Unborn Baby Sharks Can Swim Between Uteruses And Snack On Their Siblings

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Sharks are pretty incredible creatures. And the more we learn about them, the more peculiar they become. You might know, for example, that certain species give birth to live young and that some female sharks have a left and right uterus. What you might not know is that shark fetuses can swim from one uterus to the other. And they can do so pretty quickly.

As reported in the journal Ethology, sonograms of pregnant tawny nurse sharks conducted at the Okinawa Churaumi Aquarium in Japan revealed the incredible feat. The reason for this is unclear but the explanation put forward by the researchers is quite gruesome. Some sharks feed their fetuses through oophagy, meaning the not-yet-born baby sharks feed on eggs in the womb, and sometimes even their siblings. The ability to go and hunt for more eggs might be key to a better chance of survival.

This behavior inside the uterus is quite unlike what we have witnessed in mammals. Mammal fetuses are quite stationary while these shark pups can swim up to 8 centimeters (3.1 inches) per second. The research took several years to complete and focused on three pregnant tawny nurse sharks. The team collected 40 ultrasounds clips, showing the actual movements and also the variety of scenarios that developed inside the womb. One mother had embryos that swapped side three times, another's young did so 24 times. The researchers even witnessed a pregnancy where the pups went from two in each uterus to a single (but well-fed) pup.

The team also discovered that tawny nurse sharks can sometimes open their cervixes to allow their pups to place their snouts outside the uterus if they want to. This is very different from what happens in mammalian pregnancies. The cervix of expectant female mammals is tightly shut until birth.

This study was only possible thanks to the development of the underwater ultrasound. This device allowed observation of the pregnancy in the less-stressful environment of the animal’s tank. Previous observations have involved actually removing the shark from the water, clearly not an ideal place to study "regular behavior". To work, the machine had to be both waterproof and able to withstand the pressure at the bottom of the tank. The new ultrasound is a necessary requirement for the underwater OB-GYN and will allow for even more interesting studies.

 [H/T: Live Science]


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