Sharks exhibit a remarkable array of reproductive strategies, setting them apart from many other marine animals. Unlike creatures that release vast numbers of eggs and sperm into the water, sharks have evolved diverse and complex methods to ensure offspring survival. These varied approaches highlight the adaptability of sharks across different environments. Understanding how sharks reproduce reveals the intricate biological mechanisms that have allowed these ancient predators to thrive for millions of years.
Egg-Laying Sharks
Some sharks reproduce by laying eggs, a method known as oviparity. After internal fertilization, the female encases her fertilized eggs in a tough, protective covering, often called “mermaid’s purses” due to their leathery appearance. She deposits these cases in safe, sheltered locations, securing them to the seabed, reefs, or seaweed with specialized tendrils or sticky filaments. Horn sharks, for example, lay spiral-shaped eggs wedged into rocky crevices to protect them from predators, while catsharks are also common oviparous species.
Inside the egg case, the embryo develops, nourished by a yolk sac. This period can span several months; horn shark embryos typically hatch after 6 to 10 months. Upon hatching, a miniature, fully formed shark emerges, ready for independent survival, as no parental care is provided. Approximately 40% of all shark species use this strategy.
Live Birth From Internal Egg Hatching
Many shark species use ovoviviparity, where fertilized eggs develop internally within the mother’s reproductive tract and hatch inside her uterus, leading to live birth. Initially, embryos are nourished by their yolk sacs. This method lacks a direct placental connection between the mother and her developing offspring. As embryos grow, some species provide continued sustenance beyond the initial yolk supply.
Oophagy, the consumption of unfertilized eggs produced by the mother, is one strategy. Sand tiger sharks are known for an extreme form where developed embryos consume unfertilized eggs and engage in adelphophagy (intrauterine cannibalism) by eating weaker siblings. This intense competition results in very small litters, often just one or two pups per uterus.
Nurse sharks also exhibit ovoviviparity. Great white sharks nourish embryos with lipid-rich “uterine milk” before oophagy begins. Ovoviviparous young are typically fully functional and relatively large, increasing their immediate survival chances.
True Live Birth
Viviparity, or true live birth, is the most complex shark reproductive strategy, with embryos developing entirely within the mother’s uterus. Direct nourishment comes via a specialized placental connection, often a yolk-sac placenta, where the yolk stalk associates with the uterine wall to transfer nutrients and oxygen. Some viviparous species also secrete “uterine milk,” a nutrient-rich fluid absorbed by developing embryos.
This maternal investment ensures continuous nutrient and oxygen supply, and waste removal. Similar to mammals, a temporary umbilical cord-like structure connects the pup to the placenta, leaving a small “belly button” scar that fades post-birth. Young are born fully developed and immediately capable of independent survival, a significant advantage.
Hammerhead, blue, and bull sharks are prominent viviparous examples. Bull sharks have 10-11 month gestation, birthing 1-13 pups in shallow nursery areas. Blue sharks typically have 9-12 month gestation, with litters usually 15-30 pups (range 4-135). This substantial maternal investment generally results in smaller litter sizes than egg-laying species, but significantly increases individual survival rates.