A Shark's Pregnancy Lasts Much Longer Than You Think
If you thought nine months was a long time for a human to be pregnant, be thankful we aren't sharks. Many species of shark have pregnancies lasting an average of 11 to 12 months long (via CNN). What's more, this isn't even the longest a shark can be pregnant, with some species of shark carrying their pups for up to three years.
The shark with one of the longest gestational periods is the Pacific spiny dogfish, whose pregnancies last around 22 to 24 months. According to the Monterey Bay Aquarium, this is also one of the longest gestational periods of any vertebrate animal. However, some newer research suggests that sharks like the frilled shark and basking shark may have even longer pregnancies. According to the Government of Canada, basking sharks give birth to live pups after carrying them for 2-3 years. Meanwhile, frilled sharks pregnancies may be as long as three and a half years (via Marine Bio).
Some species of shark have live births, while others lay eggs
What's even more interesting about shark pregnancies is the fact that sharks can lay eggs or give birth to live pups depending on their species. Some sharks even have eggs yet still give birth to live pups. In fact, marine biologists have defined three types of shark pregnancies, oviparous, viviparous, and ovoviviparous.
According to How Stuff Works, about one-third of shark species are oviparous, laying eggs and then leaving them to develop on their own in the ocean. Meanwhile, another one-third of shark species are viviparous, carrying a pregnancy and giving birth to live pups like most mammals do (via Biology Dictionary). Lastly, other sharks are ovoviviparous, having the most unique pregnancies. These sharks carry their eggs inside their bodies until the pups are ready to be born (via Merriam-Webster). The eggs will hatch inside the mother shark, where she continues to carry the pups until they are fully developed and ready to be born.
Some sharks can lay impressive numbers of eggs
While the number of eggs or pups that a shark can have at a time depend on the species, some sharks are capable of extremely large numbers in a single litter. For example, the blue shark has litters of 25 to 50 pups, with some litters as large as 135 pups (via Mass.gov). These sharks are viviparous, so they carry all of those live pups in them, giving birth to dozens of live pups.
Another impressive shark pregnancy is that of the shortspine spurdog, a common deep-water shark. These sharks are able to get pregnant immediately after birth through a conveyer belt-like process whereby fully formed pups sit at the bottom of the uterus while eggs sit at the top of the uterus waiting to be fertilized (via Research Gate). In addition, some deepsea sharks — of which the shortspine spurdog is one — have two uteruses, meaning they can be perpetually pregnant in both uteruses, according to Oceana.