Describing a new species for science is not quite as easy as it was in the days of 17th- or 18th-century naturalists. But that just means we have to look a little more closely. Such as, into an organism's DNA.

And rather than hunting through the dense jungles for years, scientists can, with a little genetic scanning of organisms already preserved in museums and university labs, turn up surprising new species—sometimes ones that have been living in plain sight for millions of years.

A new study, published online June 25 in PLoS ONE, finds that the common Sydney octopus (Octopus tetricus—which, on the west coast of Australia is sometimes called the common Perth octopus—and also occasionally goes by the awesome common name of the gloomy octopus) is not a single species, but in fact is two genetically distinct species. The researchers also discovered that another octopus species (O. gibbsi), which lives off the coast of New Zealand is no species at all, but rather just a remote population of eastern common Sydney octopuses.

The Australia-based scientists studied small tissue samples from 13 different common Sydney octopuses from the east coast, 17 from the west coast and 4 O. gibbsi that had been collected from the northern coast of New Zealand. (All of the samples came from museum or laboratory collections.) The researchers looked at partial sequences of five different genes and found the west coast octopuses to be a unique species—and O. gibbsi to be confidently part of the east coast clan.

The split, it seems, is hardly recent in evolutionary terms. The researchers estimate it to have occurred between 3.2 million and 6.9 million years ago—somewhere between the time our first hominin ancestors split off from the chip and bonobo line and when the famous Australopithecus Lucy was walking the plains of Africa on two legs.

That genetic divergence corresponds in time to the appearance of a land bridge that connected Australia and Tasmania—effectively splitting off what might then have been a single species of octopuses. Later periods of glacial cooling could have then pushed the two respective octopus species north, toward warmer waters off the coasts, separating them further, the researchers posit. Even today, the southern coast of Australia has areas, such as the Great Australia Bight, that bring in cold waters that might serve as a temperature barrier to further foster separation.

Although the females from all of these groups appeared quite similar, the males had some defining features—namely in their reproductive arm, the hectocotylus. In the western octopuses, the males had "significantly greater sucker numbers" on this arm than did the eastern octopuses, the scientists noted.

But why bother with these seemingly trite taxonomic distinctions? Not only does it help scientists properly describe these creatures, but it might also help to keep their and other marine life populations stable. As the researchers point out, it's easier to keep an eye on something if you can properly define it: "taxonomic resolution within this group will aid in the management of these marine resources," they wrote.

In the course of the study, the scientists also make the first confirmed report of the common Sydney octopus in Tasmania, where ocean waters are warming at four times the global average. This heating up has already brought in other new sub-tropical species, such as new box jellyfish and lobsters. Discovering just what a new species of octopus will mean for "native ecosystems and commercial fisheries," the researchers wrote, "should be given high priority."

In the meantime, the ousted west coast octopus population—which may now be feeling gloomy, indeed—is in need of a new name. My vote goes for O. princeius (common name: the octopus formerly known as). Any other suggestions?

Read about how octopuses can actually change their own RNA in Octopus! The Most Mysterious Creature In the Sea.