If you thought it is safe enough to swim again in the ocean with sharks, then how about meeting a two-headed shark while enjoying your swim? This can be a terrifying thought. Scientists have recently found such one-of-a-kind, but lucky enough, it's a really rare case, and it won't eventually survive into an adult shark. 

Researchers from the University of Malaga in Spain have been studying the cardiovascular system development of the near-threatened Atlantic sawtail catshark (Galeus atlanticus) on its embryonic stage when they have figured out an unusual specimen. Inside the translucent egg was a two-headed catshark.

Featured on ScienceAlert, the article mentioned that the event was truly rare, and it was the first time to ever happen in an egg-laying shark. “Each head had a mouth, two eyes, a brain, a notochord [like a spinal cord] and five gill openings on each side... [there] were two hearts, two oesophaguses, two stomachs, two livers,” the authors mentioned on the Journal of Fish Biology as cited by Vox.

Michelle Heupel, a researcher from the Australian Institue of Marine Science, tried to elaborate more about the discovery made by the team. "Survival after birth may occur, but would likely be very brief. It is unclear whether the two heads will preclude swimming and prey capture, and whether joined internal organs will function adequately," she said.

She also clarified that the tanks where the embryos developed had no agents that can cause the said defect, and based on their leads, the cause was genetics. "We see two-headed sharks occasionally," Florida Museum of Natural History director George Burgess told the National Geographic. "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."

According to ScienceDirect, the study used 797 embryos for observation, and one of them was the two-headed shark. To preserve it for future studies, the team euthanized the embryo in formaldehyde and alcohol.