Some Unborn Sharks Swim from Uterus to Uterus Inside Their Mom

Scientists have been studying the phenomenon for two years.

Swimming In Shark Infested Waters At Night
Getty ImagesBarcroft Media

Up there with tiger tooth extraction and female bonobo ejaculation sampling is giving a pregnant shark an ultrasound. Even if the shark isn’t interested in eating you, you first need to build a waterproof ultrasound machine, which is exactly what Kiyomi Murakuma did in 2016 at the Okinawa Churaumi Aquarium. What she saw upended the idea that embryos of live-bearing vertebrates are mostly sedentary. .

This sparked a two-year during which researchers monitored the embryonic activity of a number of pregnant tawny nurse sharks. They noted single embryos migrating regularly between the left and right uterus. In females pregnant with multiple shark babies, the embryos sometimes dispersed themselves evenly between uteri, and other times gathered in uneven groups on the left or right. In one case, all four shark embryos were crammed into one uterus.

The scientists looked further to understand the cause of this unusual uterine migration. It turns out that tawny nurse shark embryos eat their mothers’ unfertilized eggs. Once they’ve feasted on the eggs in one uterus, they swim next door to enjoy a second helping. The shark embryos actually appeared to be competing, as they might outside the womb, for limited food resources.

This behavior is distinct from other shark species, like the sand tiger shark whose embryos cannibalize their siblings in utero and thus eliminate the competition from the outset. This explains why sand tiger sharks give birth to one very well-fed pup, which is already three feet long at birth.

Murakuma and her colleague Taketeru Tomita, codeveloper of the underwater ultrasound machine, witnessed another surprising behavior while observing the mother carrying quadruplets. One of her four embryos actually poked its head out of her cervix and then retreated back inside. Why would it do this? To literally “test the waters”?

The team has no answers yet, but the additional discovery points to just how little scientists know about shark gestation. There are so many studies yet to conduct, but it’s becoming more and more difficult to conduct them due to the scarcity of the study population. The tawny nurse shark is classified as “,” one notch away from endangered. Even robust populations are impacted by nuclear testing sites and hunting, among other manmade threats.

If we hope to understand the embryonic mysteries of live-bearing, or viviparous, sharks, we ought to get serious about protecting them and the waters where they live.

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