Shark reproduction is remarkably diverse, offering no simple answer to whether they are born in eggs. Unlike most fish that scatter eggs or mammals that give live birth, sharks employ a wide array of strategies to bring their young into the world. These varied methods reflect millions of years of adaptation, ensuring the survival of offspring in marine environments. Examining these distinct processes reveals the unique ways shark pups begin their lives.
Sharks That Lay Eggs
Some shark species are oviparous, meaning they lay eggs that develop and hatch outside the mother’s body. These eggs are encased in a tough, protective structure often called a “mermaid’s purse.” This leathery case shields the developing embryo and typically features tendrils or spiral ridges that anchor it to underwater structures like seaweed or rocks, preventing it from being washed away by currents. The embryo inside the egg case receives all its nourishment from a yolk sac.
Development within these egg cases can take several months before the young shark hatches. Horn sharks and catsharks are examples of oviparous sharks. Horn sharks deposit their spiral-shaped egg cases into rock crevices, while Port Jackson sharks may carry their egg cases in their mouths to find a safe spot before depositing them. Once hatched, these miniature sharks are immediately independent, receiving no further parental care.
Sharks That Hatch Internally
Many sharks are ovoviviparous, a reproductive strategy where fertilized eggs are retained within the mother’s body, and the embryos hatch internally before being born alive. This method provides the developing pups with protection from external predators and environmental hazards during their growth. The embryos initially rely on a yolk sac for nourishment.
As the yolk supply diminishes, some ovoviviparous species have evolved additional methods for embryonic nutrition. One strategy is oophagy, where the developing embryos feed on unfertilized eggs produced by the mother. In more extreme cases, known as adelphophagy or intrauterine cannibalism, the strongest embryos consume their weaker siblings within the uterus, ensuring only the most robust individuals are born. Great white sharks, mako sharks, and tiger sharks reproduce through ovoviviparity, giving birth to live, well-developed young.
Sharks With Live Birth
Viviparity is a third reproductive method in sharks, involving live birth where the developing young receive direct nourishment from the mother, similar to mammalian reproduction. In these species, a placental connection forms between the mother and the developing pups. This “yolk-sac placenta” allows for the direct transfer of nutrients and oxygen from the maternal bloodstream to the embryos, sustaining their growth throughout gestation.
This direct maternal nourishment contrasts with ovoviviparous reproduction, where the primary nutrient source is typically the yolk sac or unfertilized eggs, without a direct physiological connection to the mother’s circulatory system after the yolk is depleted. Viviparous sharks, such as bull sharks, hammerhead sharks, and blue sharks, give birth to relatively few, but highly developed, pups. The extended period of maternal care and direct nutrient supply through the placenta means the young are born larger and more capable of independent survival immediately after birth.