University of Malaga scientists were studying the cardiovascular systems of Atlantic sawmill catsharks (catshark (Galeus atlanticus) when they found one with two heads. This is the first time that dicephaly (two-headedness) has been seen in an egg-laying shark. From National Geographic:
The causes of dicephaly aren't known, but the researchers—led by Valentín Sans-Coma of the University of Malaga—suspect that genetics are the most likely culprit (rather than some environmental factor, à la Blinky, the three-eyed fish, from The Simpsons)…
"We see two-headed sharks occasionally," says George Burgess, director of the Florida program for shark research at the Florida Museum of Natural History. "It's an anomaly, caused by a genetic misfire. There are lots of different kinds of genetic misfires, and most don't make it out of the womb."
"There's a reason you don't see a lot of sharks with two heads swimming around: they stand out like a sore thumb, so they get eaten," adds Burgess. "They would have trouble swimming and probably digesting food."