Periodically, batches of seals start washing ashore with strange corkscrew wounds.  No one has ever conclusively proven what causes these wounds, or why, although there are many theories.

The first notable cases of this "corkscrew seal attack" happened at Sable Island, a tiny fingernail clipping of an island in the Atlantic Ocean, 180 miles east of Halifax, Nova Scotia.  The uninhabited island is a wildlife preserve protected by the Canadian government.

Shark predation on Sable Island seals is nothing new - after all, wherever there are seals, there will be sharks.  But in the late 1990s, researchers noticed that a lot of seals were turning up with particularly strange wounds.    The clean-edged cut is a corkscrew-shaped slash, which is not characteristic of any known shark attack.

Deepening the mystery, the corkscrew attacks happened most often in winter, when the sharks which prey on seals were not likely to be around.

Seal and shark researchers across the globe were enlisted to help solve the mystery.  Circumstantial evidence implicated the Greenland shark, although this theory has never been confirmed, and many people remain skeptical.

The Greenland shark is a large, sluggish shark which is uniquely blind.  Each Greenland shark is eventually colonized by a parasitic copepod which attaches itself to the shark's eye, and eats its cornea.  To compensate for the blindness it causes its host, the copepod is bioluminescent, which allows it to act as a lure.  The copepod lures the fish near, and the Greenland shark snaps at them.

Greenland sharks are coldwater sharks, and habitual dwellers of the deep ocean.  To be fair, you don't really need eyes to see down there, since it's so very dark.

However, in winter with the colder surface temperatures, the Greenland shark ventures closer to the surface.  According to the "Greenland shark attack theory," the seals are attracted by the glowing parasitic copepod lure, but manage to twist out of the shark's jaws before it can properly eat them.

Another batch of seals is turning up dead with this spiral cut, this time in the UK.  British tabloid The Sun reports that "Horrified scientists have found around 60 carcasses so far."  

The cuts look so clean and uniform that many people assume that the cause is mechanical.  Except that with the direction of the cuts, the seals would have to be swimming into a set of blades or a turbine head-first, which seems unlikely to say the least.

Lorne Coleman of Cryptomundo posits that this might be the work of Megalodon, the gigantic shark long thought to be extinct, rising from the seas only in a series of bad made-for-cable movies on the Syfy channel.  I think Coleman is being tongue-in-cheek, but one still has to point out the obvious: Megalodon would likely swallow a tiny seal whole, the way we would eat a single peanut.

Saddest of all, this strange plague is hitting a seal population which is already on the ropes.  Seals on the north coast of the UK were hit hard by a distemper outbreak which killed up to 60% of the population in some places.

Photo credit: Flickr/Tambako the Jaguar