Sharks, members of the class Elasmobranchii, exhibit remarkable diversity in their reproductive methods. The question of whether sharks are born in eggs does not have a simple answer because different species employ three distinct strategies: oviparity, ovoviviparity, and viviparity. This reproductive flexibility, ranging from external egg-laying to live birth, reflects the group’s long evolutionary history. Understanding these methods reveals how these predators ensure the survival of their offspring.
The Egg Layers: Oviparity
Oviparous species lay eggs outside the mother’s body for external development. Approximately 40% of all shark species use this method, encasing the fertilized egg in a protective structure. This casing is typically composed of tough, leathery protein and is commonly referred to as a “mermaid’s purse.” The appearance varies; for example, catsharks produce cases with long tendrils that anchor the egg to marine structures like seaweed. Horn sharks lay spiral-shaped cases that they sometimes wedge into rocky crevices for added protection.
The embryo inside the mermaid’s purse is nourished entirely by the yolk sac, without any direct input from the mother after deposition. Development time ranges from a few months to over a year, depending on the species and water temperature. Once fully developed, the pup chews its way out of the case as a small, self-sufficient version of the adult, receiving no further parental care.
Internal Incubation: Ovoviviparity
Ovoviviparity is the most common reproductive strategy among sharks. It involves eggs developing and hatching inside the mother’s reproductive tract, resulting in a live birth. This method is also called aplacental viviparity because the young lack a placental connection to the mother. The fertilized egg is retained in the oviduct, where the embryo initially relies on a large yolk sac for nutrition. Once the yolk is depleted, pups may receive additional nourishment, often as uterine milk or fluids secreted by the uterine lining.
Many ovoviviparous species, such as the Great White Shark and Shortfin Mako, practice oophagy, or egg-eating. In this process, hatched embryos consume a continuous supply of unfertilized eggs produced by the mother, allowing them to grow larger before birth. A more extreme variation, known as adelphophagy or intrauterine cannibalism, occurs in species like the Sand Tiger Shark. Here, the first embryo to develop in each uterus consumes the other developing embryos and unhatched eggs. This intense competition results in only one or two large, highly developed pups being born.
True Live Birth: Viviparity
Viviparity, or placental viviparity, is the most advanced form of reproduction in sharks, closely resembling the process in most mammals. Embryos develop inside the mother and receive nutrients directly through a specialized connection. This connection forms when the yolk sac attaches to the uterine wall, creating a yolk sac placenta. The placenta transfers oxygen and nutrients directly from the mother’s bloodstream to the developing embryo. Species like Hammerhead Sharks, Bull Sharks, and Blue Sharks give birth to fully formed and relatively large pups, which are born with a visible umbilical scar where the placental connection was severed.
Life After Birth: Shark Nurseries
Newborn sharks, called pups, require a safe environment to grow and survive their vulnerable early life stage. Many species utilize specific, protected habitats known as shark nurseries immediately following birth. These areas are typically shallow coastal bays, estuaries, or mangrove forests where the water is warmer and food is abundant. The primary function of a nursery is to offer a refuge from larger predators, especially adult sharks. Pups often spend their first few years in these habitats, which are repeatedly used across multiple years. Protecting these nursery habitats is a conservation priority because the survival rate of pups heavily influences the stability of the adult shark population.