Are two heads always better than one?

Scientists from Spain are asking that question after they published findings this month documenting the first ever case of a two-headed shark among egg-laying shark species. The research, published in the Journal of Fish Biology, describes an Atlantic sawtail catshark embryo.

This catshark (Galeus atlanticus) lives only in the western Mediterranean, at depths of 330 to 710 meters (1,082 to 2,329 feet), and is considered near threatened. While studying the fish’s cardiovascular system, the scientists found an embryo with two heads, each with its own mouth, set of eyes, brain, and gill openings. The embryo had a single intestine but two sets of stomachs and livers.

When an animal has two heads it is said to exhibit dicephaly. The condition is relatively rare in the animal kingdom but has been seen in many different groups, from snakes to dolphins to people. (See the two-headed bull shark found by fishermen off the Florida Keys.)

Dicephaly has been recorded in other sharks that either bear live young (viviparous) or lay eggs that develop, and then hatch, inside the mother (ovoviviparous)—but never in true egg-laying sharks (oviparous).

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Seven other two-headed sharks of various species had been found with two complete heads. Another shark had been found with a duplicated face. (See photos of a rare cyclops shark.)

The Spanish team had collected 797 embryos for study of their cardiovascular systems, and only one displayed the abnormal feature, leading them to conclude that the incidence of dicephaly in egg-laying sharks is likely quite low.

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).

View Images Scans reveal internal structure of the unusual shark. Photograph Courtesy Journal of Fish Biology

"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."

When discussing another find of a two-headed shark, biologist C. Michael Wagner of Michigan State University previously told National Geographic that it is important not to jump to conclusions about such rare finds.

“In and of itself, this single natural history observation does not tell us anything earth-shattering about the health of the world’s oceans,” he said.

If they make it to birth, two-headed sharks probably don’t survive long in the wild, Wagner added, since they likely have a hard time swimming and finding food.

"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."