Two-headed Sharks Becoming Increasingly Common

Two-headed Sharks Becoming Increasingly Common

 Two-headed sharks may seem like something out of a ʙᴀᴅ sci-fi movie. However, several have recently been found; and it’s becoming increasingly more common.

Last month, another curious two-headed specimen was discovered — an Atlantic sawtail catshark embryo with two heads. This is special because it’s the first found from an oviparous, or egg-laying, shark. Most two-headed shark fetuses that have been found are blue sharks, since they carry up to 50 babies at one time. With that large number of offspring, there is more of a chance of ᴍᴜᴛᴀᴛɪᴏɴ. This phenomenon is more properly known as polycephaly, the ɢᴇɴᴇᴛɪᴄ abnormality that causes an embryo to develop two heads. But the frequency of cases seems to be increasing. Pᴏʟʏᴄᴇᴘʜᴀʟʏ occurs when twins fail to separate fully in the womb. This phenomenon can affect any living creature, including humans. The blue shark seems to be particularly prone to polycephaly, however. Ehemann’s 2016 study documented the eighth known case in blue sharks. The leader of the 2011 study, Felipe Galván-Magaña, hypothesized that this propensity is related to the blue shark’s high embryo count. The sharks can produce up to 134 pups at a time. The ᴍᴜᴛᴀᴛɪᴏɴ that leads to the growth of two heads is called axial bifurcation, and it has been seen in other animals – even humans. In these sharks, if they sᴜʀᴠɪᴠᴇᴅ birth, it is not likely that they would live long in the wild. Unlike the ᴛᴇʀʀᴏʀs of one’s imagination, no two-headed sharks have been discovered swimming around. More shark embryos and fetuses are being located, though, and here’s the catch — nobody knows why.

Now, scientists are trying to figure out why this is happening. The most popular theory is ᴏᴠᴇʀꜰɪsʜɪɴɢ, which would lead to a smaller ɢᴇɴᴇ pool and higher susceptibility to ɢᴇɴᴇᴛɪᴄ ᴍᴜᴛᴀᴛɪᴏɴs such as axial bifurcation. Other possible causes include ᴍᴇᴛᴀʙᴏʟɪᴄ ᴅɪsᴏʀᴅᴇʀs, ᴘᴏʟʟᴜᴛɪᴏɴ, and ᴠɪʀᴀʟ ɪɴꜰᴇᴄᴛɪᴏɴs. Learning more about why this happens is a step towards preserving these ꜰɪᴇʀᴄᴇ ocean ᴘʀᴇᴅᴀᴛᴏʀs.