The seemingly ubiquitous common octopus (Octopus vulgaris) is our platonic octopus ideal. Even if Plato didn't write about it, Aristotle did. And since then, it has been the most widely studied (and consumed) species.
But contemporary science is complicating things, a new paper, published this month in the journal Frontiers in Ecology and Evolution, details.
The common octopus lives off the coasts of Africa, the Americas, Asia and Europe—from the seas north of the UK, through the equatorial tropics, to the southern tip of Africa.
The vast distances between these far-flung populations have led to speculation that perhaps the vulgaris clan may have actually already speciated, splitting off one or more groups on their own evolutionary trajectories. Over recent years, in fact, researchers have proposed up to 10 separate groups within the common octopus "species complex."
Getting to the bottom of this uncommon mystery is of interest to scientists and fisheries managers alike. The UN's Food and Agricultural "statistics reveal that there are real problems in the identification of cephalopod species caught by the fisheries," the authors note.
To complicate things further, a species that separated into distinct pockets can develop different physical characteristics while remaining similar enough genetically. Or things can swing the other way, with groups undergoing substantial genetic changes while keeping up basically the same appearances.
Early mitochondrial testing has found genetic differences among common octopuses that live off the coasts of Japan. But these small found variations still were not enough to kick it out of the clan. Deeper testing and more samples are needed to settle the debate.
And depending on which genetic tests are used, researchers find either no difference, or several differences in populations within the Mediterranean region. Scientists had similar outcomes when sampling populations around the western coast of Africa. One possible explanation for different genetic lineages found in one small study is that octopuses could have been deposited from afar after unwittingly stowing away in ships' ballast water holds.
In the Americas things get even trickier, with some octopuses—identical to the O. vulgaris—being labeled as O. americanus even though there is no type specimen for such a species. But, as the authors note, "to date, no genetic studies have been conducted in this area to clarify the relationship."
The ever-vexing O. rugosus was described as a distinct species more than two centuries ago. And its defenders point to shorter arms and different skin texture than the common octopus. But many doubt this other widespread octopus to be anything other than more members of team vulgaris.
So who is the common octopus? This master of disguise continues to elude scientists.
"Genetic approaches will [be] a useful tool to investigate biodiversity," note the authors—as well as to "define the [wild] stocks in order to prevent their overexploitation."
Read more about how octopuses have spread throughout the globe in Octopus! The Most Mysterious Creature In the Sea.
Illustration courtesy of Ivan Phillipsen