Two-Headed Sharks Keep Popping Up—No One Knows Why

[Premium Aquatics]

For those of you who haven't read the PA staff bios, I am a marine bio major and I thought I would share this article for all of you shark fans or science nerds like myself

Two-Headed Sharks Keep Popping Up—No One Knows Why

P.S. I am sure some of you are getting curious about our Black Friday Sales. I honestly do not know what the boss is planning yet, but I will make sure everyone knows when I do

[Bruleyii]

The only logical explanation is aliens.

[sethsolomon]

Maybe all the cesium that has been introduced into the water from the Japan nuclear power plant?

[Origami]

Maybe all the cesium that has been introduced into the water from the Japan nuclear power plant?

Many of the citations in the article are before the nuclear incident in Japan. One comes from 2008 and another from 2011. Another story cited a date of "a few years ago." 

 

There's a lot of potentially valuable information missing from the article. We don't know the geographic distribution or the frequency of discoveries of this sort. The article does say, "Two-headed sharks have been full and far between, so it's tough to know what's behind the mutations. "

 

In each citation, the two-headed shark was either in the womb or, in one case, in an egg case. We don't know if genetic testing has been done to establish if both heads come from the same or from different embryos (think Siamese twins). The article speculates about overfishing as a cause, which is plausible. It also offers viruses, metabolic disorders, and pollution as possible causes. It could also just be part of the normal rate of birth defects in the various species. After all, these individuals most likely would not survive beyond the womb and their bodies rapidly scavenged. 

[Joshifer]

It's the beginning of sharknado

[Premium Aquatics]

 

There are other articles out there, some sites not as reliable as National Geographic though. They have been finding them in different areas. I think one article I read did mention that it was more of a twin situation because the embryo couldn't fully split. Since some sharks migrate pollution, climate change, etc.. could be altering their migration patterns and leaving them only to inbreed. You mentioned that the embryos would not survive beyond the womb, and so far that is exactly the case. There was an article where there was a live fetus with two heads, but after it was removed the from the womb it did not survive for very long.

[Origami]

There are other articles out there, some sites not as reliable as National Geographic though. They have been finding them in different areas. I think one article I read did mention that it was more of a twin situation because the embryo couldn't fully split. Since some sharks migrate pollution, climate change, etc.. could be altering their migration patterns and leaving them only to inbreed. You mentioned that the embryos would not survive beyond the womb, and so far that is exactly the case. There was an article where there was a live fetus with two heads, but after it was removed the from the womb it did not survive for very long.

You may not want to forget that Rupert Murdoch's 21st Century Fox bought a controlling interest in National Geographic in September 2015. It's not exactly the same publication publishing to the same standards as it held for many decades before.

 

^^ That's more a comment on the "reliability" of NG as a source that is slowly being swallowed up by the Fox media empire.

 

As for the other elements: Yes, inbreeding could be responsible. However, an equally likely possibility is that this is just part of the natural background noise of genetic disorders - that is, birth defects in these species. There does not seem to be enough of a sampling to indicate that there's been a statistically significant increase in this defect or in any other defect not mentioned. That's all I was getting at - there's a lack of solid evidence (cited) to support any sort of conclusion other than, "Hey, look what we found."

 

For example, in humans, conjoined twins occur about once every 200,000 live births. The survival rate is (according to the web) between 5 and 25 percent. For a predator like a shark to survive this condition in the wild, it would have to overcome a huge problem: Swimming with effectiveness to pursue and catch prey. So, even it one survived hatching or birthing, it would very likely starve, drown (they need to move to drive water over their gills), or be eaten. I would venture a guess that it's survival rate would be much closer to zero. That would make discovering one outside the womb much more unlikely.