Baby Shark! Shark Reproduction Continued

March 11, 2019

Diving

Shark reproduction is pretty strange to begin with. But for sand tiger sharks, it is even weirder.

Sand tigers are the largest sharks you’ll see at the Greater Cleveland Aquarium. Their mating behavior is pretty ferocious.

We don’t know what prompts female sand tigers to be ready to mate. They must be mature enough in age—somewhere in their teens—and the females must also be ready to mate, which happens about every two years. We are not sure how (or if) females select mates. Scientists are still studying this!

When sand tigers mate, the much smaller males must bite and forcibly flip over the much larger and thicker-skinned females.

But before the sharks get to that point, there is a period of courtship behavior—sort of like shark dating.

Males might follow a female, swimming behind and slightly below her in the water. Then he might escalate to tailing, during which he very closely follows her. The next step is nosing. The male will follow the female very closely, with his nose, or rostrum, very close to her cloaca. (The cloaca is a shark’s reproductive canal and its way to poop. Both males and females have a cloaca.) Then the male will nudge the female, then bite her fins or body—all the while hoping to bite her and flip her over to mate.

If the male is successful, he will bite the female on the pectoral (arm-position) fin and try to flip her over. She then will become catatonic and will allow mating.

But, if the female is not interested at any point, she may bite the male right back! And remember, the females are much larger and thicker-skinned than the males. Female sand tigers can also circle close to the bottom to prevent males from approaching her to bite.

If the female is interested in mating, she may point her nose downward and allow the male a better chance to bite her.

Then the male sand tiger can hold the female’s cloaca with his clasper. Claspers are two finger-like appendages that all male sharks, skates, and rays have behind their pelvic fins. The male can then deliver his sperm, cloaca to cloaca before swimming away.

The female can store the male’s sperm for an unknown amount of time—perhaps a year, perhaps more—until it seems a good time to carry pups.

Now things get even stranger.

The female sand tiger has two uteruses. So sand tigers can carry two totally separate pregnancies.

Since sand tigers are ovoviviparous, they lay eggs internally and then give live birth. But for sand tigers, life starts under difficult circumstances. Around 20 eggs may be fertilized in each uterus. Then the eggs develop into shark embryos. The embryos grow bigger and consume all the nutrition in their individual eggs.

Then the biggest, strongest sand tiger pup in each uterus eats all of its siblings. Intrauterine cannibalism! That is what scientists call it.

The surviving sand tiger pup eats whatever new eggs keep arriving in the uterus until it is ready to be born.

After eight or nine months’ gestation, the pup is born. It will be more than three feet long and totally independent immediately.

Scientists are still learning about sand tiger shark reproduction. At the Greater Cleveland Aquarium we have seen mating behavior, but no pups yet.

– Nora Morrison

Baby Shark! Shark Reproduction

March 7, 2019

Diving

Written by Greater Cleveland Aquarium

In the Shark Gallery seatube, we often hear guests singing “Baby Shark” and sometimes we are asked how baby sharks are born. The answer to that is really fascinating.

There are around 500 species of sharks that we know of, and sharks are extremely varied in their size, body shape and how they reproduce.

All baby sharks begin when a male shark and a female shark mate. How sharks find each other, how they select mates and why they decide to mate is still something scientists are investigating. The male normally bites the female to hold onto her and flip her over, and then uses his fingerlike appendage, called clasper, to deliver sperm. Since female sharks are generally bigger than male sharks, this can be difficult for males.

The female shark can store the male’s sperm until the time is right to fertilize the eggs–sometimes even waiting for years! Then things get even stranger.

Baby sharks, which are known as pups, can be born in three quite different ways.

First, some sharks lay eggs. We call this oviparous. The “mommy shark” lays an egg case in a good spot and swims away. The egg case, which is sometimes called a “mermaid’s purse,” can be perfectly camouflaged to blend in with the sea floor or algae. The egg includes all of the nutrition the pup will need to grow from a fertilized embryo to a fully functioning shark pup. When the pup is ready, it emerges from the egg case and is totally independent.

Second, some sharks grow from eggs–but inside the mother shark’s body. This is called ovoviviparous. In this type of reproduction, there is no placenta to link the “mommy shark” and the “baby shark.” The shark pup gets all of its nutrition from its own egg yolk, other egg yolks, or (yikes!) from eating its fellow fertilized eggs or other pups. Ovoviviparous sharks give live birth to a fully independent pup. This is how sand tiger sharks, like the ones you can see at the Greater Cleveland Aquarium, reproduce.

And third, some sharks do have a placenta and directly support the shark embryo until it is ready to be born as a pup. This is called viviparous, and is also how humans are born. When the shark pup has matured enough, it is born and swims away. This is how sandbar sharks, which you can also see at the Aquarium, have pups.

All sharks are born ready to fend for themselves! As soon as they’re born, shark pups are ready to swim, hunt and grow.

-Nora Morrison