Sharks exhibit a remarkable array of reproductive strategies that set them apart from most other fish. Unlike bony fish that typically release eggs and sperm into the water, all shark species engage in internal fertilization. This fundamental difference underlies the diverse ways they bring new life into the marine environment.
The Courtship and Mating Process
The initial stage of shark reproduction involves courtship leading to internal fertilization. Male sharks possess specialized organs called claspers, which are modified extensions of their pelvic fins used to transfer sperm into the female’s cloaca. Females have oviducts leading to this opening. This internal fertilization process is consistent across all shark species.
Mating can be an aggressive interaction. Males often grasp the female by biting her pectoral fins, back, or flanks to maintain position during copulation. These bites can leave visible marks on the female, and in some species, like the blue shark, females have evolved significantly thicker skin as an adaptation to these mating behaviors. After successful sperm transfer, fertilized eggs begin their development inside the female.
Sharks That Lay Eggs
Some shark species reproduce by laying eggs, a method known as oviparity. These oviparous sharks encapsulate their fertilized eggs in protective structures called “mermaid’s purses.” These egg cases are typically leathery and vary in shape and size depending on the species. For example, horn sharks lay spiral-shaped egg cases that wedge into rocky crevices, while swell sharks produce cases with long filaments that help them attach to underwater objects like seaweed.
The female shark deposits these egg cases on the seafloor, among coral reefs, or attached to submerged vegetation. The embryo inside the egg case receives nourishment from a yolk sac, developing over several months to over a year, depending on the species and environmental conditions. Oxygenation occurs through slits in the egg case, aided by the developing embryo’s tail movements that increase water flow. Examples include catsharks, bamboo sharks, and zebra sharks.
Sharks That Give Live Birth
Most shark species give birth to live young, a strategy encompassing two distinct methods: ovoviviparity and viviparity. Ovoviviparity is the most common live-bearing method among sharks. In this process, eggs hatch inside the mother’s oviduct, and embryos remain within her body. Pups receive initial nourishment from their yolk sacs.
Once the yolk is depleted, some ovoviviparous species, such as sand tiger sharks, exhibit intrauterine cannibalism, where embryos consume unfertilized eggs or weaker siblings for continued nourishment. This results in fewer but larger, more developed pups at birth. Great white sharks and tiger sharks are also examples of ovoviviparous species.
Viviparity, or true live birth, is a form of reproduction where embryos develop inside the mother and receive direct nourishment. This is facilitated by a yolk sac placenta, which forms a connection between the embryo and the uterine wall, similar to mammalian reproduction. Through this placental connection, nutrients and oxygen are transferred from the mother’s bloodstream to the pups, and waste products are removed. Some viviparous species also secrete “uterine milk” for additional nourishment. Hammerhead sharks, bull sharks, and blue sharks exhibit viviparous reproduction.
Hatching and Birth
The final stage of shark reproduction culminates in the emergence of young, whether from an egg case or directly from the mother. For oviparous species, after months of development, the shark pup uses an “egg tooth” or specialized structure to break free from its leathery egg case. This process can take several hours, after which the young shark is immediately independent. Empty egg cases can often be found washed ashore.
For live-bearing species, birth involves the expulsion of pups from the mother. These young sharks are equipped with the instincts and physical abilities necessary for immediate survival. Sharks do not provide parental care after hatching or birth. Newborn pups instinctively swim away, often seeking shallow nursery habitats like mangroves or sandy bays, which offer protection from larger predators and a rich food supply.