What is dead may never die.Scott Portelli

Every winter in Spencer Gulf, a large inlet intruding into Australia’s south coast, hundreds of thousands of giant cuttlefish gather to breed. They’re about the size and weight of a corgi, with ever-changing displays of shadow and colour rippling across their dynamic skins. At the height of the breeding season, these amorous, multi-armed, living rainbows can get so numerous that there’s one of them in every square meter of water.

But lately, these mating swarms have dwindled to a small fraction of their former glory, and no one knows why. Pollution, warming waters, and a dearth of prey are all possibilities. But Bronwyn Gillanders from the University of Adelaide suspected that the decline might just be part of a natural cycle, a downward trend stuck between upward ones. She couldn’t test that idea, since no one had any long-term data on giant cuttlefish numbers. But such data did exist for other cephalopods—octopuses, squid, and other species of cuttlefish. Gillanders’s team member Zoe Doubleday pulled it all together, by scouring earlier studies and contacting other scientists.

To her surprise, she didn’t find evidence of either cycles or declines. Instead, she found that since the 1960s, cephalopods numbers have been increasing.

There had been anecdotal murmurs about such an upward trend before, but Doubleday’s data threw it into stark relief. The rise was obvious when the team analyzed data from both fisheries and other sources. It was there in both northern and southern hemispheres. It applied to species that stick to the same patch of ocean ocean floor, those that swim in the water layer just above the bottom, and those that patrol large stretches of open ocean. And it applied to every major group of cephalopod: For the most part, cuttlefish are doing well, squid are on the up, and octopuses are ascendant.

“You wouldn’t have expected to see the same trend across these different groups,” says Gillanders. “It does potentially suggest that a large-scale, global phenomenon is affecting all of them.”

What could that be?

Changing climate is an obvious answer. Like the rest of the planet, the oceans are getting warmer. Higher temperature could accelerate the life cycles of cephalopods, allowing them to grow faster and breed sooner.

It’s also likely that they are the unexpected beneficiaries of our knack for yanking pretty much everything else in the oceans. Think about all the fish species that we eat—cod, haddock, salmon, sole, and the like. Collectively, their numbers have halved since 1970. We also tend to fish down the food chain, taking out large predators like sharks, tuna, and swordfish first, before moving our way down medium-sized and smaller species. As these hunters are removed, their prey can flourish, cephalopods included. Overfishing, in other words, releases the krakens.

“It makes sense once you stop and think about how ocean food webs work, but it goes against the conventional wisdom that global fisheries target pretty much everything and, unless well managed, tend to overfish things,” says Benjamin Halpern from the University of California, Santa Barbara.

Cephalopods can respond to such changes quickly because their lives are as dynamic as their skins. They have fast growth rates, short lives, flexible reproductive cycles, voracious appetites, and broad-ranging tastes in prey. “These are characteristics which provide the capability to rapidly fill ecological niches that have been vacated by other groups,” writes Paul Rodhouse from the British Antarctic Survey.

And what about the giant cuttlefish that started all of this? “They turned around,” says Gillanders. They’re not back at their previous levels, but “in the last two years, we saw an increase in abundance.”

That would seem like good news, but no one’s cheering. The data show that “global cephalopod fishery catches seem to have leveled off recently,” Rodhouse says, “So, this is not a sensational ‘cephalopods are taking over the world’s oceans’ story.”

The consequences of a cephalopod boom are also hard to predict, because the animals themselves are so unpredictable. They could slash the numbers of the small fish and shellfish that they hunt. They could fuel a resurgence of larger predators that hunt them. They could face an imminent bust if they attract the attention of people who no longer have fish to catch.

“I guess if you're a squid or octopus fisherman, these increases may seem like a great thing,” says Halpern. “But such dramatic global changes are quite worrisome. When we change the oceans this much, we move things into a new state—one that we know much less about. We might have more squid on our plates in the short run. What are we risking losing in the long run?”

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