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Biologists discover a two-headed shark fetus growing in their lab

Everything is fine.

Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

Here’s one for your nightmares. Biologists at the University of Málaga in Spain have discovered a shark embryo with two heads.

The biologists were sorting through hundreds of Atlantic sawtail catshark embryos for a study on how the sharks’ cardiovascular system develops, when they spotted the creature growing inside a translucent egg.

“Each head had a mouth, two eyes, a brain, a notochord [like a spinal cord] and five gill openings on each side,” the authors write in the latest issue of the Journal of Fish Biology. Behind the gills, the two heads fused into a single body, inside which “were two hearts, two oesophaguses, two stomachs, two livers.” (The creature shared an intestine and one set of kidneys and reproductive organs.)

This is the first time a two-headed conjoined twin has ever been recorded in an egg-laying shark, the report states.

Previously, scientists have found conjoined sharks in species that give birth to live animals. And even in those cases, it has been exceedingly rare: Only seven other similarly fused two-headed shark specimens appear in the scientific literature, they note. (A very few two-headed sharks have been found in the wild. Click if you dare.)

Here’s what the researchers saw. It’s kind of cool and kind of gross.

The two-headed shark fetus, next to its yolk.
Journal of Fish Biology

We’ll never know if this specimen could have survived and lived with two heads. The researchers euthanized and preserved it in formaldehyde and alcohol in the course of their study.

And for that matter, no one knows if a two-headed shark could survive to adulthood, David Shiffman, a shark biologist, writes in Hakai Magazine. (The ones that have been found have either been fetuses or juveniles.) Shiffman spoke with Michelle Heupel, a researcher at the Australian Institute of Marine Science, to elaborate:

“Survival after birth may occur, but would likely be very brief,” says Heupel. “It is unclear whether the two heads will preclude swimming and prey capture, and whether joined internal organs will function adequately.” Despite this lethality, she noted that such deformities are extremely rare and therefore do not pose a conservation threat to shark species.

A cross section of the two-headed shark embreyo.
Journal of Fish Biology

Two-headed conjoined twins are rare in nature for any species of vertebrate (including humans). It’s still an open question of what causes shark fetuses to fuse, the researchers note, saying there were no teratogenic — birth defect–causing — agents in their incubation tanks.

It’s a nice reminder that there are unanswered questions about nature lurking in even the weirdest, smallest places.