Researchers working on a cardiovascular study in Spain have discovered a rare two-headed shark embryo. The animal in question was an Atlantic sawtail catshark, a little-known species found in just a small area of the Atlantic Ocean. Each of its heads had a mouth, two eyes, a brain and five gill openings on each side. 
Image: Sans-Coma et al./Journal of Fish Biology
Now, before we get into the nitty-gritty of the fish's strange body, we should note that the researchers believe the cause of the deformity was "a genetic fluke rather than environmental conditions", as shark scientist David Shiffman points out over at Hakai Magazine (read: the embryo is almost certainly not a radiation mutant).

Because this is the first time that a two-headed pup has been documented in an egg-laying species of shark – Atlantic sawtail catsharks (Galeus atlanticus) are oviparous – there's a lot we don't know about how an animal like this would fare in the wild. It's unlikely that it could have survived to adulthood, however. "There were two hearts, two oesophaguses, two stomachs, two livers, but a single intestine,write the authors of the study, which was led by Valentín Sans-Coma, a zoologist at the University of Malaga. 

Dicephaly (that's the condition of having two heads) has been observed in all major groups of vertebrate animals, most commonly in reptiles like snakes and turtles. While many of these animals do survive initially, their lives are often cut short, as was the case with the conjoined whales that washed up in Mexico back in 2014:
In fish, the mutation presents a number of problems. For a start, landing prey isn't easy when your swimming skills are hindered by the extra weight, as well as the effect that two heads and extra fins have on a normally streamlined shape. In some instances, the spine of a two-headed shark will twist into a helix, which makes it even harder to behave normally in the wild. "The right head was somewhat longer than the left," adds the team. 

An inability to swim could explain why the 
eight truly dicephalous sharks found to date have been discovered in an embryonic state. You might remember this bit of bull shark "double trouble", found by a fisherman working off the Florida Keys:
Image: Wagner et al./Journal of Fish Biology 
Image: Wagner et al./Journal of Fish Biology
In the case of the bull shark, the deformity was caused by "axial bifurcation", where the embryo fails to split into two separate individuals (the most common cause of two-headedness). It's likely that the Spanish catshark embryo gained its extra head for the same reason, but the research team hopes their investigation of its genetics will help them to learn more. It's possible that some shark species are more prone to this kind of mutation, or that other factors like viral infections or metabolic disorders play a part. 

The embryo was just one of 797 the team examined, and by these numbers, only 0.13% of Atlantic sawtail catsharks exhibit the bizarre condition. 

Top header image: Sans-Coma et al./Journal of Fish Biology