On the march: cuttlefish are masters of all they survey David Wiltshire

Octopuses and their tentacled brethren are taking over the seas, as ocean temperatures climb and humans snaffle up their natural predators.

Zoe Doubleday, a marine biologist at the University of Adelaide in Australia, and her colleagues were studying an iconic local species, the giant Australian cuttlefish, which had been in decline for several years.


Doubleday wanted to see whether it was part of a larger cyclical trend in global populations, so she looked at data from surveys and from cephalopod fisheries and cephalopod bycatch in finfish fisheries between 1953 and 2013.

To her surprise she found a consistent increase in cephalopod populations over the past six decades, in species from all over the world and in every habitat, from the deep ocean to the near-shore shallows.

“When we looked at the data from around the world, it was a different story,” she says. “It wasn’t just about cuttlefish any more.”

Since 2013, the giant Australian cuttlefish has also recovered, and Doubleday thinks the previous drop was part of a natural fluctuation.

The exact cause of the global increase still needs to be pinned down, but there are a couple of strong contenders.

One is that as overfishing reduces the number of fish in the sea and cephalopods benefit from the removal of predators and competitors. Cephalopods are able to adapt quickly to take advantage of new opportunities. “When you clear a garden, the first things that start to grow are weeds. Cephalopods are like that,” Doubleday says.

Rigoberto Rosas-Luis, a biologist at the Laica Eloy Alfaro de Manabí university in Manta, Ecuador, agrees. He says that the huge numbers of juveniles produced by cephalopods, their short life cycle, and the fact that they aren’t very picky eaters make it easy for them to colonise new areas. “A generalist organism is more capable of facing variations in the ecosystem,” he says.

Rising ocean temperatures could also be to their advantage, because higher temperatures are thought to accelerate their life cycle, making it even easier to adapt to changing environmental conditions – as long as the temperatures don’t rise beyond their maximum tolerance, and they can still find enough to eat.

Eat, prey, love

Is the rise of the cephalopods a good thing? No one knows yet. They are voracious predators so their rise could hamper prey species. But they also fall prey to many marine animals themselves as well as providing food for humans, who could benefit from the higher abundance.

Doubleday hopes that this research will be a starting point to begin looking at cephalopod population trends in more detail, and how they relate to the global environment.

“Cephalopods are a bit of a research underdog, compared with other marine animals,” she says. “So I hope we can use this as a springboard to do some more work.”

“It is a good example of how the ecological changes in the world’s oceans are being driven by humans through the long-term effects of fisheries and probably global climate change but to date the latter is less well established,” says Paul Rodhouse, a biological oceanographer at the British Antarctic Survey. But he adds that the global cephalopod fishery catches seem to have levelled off recently, so it may be premature to declare they are taking over the seas.

Journal reference: Current Biology, DOI: 10.1016/j.cub.2016.04.002

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