Sharks are agile swimmers, even before they are born. Underwater ultrasound scans have revealed that shark fetuses can swim from one of their mother’s twin uteruses to the other.
Most mammal fetuses remain sedentary in their mothers’ wombs. Like mammals, some sharks give birth to live young instead of eggs, leading scientists to assume that fetal sharks probably stay put too. Taketeru Tomita at Okinawa Churashima Research Center in Japan and his colleagues have now discovered that this isn’t the case.
Using a newly developed underwater ultrasound machine, the team monitored three pregnant tawny nurse sharks living in the Okinawa Churashima Aquarium.
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Female sharks have two connected uteruses, aligned side by side. Conducting 44 ultrasound scans throughout the sharks’ pregnancies, the team found that the number of fetuses in each of a shark’s two uteruses kept changing, from two months into pregnancy onwards.
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This suggests that the fetal sharks swim from one uterus to another. To swim between uteruses, a fetus must navigate through a tight, U-shaped bend. The ultrasound footage showed one fetal shark wiggling its tail as it moved from one uterus to another at a speed of 8 centimetres per second.
It is unclear why these animals begin swimming inside their mother before birth, but Tomita thinks they may be looking for food.
Like some other shark species, fetal tawny nurse sharks are known to feed on undeveloped eggs. It is possible that fetal sharks move between uteruses in search of more eggs to eat. Tomita thinks the eggs eaten in the womb are probably unfertilised, but we don’t know for sure.
The fetal sharks may find their way between uteruses by swimming along the uterine walls, perhaps using touch to help them navigate, says Tomita. “It is unlikely that they can see in the dark uterine environment,” he says.
The finding was a surprise for the team. “This phenomenon was completely unexpected,” says Tomita.
Ethology
