It adds a whole new layer to that insufferable Baby Shark song we’ve all been forced to listen to.
Some species of shark actually cannibalize their brethren in the womb.
Actually, we’ve known for a while that sharks feast on their siblings in the womb. Sand sharks, for example, can carry up to twelve embryos at a time, but often give birth to only two sharks. This is because sharks can carry the embryos of more than one mate during one pregnancy and the largest and strongest embryo feasts on its weaker brothers and sisters.
Talk about sibling rivalry!
But new research shows that this competition is even more vicious than we thought. A new kind of ultrasound device has revealed to biologists that these shark embryos can actually travel between wombs to feast.
Here’s what you need to know:
Sharks have more than one uterus. The sand shark and nurse shark, for example, have two.
While we normally think of gestation as a rather peaceful experience where the future baby develops and maybe gives a few kicks or flips towards the end, that’s not the case for sharks. These guys prepare for a life of hunting before they’re even born.
Researchers from Okinawa Churaumi Aquarium in Motobu, Japan used an underwater ultrasound device on pregnant tawny nurse sharks (Nebrius ferrugineus) and discovered that not only did unborn sharks feast on their wombmates, but they could actually move to the mother’s other uterus to feast on other embryos.
“Our data shows frequent embryonic migration between the right and left uteri, which is contradictory to the ‘sedentary’ mammalian fetus,” the team wrote.
Over the course of a few years, they took ultrasound clips of pregnant nurse sharks in captivity and showed that these bad boys could switch uteruses up to 24 times during a pregnancy, moving at a rate of up to 8 centimeters per second! That’s some serious speed if it’s happening inside a mother’s body.
Check out this ultrasound photo of a sand shark in the womb:
The new technology actually caught the act on tape – up until now, scientists only knew about the phenomenon because a later ultrasound snap would show that the number of shark embryos in one uterus had gone down while the count in another uterus had gone up by the same amount.
Scientists believe this behavior takes place in tiger sharks as well, though it’s only been observed during a surgical procedure and there’s no way to prove that it wasn’t because of human intervention, as opposed to being a natural occurrence in that species.
In many shark species, the movement of embryos is limited to the opening and closing of the mouth – and not to eat their siblings, but simply to aid in respiration. So this behavior doesn’t take place in all sharks.
While some of this behavior is likely due to competition between gene pools (remember that the embryos have different fathers), it might also take place because of the unique way that sand shark embryos get their nutrients inside the womb – it’s a process called “oophagy” in which they eat eggs (normally unfertilized ones) produced by their mother’s ovaries while still inside the uterus. But sometimes the nearest egg is in the uterus next door and happens to be another embryo!
The curious case of the sand shark doesn’t end there. We also have proof that shark embryos can poke their heads out of their mother’s cervix and take a look around at the world outside before they are born! The cervix of this shark species does not remain watertight, allowing a window for embryos to nudge their way through.
All of these new finds show us just how little we know about some creatures, even when we’ve studied them for decades. But as we develop more new technology, we’re bound to get many more interesting stories about how things really work in the animal kingdom.
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