Sharks Compete for Food in Utero, Plot Twist: They’re on the Menu

Image from Tomita et al., 2018.

Unborn Baby Sharks Will Swim Between Uteruses to Devour Their Brethren

This week, we’re re-branding R.A.D. as Hell to be M.E.T.A.L. as Hell (acronym forthcoming) which should be self-explanatory if you read the title of the above article. And, honestly, the title kind of says it all, too. You probably have questions, though, so let’s go from the top.

Female sharks have some pretty intense (like camping!) reproductive systems, for example: the above-mentioned twin uteruses seen in some species. It’s also fairly common for sharks to be ovoviviparous, which is a really fun word to say out loud and you should absolutely try it. Ovoviviparous means that the embryonic sharks are housed in eggs, not a placenta, but the mama doesn’t lay the eggs – they mature and hatch inside her body and she gives live birth. It’s kind of a best-of-both-worlds scenario between the placental gestation of mammals and the eggs employed by pretty much everything else.

In-between hatching and birth, those toothy little torpedoes need something to nourish them, and since they don’t have their own egg yolk any longer they tuck into their mother’s stash of unfertilized eggs – and each other (properly termed oophagy and adelphophagy, respectively). But that’s old news. What warranted a scientific paper and this Science Alert article is the uterus hopping. To gather this evidence, the scientists from Japan’s Okinawa Churaumi Aquarium water- and pressure-proofed an ultrasound machine and then took more than 40 ultrasounds of three different tawny nurse sharks. Previous evidence for pre-birth migration had come from sharks in surgery or otherwise stressful situations, so being able to conduct the ultrasounds underwater in the sharks’ aquarium exhibit was key. The scientists were able to see in the ultrasounds over several months as the number of shark pups one mother was carrying went from four down to three, then two, until only one happy, siblicidal cannibal remained. It’s believed to be an adaptation to produce high quality offspring that are more likely to survive. Metal as hell, y’all.

We’re all dealing with a nightmare situation right now as COVID-19 marauds across the globe. Everyone could use a little extra light in their life. If baby sharks eating each other in the womb is that light for you, welcome home, friend.

Johnny Venger could really use a hand coming up with a good M.E.T.A.L. acronym.