a dead octopus weighing about a third of a kilogram—despite, herself, missing a few arm tips.
These amazing, if occasionally gruesome, observations reveal that octopuses chose an octopus meal even if there were plenty of other, less feisty food options, such as mussels. But, as the authors point out, even the more docile mussels require more energy to extract than a smaller octopus might to get the same amount of meat. And octopus meat, the scientists note, is higher in protein per ounce than that of mussels.
Additionally, the octopus predator, after bringing back its prey, sealed off its den opening with rocks. This allowed the eating octopus to feed in relative safety and privacy—another advantage of a single, large catch over having to crack and carry smaller bivalves.
In many species of octopus, the smaller male octopus has been known to be vulnerable to a larger female during mating. Earlier this year, for example, researchers described day octopuses (Octopus cyanea) mating, after which, the larger female strangled the male and dragged him to her den (presumably to turn into dinner).
It remains to be seen whether these common octopuses are killing their fellow octopods via suffocation or via venom.
Read more about surprising octopus behavior in Octopus! The Most Mysterious Creature In the Sea.
Illustration courtesy of Ivan Phillipsen
