Are jellyfish massing against humankind? Not really; it just seems that way. They may be sending us a message, though.

Enormous aggregations of the diaphanous sea creatures have been wreaking havoc from shore to shore ― clogging water intake valves on seaside power plants, destroying fish farms, crowding fishing nets and, yes, stinging people (sometimes fatally). It’s nothing personal. Jellyfish, lacking brains, do not wish us harm. They’re merely going about their lives as they have for more than half a billion years: swimming, eating and reproducing, lately in mind-boggling numbers.

If anyone is to blame for recent destructive jellyfish “blooms,” as their regional population explosions are called, it is not them, but us. That’s because jellyfish can tolerate waters that are warm and polluted ― conditions that human activity promotes. And as people have fished predators and competitors from their midst, jellyfish reign.

One question is whether we are witnessing an irreversible jellyfish takeover of the seas or just a natural waxing of the jellyfish population that will eventually wane. Scientists don’t know. But finding the answer is crucial to keeping the oceans safe for every sort of undersea life.

One reason for the mystery is that jellyfish, lacking skeletons or shells, leave little trace in the geologic record. Another is that they have not been studied much; fishery biologists have long ignored jellyfish as a small nuisance.

One recent study of the existing scientific data found “no robust evidence for a global increase in jellyfish.” The researchers noted that populations oscillate in 20-year cycles, and an expansion that began during the 1990s is only making it seem as if jellyfish are taking over. Another study, which looked at not only recorded data but also anecdotal reports from experienced fishermen, found that jellyfish populations have increased beyond what natural cycles can explain.

Either way, it’s clear that at least in individual ecosystems, poor human stewardship of coastal waters has been giving jellyfish an edge. Consider, for example, the Black Sea, a repository for the sewage and agricultural runoff of 22 countries, where one species of comb jellyfish began taking over in 1982. The pollution had lowered the water’s oxygen level, giving the low-energy jellyfish an advantage over other creatures. And fishermen had taken out the tuna, sturgeon, mackerel and other species that either ate or competed for food with jellyfish.

At one point, the comb jellyfish achieved a mass of more than 1 billion tons ― more than 10 times the weight of all the fish caught in the world each year. The population began to recede only after a different species of comb jellyfish arrived, via ballast water, and began eating the first kind.

In the Sea of Japan, there have been six blooms of gigantic Nomura jellyfish since 2002 ― something that happened just three times in the 20th century. In Antarctica, melting sea ice is helping jellyfish thrive. In the Bering Sea, it’s overfishing. And so on.

What can be done to keep jellyfish in check? Not a whole lot. South Korean robots that can chop up thousands of them an hour may be able to protect power plants, but even a single local population can include hundreds of millions of jellyfish, so the robots won’t make a huge difference. Nor can we eat our way out of the problem, as only a handful of species are palatable to humans.

The ways to deal with this excess of jellyfish are more indirect. More data about how jellyfish populations wax and wane, and move about, might make it possible for scientists to predict when coastal regions can expect problems. (The rest of us can help by reporting our observations, and stinging experiences, to jellywatch.org.)

The other response is to do all that stuff we should already be doing anyway: Stop polluting, overfishing and emitting so many of the greenhouse gases that contribute to global warming. If keeping the world fit for human habitation isn’t a good enough reason, then do it to prevent the jellyfish, however innocently, from taking over the seas.

Editorial (Bloomberg)