All sharks mate through internal fertilization. The male inserts one or both of his claspers into the female's reproductive tract and deposits sperm. From that point, the fertilized eggs may be laid by the mother, or they may develop either partially or fully inside the mother. The young get their nourishment either from a yolk sac or other methods, which are described in more detail below.
Of the approximately 400 species of sharks, about 40% lay eggs. This is called oviparity. When the eggs are laid, they are in a protective egg case (which sometimes washes up on the beach and is commonly called a "mermaid's purse"). The egg case has tendrils that allows it to attach to a substrate such as corals, seaweed or the ocean bottom. In some species (such as the horn shark), the egg cases are pushed into the bottom or into crevices between or under rocks.
In oviparous shark species, the young get their nourishment from a yolk sac. They may take several months to hatch. In some species, the eggs stay inside the female for a period of time before they are laid, so that the young have a chance to develop more fully and spend less time in the vulnerable, immobile egg cases before they hatch.
Types of Sharks That Lay Eggs
Shark species that lay eggs include:
- Bamboo sharks and carpet sharks
- Horn (bullhead) sharks
- Swell sharks
- Many catsharks
About 60% of the shark species give birth to live young. This is called viviparity. In these sharks, the young remain in the mother's uterus until they are born.
The viviparous shark species can be further divided into the ways the young sharks are nourished while in the mother:
- Some species are ovoviviparous. In these species, the eggs are not laid until they have absorbed the yolk sac, developed and hatched, and then the female gives birth to young that look like miniature sharks. So, these young sharks get their nourishment from the yolk sac, similar to sharks that form in egg cases, but the sharks are born live. This is the most common type of development in sharks.
Examples of ovoviviparous species are whale sharks, basking sharks, thresher sharks, sawfish, shortfin mako sharks, tiger sharks, lantern sharks, frilled sharks, angelsharks and dogfish sharks.
- In some shark species, the young developing inside their mother get their primary nutrients not from a yolk sac, but by eating unfertilized eggs (called oophagy) or their siblings (embryophagy). Some sharks produce a large number of infertile eggs for the purpose of nourishing the developing pups. Some produce a relatively large number of fertilized eggs, but only one young survives - the strongest one eats the rest. Examples of species in which oophagy occurs are the white, shortfin mako and sandtiger sharks.
- There are some shark species that have a reproductive strategy similar to humans and other mammals. This is called placental viviparity and is reserved for about 10% of the shark species. The egg's yolk sac becomes a placenta attached to the female's uterine wall and nutrients are transferred from the female to the pup. This type of reproduction occurs in many of the larger sharks, including bull sharks, blue sharks, lemon sharks, and hammerhead sharks.
- Canadian Shark Research Laboratory. Reproduction. Accessed January 31, 2012.
- Compagno, L., Dando, M. and S. Fowler. 2005. Sharks of the World. Princeton University Press.
- Florida Museum of Natural History. Shark Basics. Accessed January 31, 2012.
- Greven, H. 2009. Viviparous Sharks. Shark Info / Prof. H. Greven. Accessed January 31, 2012.
- Hamlett, W.C. Reproductive Modes of Elasmobranchs. Florida Museum of Natural History. Accessed January 31, 2012.
- Martin, R.A. Elasmobranch Reproductive Modes. ReefQuest Centre for Shark Research. Accessed January 31, 2012.