Sharks reproduce by a variety of different methods. Some lay eggs, while others give birth to live young.
How Sharks Are Born
Sharks give birth in one of three different major ways.
Some shark species lay eggs, although these eggs are not always external. In some species, eggs are produced but they remain within the female shark’s uterus so that they are protected; these shark species typically then bear what looks like live young because the young have hatched within the female’s uterus before they are delivered.
Some sharks give birth to live young that have developed within their uterus.
The eggs that sharks lay aren’t like the typical eggs you find in a grocery store: in fact, the egg cases are often dubbed “mermaid’s purses,” based on their slight resemblance to purses that people carry. Skates or rays also produce eggs in a similar protective egg case.
The egg case usually contains a single fertilized egg, although, in rare circumstances, multiple eggs may be in a case. There is a yolk sac within the egg case that nourishes the developing shark or skate.
When sharks lay eggs, there is no parental care after the eggs have been released. Instead, the young are on their own from the moment they are born.
Examples of sharks that reproduce by producing egg cases include dogfish and angel sharks.
You can often tell the difference between a shark’s egg case and that of a skate, although both are thick and protective cases. The shark egg case generally has small tendrils or fibers that are used to help anchor it to the ground or seaweed so that it doesn’t float away and get washed ashore.
There’s also a relatively unique egg case produced by Bullhead Sharks. These marine organisms produce a corkscrew or spiral-shaped egg, which is quite different from the traditional mermaid’s purse egg case.
In contrast to egg-laying or oviparous shark species, many sharks giving birth are viviparous and bear live young. Roughly 70 percent of shark species are viviparous, and these sharks can be separated into two groupings of sharks: those that have a placenta and those that don’t.
As you can see, there’s a great variety in how sharks reproduce, but the differences don’t stop there. Within the aplacental species that don’t use a placenta, there are shark species that use a yolk sac while the young are developing within the female shark and there are those that use egg capsules that are unfertilized but yolk-filled.
The tiger shark even has its own way of young developing: the largest two embryos (who are usually the first fertilized) will eat the other embryos within the female tiger shark, leaving just the two to be born, having received nourishment from their siblings. The term for this process is intra-uterine cannibalism.
Like egg-laying sharks, the live young-bearing sharks provide no care to their young once they have been born. That said, in some cases, the female shark’s uterus produces a substance that is secreted and may provide nourishment to the young, referred to as uterine milk, which is quite unlike the milk that mammals produce.
Threats to Sharks
Sharks are under threat to their existence because they are slow-growing and may be hunted by humans. Changes in marine environments can also drive sharks away from areas that they naturally lived in.
Some of the reproductive trends that cause sharks to be under threat include their slow development. They often mature slowly, so it is a long time before they reach sexual maturity: for example, the Greenland Shark may live for 400 years but doesn’t start to reproduce until it is 150 years old!
At the same time, sharks typically have long pregnancies, often longer than the 9 months that people face, with average times between 9 and 12 months. During their pregnancies, they also usually only produce a few young, although some such as the Blue Shark may produce over 100 young at one time (this is still significantly less than a bony fish who might produce millions of eggs at one spawning.)
It’s also important to note that some species don’t reproduce every single year even if they could. While some can get pregnant immediately after delivering young, others may not get pregnant for 1 to 2 years after delivering young or laying eggs.
Depending on the species of shark, the sharks lay eggs or the sharks give birth to live young. While scientists know a fair amount about how sharks reproduce, there are still some mysteries, such as within deep-sea sharks, where little is known about their behaviors.
If you happen to walk along the beach and find a mermaid’s purse, look closely at it and try to determine if it belongs to a shark or a ray. Remember, the ray tends to have a rectangular shape with “horns” located at each of the corners, while sharks are often curlier with fibrous tendrils.