How Are Sharks Born?

Sharks can be hatched from eggs, born live, or hatched in the womb and then born live -- if their siblings don't eat them first. Learn about shark reproduction and birth in this episode of BrainStuff, based on this article: https://animals.howstuffworks.com/fish/sharks/shark-pup.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to Brainstuff, a production of iHeartRadio, Hey, brain Stuff,
Lauren vogelbaumb here. Reproduction and birth are pretty much always
weird miraculous, sure, but none of us animals, human or otherwise,
get carried in on storks. But if you've ever tried
to explain the process to a small child, just be

(00:24):
glad that we're not sharks. A shark can be born
three different ways, including live birth, hatching from an egg,
and an egg slash live birth combination. Plus in some
shark species, you have to survive gestation without being eaten
by your developing siblings. We'll get to that in a minute,

(00:45):
but first let's look at how sharks get pregnant. As
opposed to other fish. Sharks use internal fertilization in a
process that can appear violent to humans. The male shark
often bites the female's fins or back to keep her
in place, then aligns their reproductive organs and inserts his
clasper into her cloaca. The clasper, which in some species

(01:07):
is spiny and barbs that stays in place a transfers
sperm into the female, some of which may fertilize eggs
in her oviduct at that point, the male shark exits
the story, never to be seen again. The female may
mate with a few different males in order to produce
a single litter. Gestation periods for sharks vary from about

(01:28):
five to six months to two years. The spiny dogfish
shark has the longest gestation period known not just of sharks,
but of all vertebrates, at twenty four months. Here's a
rundown on those three different ways that sharks swim into
the world. First up, we've got viviparity, or live berths.

(01:48):
Viviparous sharks carry their embryos throughout the entire gestation period
and give birth to live shark pups, similar to how
mammals give birth. The embryos are attached inside the womb
with a oak sack placenta, which is how they receive
their nutrition. In some species, females also secrete uterine milk,
which provides more nutrients to the oak sac. Viviparity is

(02:11):
practiced by bigger species, such as blue and hammerhead sharks.
Then there's oviparity, or egg laying. Oviparous sharks lay eggs
which are protected by a tough egg case. These egg
cases are tubes sometimes called mermaids purses because they often
look like flat pouches complete with stringy ends on their corners.

(02:35):
They also look a little bit like ravioli or a
face mask. The female shark deposits the egg cases in
the sea in spots that are hopefully safe from predators.
For example, horn sharks leave the cases in rock crevices,
where they harden into twisted spirals that are difficult to remove,
although snails and seals have been known to break the shell.

(02:57):
Port Jackson sharks do the same thing, carrying the egg
cases in their mouth until they find a safe spot.
From there, the shark pup is on its own. The
embryo is nourished by the yolk in the eggsack and
choose itself out when it's fully developed. Oviparous sharks develop
more embryos per litter than other sharks, but many won't
survive due to predation. Finally, there's what's called ovoviviparity, which

(03:23):
is a combination of the previous two. Ovoviviparous sharks produce eggs,
but instead of laying them to hatch outside the body,
they carry the eggs inside themselves. When an egg hatches,
the shark pup continues developing inside the female shark until
it's born. For the first part of the gestation period,
the embryos receive nourishment from their yolk sack, and once hatched,

(03:46):
the lining of the uterus likely provides uterine milk or
some other kind of nutritious fluid, but there's never a
placenta directly connecting the embryo to its mother. In some
ovoviviparous species, embryos get a dish nutrition from eating their
siblings in the womb. Yep. The eggs in ovoviviparous sharks

(04:06):
hatch at different times, and the shark pups sometimes practice
what's known as intrauterine cannibalism, or eating the other eggs
fertilized or unfertilized in the womb. Only about fourteen species
are known to do this, but the best known intrauterine
cannibal is the sand tiger shark. Although the sand tiger
shark has two uteri and produces many eggs, each litter

(04:30):
yields just two pups, one from each uterus. That's because
as the sharks develop their embryonic teeth, they start to
eat the other embryos and any unfertilized eggs. It's survival
of the fittest until only one pup remains because of
their pre birth diet. The sand tiger pups and to

(04:50):
the world bigger than other pups. They measure approximately three
feet or a meter long. The number of shark pups
in a litter varies among species, even among the three
main categories. The viviparous blue shark has been known to
give birth to one hundred and thirty four pups in
one litter. The whale shark has given birth to three hundred,

(05:10):
but such high numbers are rare. However, whether hatched from
an egg or born live, shark pups emerge as miniature
versions of the sharks they may become, which is a
good thing because they receive no further assistance from their
parents after they're born or hatched. A shark pup's success
in life is largely determined by its size of birth

(05:31):
and whether the female shark has used a nursery area
that is a shallow part of the sea with fewer
predators than the open sea. A Some shark species grow
very slowly, putting them in danger of being eaten by
bigger sharks for longer. If few pups survive to reach
maturity and to reproduce themselves, whether due to natural predators,

(05:52):
a lack of food, or other pressures like human interference,
their species could become endangered. That's why scientists are researching
how sharks reproduce and give birth, and are even experimenting
with artificial wombs four species of concern. The idea is
that when fishers accidentally catch and kill pregnant sharks, the

(06:12):
living embryos could be rescued and brought to term in
these human made environments and then hopefully released into the wild, or,
in the case of those sand tiger sharks, a few
embryos could be rescued from the pup beat pop womb
to develop in solitary safety. Today's episode is based on

(06:34):
the article how are shark pops Born? On HowStuffWorks dot com,
written by Molly Edmunds. Brainstuff is production of iHeartRadio in
partnership with how stuffworks dot Com and is produced by
Tyler Klang. Four more podcasts from my heart Radio visit
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