Overfishing and a correlating rise in inbreeding may be one of the factors responsible for an uptick in two-headed shark sightings, according to a new paper in the Journal of Fish Biology.

The study's authors recently discovered a dual-faced Atlantic sawtail catshark (Galeus atlanticus), which joins earlier discoveries of a bull shark fetus with two heads and blue shark conjoined twins. The bull shark was examined in 2011 by staff at Michigan State University: an x-ray found that the specimen had two heads, two hearts, and two stomachs.  

A two-headed bull shark found in Key West in 2011. Image Credit: Courtesy of Shark Defense

While seemingly fit for a Syfy Channel movie, these fish typically don’t live long enough to terrorize anything: The two heads make it more difficult to swim and gather food. As NatGeo notes, most don't even survive birth. The abnormalities are often discovered by fishermen who capture sharks and then examine the offspring they’re carrying rather than ensnare live samples.

Scientists believe metabolic disorders, viral infections, and pollution could also be possible contributors.

If that wasn’t disturbing enough, in 2011 a fisherman caught a dusky shark and discovered it was pregnant with a one-eyed embryo—a.k.a. the "cyclops shark."

[h/t NatGeo]