Sharks exhibit a fascinating range of reproductive strategies. The question of whether sharks lay eggs or give live birth does not have a single, simple answer. Their methods are varied, reflecting a long evolutionary history and adaptation to different environments. This complexity highlights the unique biology of sharks.
Diverse Reproductive Strategies
Sharks employ three primary reproductive strategies: oviparity, ovoviviparity, and viviparity. All sharks utilize internal fertilization, where the male transfers sperm directly into the female. This differs from many bony fish, which typically release eggs and sperm externally. The internal development of embryos is a shared characteristic, but subsequent stages vary. Oviparity refers to egg-laying, viviparity involves live birth with a direct maternal connection, and ovoviviparity is a middle ground where eggs hatch inside the mother before live birth.
Sharks That Lay Eggs
Approximately 40% of shark species are oviparous. After internal fertilization, the female deposits these eggs, often encased in a protective structure known as a “mermaid’s purse.” These egg cases are tough, leathery pouches designed to safeguard the developing embryo. Each case typically contains a single embryo, nourished by a yolk sac.
These unique egg cases are often equipped with specialized features to help them secure to marine environments. Some have long, curly tendrils that anchor them to seaweed or rocky seafloors, preventing them from being washed away. Others might be screw-shaped, allowing them to be wedged into crevices. Species like horn sharks, catsharks, zebra sharks, and Port Jackson sharks are known for laying these distinctive egg cases. The embryo develops inside the case for several months, sometimes up to 15 months, before hatching as a miniature version of the adult, fully prepared for independent life.
Sharks That Give Live Birth
The majority of shark species give live birth, employing either ovoviviparity or viviparity. In ovoviviparous sharks, fertilized eggs hatch inside the mother’s uterus, with young developing internally, often sustained initially by their yolk sacs. After yolk depletion, some pups receive additional nutrition by consuming unfertilized eggs (oophagy). In more intense cases, such as with sand tiger sharks, developing embryos may even consume their unhatched siblings, a phenomenon known as intrauterine cannibalism. Great white sharks, mako sharks, and cookiecutter sharks are examples of ovoviviparous species.
Viviparous sharks exhibit a more mammal-like reproductive process, developing a direct placental connection to the pups. This placental link, formed from the yolk sac, allows the mother to transfer nutrients and oxygen directly to the young. Waste products from the pups are also transferred back to the mother for elimination. Hammerhead sharks, bull sharks, blue sharks, and lemon sharks are prominent examples of viviparous species. This method provides high protection and nourishment, resulting in larger, more developed pups at birth, capable of fending for themselves immediately.