Featured Image Credit: Christopher Johnson
By Eva Gruber
Two-headed sharks may sound like something out of a horror film, but in actuality, they are becoming more and more common in the real world. Many animals can produce offspring with the mutation which causes two heads – also known as dicephaly – calves, turtles, snakes, cats, dogs, and even humans.
Often, the dicephalic animals suffers from multiple health and physiological complications and they do not survive into adulthood. The two brains “argue” with each other, causing conflict in how the animal moves its body. This can become a problem when hunting for food, or even when being hunted. However, exceptions do occur – one dicephalic black rat snake lived to be around 20 years old.
Two-headed sharks have been few and far between, although in the past several years, increasing numbers of dicephalic sharks have been discovered. In 2008, a fisherman discovered a two-headed blue shark embryo in the Indian Ocean, as reported by National Geographic. A few years later, fishermen pulled in a female bull shark who had a dicephalic fetus inside her uterus.
Image Credit: Christopher Johnson
A 2011 study described several dicephalic blue sharks caught in the Gulf of California and off the coast of northwestern Mexico. It is likely that dicephaly is more common in blue sharks as they are still relatively abundant, and they produce the highest number of offspring of any shark – up to 50 – which increases the chances of any one mutation.
However, it wasn’t until this year that researchers described the first case of dicephaly in an egg-laying shark (all previous dicephalic sharks have been from live-birthing sharks). Described in the Journal of Fish Biology, Spanish researchers, while raising sharks in the laboratory for human health research, discovered a two-headed embryo inside the uterus of a sawtail catshark.
Image Credit: Journal of Fish Biology
The researchers carefully opened the egg to study the specimen, and lead researcher Valentin Sans-Coma says it’s unlikely the individual would have survived. They do not know the cause but suspect that the individual, who was one out of 800 sharks being grown in the laboratory, was likely the result of a genetic disorder.
In the wild, sharks with dicephaly – which is highly disadvantageous – could be the result of something else entirely. With increasing levels of pollution, toxins, and viruses entering marine ecosystems, along with additional stressors such as climate change, habitat degradation, and decreasing prey populations, sharks may be subjected to the negative effects of epigenetics. Another theory is in line with the general population crash of sharks caused by overfishing and finning – with less sharks, a shrinking gene pool leads to genetic abnormalities through inbreeding.
However, because these mutations are so rare and individuals cannot survive in the wild for long, the issue is incredibly difficult to study. Researchers will however be keeping tabs on every dicephalic shark that humans do come across around the world’s oceans.