An unborn baby shark swims between uteri.Okinawa Churashima Foundation

In July 2016, Kiyomi Murakumo of the Okinawa Churaumi Aquarium was giving a pregnant tawny nurse shark an ultrasound, when she saw the unmistakable outline of a moving baby shark.

It’s not unusual to see a shark swimming around. It’s far more unusual when that shark hasn’t been born yet. And in this case, the unborn shark wasn’t just fidgeting—it swam from one of its mother’s two uteri to the other.

“When [Kiyomi] told me about it, I asked her to repeat herself,” Taketeru Tomita, the aquarium’s shark expert, told me. “I couldn’t believe my ears.”

Okinawa Churashima Foundation

Murakumo’s experience wasn’t a one-off. Tawny nurse sharks grow up to nine feet in length but have a docile temperament, which makes them easy to work with. Over the next two years, the team found evidence of these in utero migrations in three pregnant females.

Sometimes, a single embryo would show up in the right uterus during one scan, and in the left one during the next. One female had four embryos inside her, sometimes split evenly between her uteri and sometimes all on one side. And on several occasions, the team actually caught the embryos swimming over, at a leisurely pace of three inches per second.

This is one of several discoveries that contradict the idea of embryos as immobile entities, incapable of anything more than gently moving their limbs or heads. For example, turtle embryos can shift around inside their cramped eggs to snuggle up against the warmest sides.  

The babies of the sand tiger shark are also mobile—for more sinister reasons. After these sharks mate, several fertilized eggs settle in each uterus. The first embryo to hatch, still inside the mother, will always attack, kill, and cannibalize the other eggs. These unborn sharks are active enough that Stewart Springer, the first biologist to discover their behavior, was bitten by one while sticking his finger into the birth canal of a recently caught female.

This grisly behavior gives sand tigers an invaluable head start in life. After gorging themselves on their siblings’ nutritious bodies, the surviving embryos—one per uterus—get unchallenged access to a steady supply of unfertilized eggs that their mothers release. On this diet, they grow at an exponential rate. By the time they’re born, they’re already three feet long, and invulnerable to many predators.

The tawny nurse shark embryos don’t cannibalize their siblings, but like the grey nurses, they also eat unfertilized eggs released by their mothers. That’s probably why they move between uteri. They’re foraging for eggs while still inside the womb—a useful skill, especially when other embryos share the same space. “Instead of the embryos eating one another, they appear to be competing,” says Toby Daly-Engel, a shark expert at the Florida Institute of Technology. “That’s just the coolest thing ever.”

Opportunities to uncover the surprises hiding inside sharks are only getting scarcer, though. The tawny nurse shark is classified as vulnerable, and like many sharks, it’s in decline. Its flesh and fins are used as food, and its skin is turned into leather. Even thriving populations have problems: At the Bikini Atoll, once the site of much nuclear testing, many tawny nurse sharks are born with a missing dorsal fin.

“This one obscure species can teach us so much,” Daly-Engel says. “The information is out there, but it’s at risk.”

No one knows if pregnant females from other shark species are also full of active, competitive babies. Certainly, few scientists have the ability to even check. Murakumo and Tomita could do so only with a bespoke ultrasound machine that they co-developed with a company that makes underwater cases for cameras. “The embryonic behaviors of live-bearing sharks are still almost unknown,” Tomita said. “Every discovery is unexpected.”

For example, in the female with four embryos, one of them briefly stuck its head out of its mother’s cervix. The team noticed its fleeting foray into the outside world on their ultrasound, and then saw its face sticking out of the female with their own eyes. What was it doing?

“It’s the mystery,” Tomita said. “We would like to know, too.”

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