When Shark Fetuses Attack

When Shark Fetuses Attack

Posted: May 21, 2015

As humans, we tend to think of the womb as a nice, warm, safe place. But if you were a sand tiger shark, the womb might seem like the most dangerous place on the planet.

I’m Anna Rothschild, and this is Gross Science.

Sand tiger sharks are large sharks that live in warm and temperate waters throughout most of the world. Females have two wombs—two “uteri.” And when a female’s pregnant, she can have about six or seven babies in each womb, each one in its own egg case.

Eventually, one of the babies will hatch from its egg. This hatchling might be little, but it already has a set of razor sharp teeth. In fact, one scientist claims he was bitten by a fetus while he was dissecting a dead female!

There’s a reason for these sharp teeth though. You see, soon after emerging, the hatchling will start devouring its growing brothers and sisters until it eats up every last one in the womb, a process called “embryonic cannibalism.” In the end the mother gives birth to just two babies—the cannibal pup from each uterus.

This might just sound like sibling rivalry gone wild, but the hatchling actually gets a huge benefit from this blood bath. By feeding on the flesh of its siblings, the hatchling grows quite large. In fact, by the time it’s born, it can be over three feet long! And this gives it an advantage in the ocean—very few predators would dare hunt a three-foot-long killer with razor sharp teeth.

There might be another reason for this strange process, as well. It turns out that female sand tiger sharks mate with multiple males, and it’s possible that they have little choice in the matter, though they might get to choose who they mate with first. Embryonic cannibalism may be a way for them to take back control. You see, females can carry the babies of multiple males at the same time. So, a female might mate with her favorite male first, to give that male’s babies a head start to becoming the cannibal hatchling.

Ew.

PRODUCTION CREDITS:

Host, Writer, Animator, Editor

Anna Rothschild

DP, Sound, Intern Extraordinaire

Kristen Clark

Many thanks to Dr. Demian Chapman and Dr. Kevin Feldheim.

Slow Bass

Music Provided by APM

IMAGES

Hatchling and Embryo

©Debra Abercrombie

Sand Tiger Sharks Mating

©Grant Kennedy

Carcharias taurus Newport

Wikimedia Commons/Jeff Kubina

“We Can Do It!”

J. Howard Miller/Public Domain

SFX

Cockroaches

Freesound/StateAardvark
(used with permission from author)

Squeak Pack/squeak_10

Freesound/Corsica_S

Running water bubbles-02

Freesound/Corsica_S

MouthPop

Freesound/HerbertBoland

Zombie pack vol. 1/Zombie eating flesh

Freesound/14red0

Burps/wetbelch4

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120 horror strings

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Swishes

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Produced by WGBH for PBS Digital Studios

IMAGE

(main image: Shark)

©WGBH Educational Foundation 2015

Want more info?

Chapman, D., Wintner, S., Abercrombie, D., Ache, J., Bernard, A., Shivji, M., & Feldheim, K. (2013) The behavioural and genetic mating system of the sand tiger shark, Charcharias Taurus, an intrauterine cannibal. Biology Letters, 9: 20130003.
http://rsbl.royalsocietypublishing.org/content/9/3/20130003