Forty years ago, annual biological surveys of Norway’s Trondheimsfjord yielded predictable results: the occasional jellyfish, as well as cod, haddock, herring, and other fish that made this 80-mile-long fjord one of Norway’s most productive commercial fisheries. Then, in the 1990s, a new species began showing up: Periphylla periphylla, otherwise known as the helmet jellyfish, or the prince of darkness for its physiological aversion to the sunlight. Imbued with a red pigment that can result in tissue damage and death if exposed to UV rays, the jellyfish comes to the surface to feed only at night, then retreats to inky depths of up to 3,280 feet when the sun rises.

Today, helmet jellyfish have become so abundant in Trondheimsfjord that trawling can bring up 8 tons of them in five minutes. The hordes of gelatinous predators vacuum up young cod and other small fish, as well as the plankton that juvenile cod feed on. Perhaps as a result, populations of cod and other commercially important fish have declined in recent years. Yet the remarkable concentration of jellyfish has also given scientists an opportunity to learn more about these reclusive creatures of the deep, and factors that influence their survival and longevity.

The ancestors of modern helmet jellies—the order Coronatae—first appear in the fossil record more than 500 million years ago. They lived through the rise and fall of dinosaurs, the reordering of continents, mass extinctions, and cataclysmic swings in Earth’s climate. Now, these types of jellyfish may be among the few species to thrive amid modern anthropogenic changes to the ocean.

One reason they’ve succeeded so tremendously in Trondheimsfjord, for instance, is that helmet jellyfish reproduce year round, emerging from their eggs almost fully developed—like the juvenile shown here—and ready to hunt. Other fjord dwellers, like cod, reproduce only once a year, and are born so small they can feed only on tiny plankton. As warming waters cause the plankton to bloom earlier, newborn cod often miss out on their peak abundance and can suffer from inadequate food supplies. Helmet jellyfish, on the other hand, are beholden to no such seasonal schedules. 

Plus, because they feel for prey with foot-long tentacles, rather than relying on eyesight, they can hunt even in polluted and turbid waters. Studies suggest that they don’t react to sound, which means rising rates of marine noise pollution may not affect them. And they can tolerate much lower concentrations of oxygen in the water than most fish, allowing them to live in the hypoxic “dead zones” that result from agricultural runoff, and to take refuge in deeper, cooler, low-oxygen waters when the sea surface heats up. 

Helmet jellyfish also employ bioluminescence to hide prey that would otherwise be visible inside their translucent stomachs, helping them avoid detection by would-be predators. The blue florescence is activated by touch, spreading from the point of contact along the animal’s nerve nets in a visually stunning display that illuminates the darkness around them. It’s one of a wide range of adaptations that enable the prince of darkness to thrive in a changing world—to “win” while so many other species are being pushed to the brink.