Sharks are a separate class of vertebrates from other fish, called cartilaginous fish. In the scientific Class Chondrictyhes, sharks separated from other vertebrates as long as 440 million years ago. Along this extensive geological timespan, sharks have adapted and evolved using several reproductive strategies differing from the bony fish.
Sharks reproduce sexually
Male sharks have a pair of reproductive organs called claspers, which are located on the underside of the body near the tail. During mating, the male uses these claspers to insert sperm into the female’s reproductive tract. Female sharks can carry fertilized eggs inside their bodies for months or even years, depending on the species. The method of delivery varies among species, with some laying eggs externally, some giving birth to live young, and others using a combination of both methods. The gestation period (pregnancy) and the number of offspring produced vary greatly depending on the species of shark. In some cases, like the blacktip reef shark, females give birth to live young after a gestation period of 8 to 9 months, bearing 2 to 4 young per litter. White sharks have a slightly longer period at 12 months giving birth to 2-6 pups. The spiny dogfish shark may have the longest gestation period of any vertebrate — over a period of 22 to 24 months.
Sharks have a unique diversity of reproductive methods among vertebrates, in some cases with the female nourishing their young via a placenta and giving live birth, resembling a mammal more than a fish!
There are three types of egg development and birthing in sharks:
- Oviparity – this is when the shark lays eggs outside of its body. The eggs are protected by a tough, leathery case and the female may lay them in a secure location to protect them from predators. Oviparous species include the bamboo shark, catshark, and horn shark.
- Ovoviviparity – in this type of development, the eggs are retained inside the female’s body, but they hatch internally before the mothers give birth to live young. This allows the embryos to have some protection and nourishment from the mother while they develop. Ovoviviparous species include the great white shark, tiger shark, and bull shark.
- Viviparity – this is when the embryos develop inside the female’s body and are nourished by a placenta. The young are born live and fully formed, and the mother may give birth to several offspring at once. Viviparous species include hammerhead sharks, nurse sharks, and lemon sharks.
In some sharks, under extreme circumstances, sharks have been documented to even clone themselves, giving birth to an identical copy of the mother. This is a form of asexual reproduction has a birth mechanism dubbed ‘automictic parthenogenesis’. This is a form of self-fertilization that mimics sexual reproduction but produces an exact genetic copy of the mother.
A 2017 paper published in the Nature journal Scientific Reports described an unusual case of a zebra shark (Stegostoma fasciatum) giving birth in the absence of any males, at an aquarium in Queensland, Australia. Despite not having had contact with a male shark for three years, the shark laid three eggs that all hatched healthy baby sharks.
This diverse methods of reproduction has supported shark species radiation and the evolutionary success of the Chondricthyes. However, late onset of sexual maturity (12-15 years for white sharks, and as long as 30 years for spiny dogfish), the long gestation periods, and their low fecundity with small litter sizes, all make sharks more vulnerable to fishing pressure. In some populations, with the combined impacts of ocean warming, habitat loss and pollution, fishing pressure may be the death knell for many species if we do not take action to protect them.
Help us keep sharks alive and thriving.