It seemed a heart-warming sight: two seals apparently frolicking in the sea before slipping below the waves off the German island of Helgoland (map) in 2013.

Then an ominous sheet of red unfurled across the waves. When the pair resurfaced, the bigger seal was skinning and eating its companion. (Also see "Did Grey Seals Mutilate Two Harbour Porpoises?")

"We thought they were playing," says marine biologist Sebastian Fuhrmann of the environmental consulting firm IBL-Umweltplanung, whose photos of the killing of a young harbor seal will appear in the March 2015 issue of the Journal of Sea Research. "It looked really cute, but in just a few seconds, it was over."

The triumphant hunter of the harbor seal was, astonishingly, a gray seal. These soulful-eyed animals have long been thought to subsist on lowly creatures such as cod. But now the gray seal seems to be morphing into the most murderous killer of the southern North Sea.

New eyewitness accounts and forensic evidence have implicated the sumo wrestler-size marine mammals in the bloody mutilation and death of harbor seals and harbor porpoises across the region. Some of the latter apparently succumb after being ambushed and held underwater until they suffocate. (Watch a video of harbor seals hunting under the waves.)

The gray seal "has the image of a nice, cuddly, friendly animal that eats fish," says marine biologist Mardik Leopold of the Institute for Marine Resources and Ecosystem Studies in the Netherlands.

But mounting evidence is suggesting otherwise.

View Images A grey seal (Halichoerus grypus), Germany's biggest predator, attacking a young harbour seal (Phoca vitulina) on the shore of Heligoland in the North Sea, Germany. Photograph by Sebastian Fuhrmann

Surprisingly Dangerous

When large numbers of mangled harbor porpoise carcasses began to wash up along the southeastern region of the North Sea a decade or so ago, no one suspected gray seals.

Porpoises can easily outswim them, and the seals hadn't been seen dining on any other creature bigger than a duck.

But clues slowly began to accumulate that gray seals in some areas are more fearsome than scientists had realized.

In 2013, a wildlife watcher saw a gray seal near the French coast suddenly pop up next to a harbor porpoise and clamp its jaws onto the porpoise's head, according to a study published in October 2014 in Marine Mammal Science.

The same year, a gray seal in German waters was seen whirling a helpless harbor seal around by the neck; a half-eaten harbor seal carcass washed ashore the next day, according to the upcoming Journal of Sea Research study.

Gray seal DNA has also been found deep inside bite marks on the bodies of badly battered harbor porpoises, according to a pair of studies published in 2014 in PLOS ONE and Marine Ecology Progress Series.

Thanks to DNA analysis, scientists can trace certain porpoise injuries to gray seals. That's because the seals' handiwork often causes large areas of missing skin and blubber, as well as three to five parallel scratches on their prey's skin, according to a November 2014 study in the Proceedings of the Royal Society B.

Though scientists have ruled out a few other culprits for the mangled corpses, including other predators such as Greenland sharks, some biologists are still skeptical that gray seals are primarily responsible. (See "Slow Sharks Sneak Up on Sleeping Seals [and Eat Them]?")

For instance, biologist Dave Thompson believes that many of the harbor seals thought to be eaten by gray seals have actually been torn apart by ship propellers, and that the gray seals scavenge them after death.

"The propeller-damaged carcasses seem to be turning up wherever we look, so the problem is very widespread and still massively under-reported," said Thompson, of the University of St. Andrews in Scotland.

Even so, it's clear "we have a new top predator in the North Sea," especially for harbor porpoises in the last four years, concludes Thibaut Bouveroux of South Africa's Nelson Mandela Metropolitan University.

"The question is why."

Cute ... and Deadly

Scientists have several theories. It's possible gray seals recently developed a taste for porpoise meat after preying on porpoises snared in fishing nets. Or, the fish that gray seals normally savor are growing scarcer.

Another idea is that these marauders of the waves have simply returned to their old habits: The mammals have recolonized the North Sea's southern stretches after being mostly wiped out in the region due to overhunting. (Also see "How a Leopard Seal Fed Me Penguins.")

Whatever the reason, other animals should keep a wary eye out for the appealing seal with the huge, liquid eyes and clownish flippers.

"Just because they're cute doesn't make them less of a predator," says biologist Abbo van Neer of Germany's University of Veterinary Medicine Hannover. "Yes, it's bloody. Yes, it's gruesome. That is just the way nature is."