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How do sharks reproduce?

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By Blaise Jones

As it is with many other aspects of shark biology, the method by which sharks reproduce varies from species to species. While all sharks begin the reproduction process via internal fertilization, the female will give birth in one of three ways: oviparous, ovoviviparous, and viviparous.

A swell shark egg, often referred to as a mermaid's purse. The growing shark pup inside can be seen through the thin outer shell. This photo was taken in the Northern Channel Islands in 2011. Photo credit: Vagabondvince310.
A swell shark egg, often referred to as a mermaid’s purse. The growing shark pup inside can be seen through the thin outer shell. This photo was taken in the Northern Channel Islands in 2011. Photo credit: Vagabondvince310.

Oviparous (laying eggs)
Oviparity is reproduction by laying eggs, which very few species of sharks do. The ones that reproduce oviparously tend to be bottom-dwelling species, such as cat sharks, horn sharks, and zebra sharks. Shark eggs function in the same way as does a chicken’s egg. The eggs serve as an external womb in which the baby shark grows until it is fully developed. The shark’s tough, leathery outer shell protects the developing embryo inside while he feeds a yolk sac that provides all his nourishment prior to birth.

Shark eggs are usually dark brown or black in color, and can vary in shape and size from small pillow-like objects to large corkscrews. One thing all shark eggs have in common are tendrils that extend off the ends, which are designed to attach to structures on the bottom of the ocean, such as rocks and coral, to prevent the eggs from blowing away in a passing current.

After anywhere between 6-10 months the eggs hatch and a fully developed juvenile shark is born. The egg case usually detaches from whatever rock it was anchored to and drifts away, sometimes ending up on the beach where passerby’s can pick them up. Most people call these empty shark eggs a “Mermaid’s Purse.”

Ovoviviparous (internal nest)
The next two methods of birthing are both live birthing, but they differ in the method of embryo development. By far the most common of the two is ovoviviparity.

Ovoviviparous birth is the gestation of eggs inside the mother’s body. Unlike mammals that have an umbilical cord, there is no direct connection between the growing fetus and the mother during development. Instead, the mother uses her body as a nest, keeping her eggs safe from harm by protecting them with her life.

Like their oviparous (egg-laying) relatives, the ovoviviparous eggs developing inside the mother’s body have yolk sacs that provide nutrients for the developing fetus. However, once the yolk runs out the eggs will hatch inside the mother’s body and the mostly-developed baby sharks will enter the mother’s uterus, where the shark will continue to develop until fully developed. When the shark fetus has received all the nourishment it can get from the uterus, the mother gives birth to a shark pup that is born ready to hunt on his own.

Viviparous (live birth)
The third method of shark reproduction is the most familiar to us mammals since it is how humans are also born. Viviparous births are live births and only a few species of shark (a fish) are able to reproduce this way since it is common for mammals (like humans, whales, and dogs).

The only sharks able to mimic mammals and give birth to live offspring are members of the Galae shark family, the most evolutionarily developed shark species. Galae sharks include some of the most well known sharks in the water: the great white, sand tiger, and hammer sharks.

The difference between viviparity and ovoviviparity is that there is a connection between the mother and the developing embryo. Viviparous sharks have a connection to their developing offspring, either via an umbilical cord, like mammals, or simply because they provide extra nutrients besides a yolk sac. These nutrients take the form of a fatty substance known as “uterine milk” though it has more in common with lard than actual milk. Sometimes the uterine milk does not provide enough nourishment for the developing shark, who will then often eat their siblings while still in their mother’s womb. This is most common in the sand tiger shark, which will only give birth to one or two 3-foot (1m) pup because he will have eaten all of his siblings prior to birth.

Viviparous sharks will carry their offspring for anywhere between 12-24 months, depending upon species and the environmental conditions surrounding them. Live birthing in viviparous sharks is a violent occurrence, with the offspring thrashing and writhing as they leave the womb. However, this is not because the baby sharks are in any pain. The thrashing is just the young sharks shaking off their connection to their mothers by breaking their umbilical cord. After all, they do not have a doctor or nurse standing by to conveniently assist in the birth.

 

 

SOURCES:

“Sharks: The Animal Answer Guide” by George H. Burgess and Gene Helfman

“The Encyclopedia of Sharks” by Steve Parker

http://elasmo-research.org/education/topics/lh_intrauterine_cannibalism.htm

http://elasmo-research.org/education/topics/lh_maternity.htm

http://elasmo-research.org/education/topics/lh_pj_eggs.htm

http://www.elasmo-research.org/education/classification/classification.htm

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