While some sharks do lay eggs, the answer to how sharks reproduce is more complex, as their reproductive strategies encompass a remarkable range of biological adaptations. Sharks exhibit diverse methods for bringing their young into the world, moving beyond simple egg-laying to include various forms of live birth.
Oviparous Sharks: The Egg-Layers
Oviparity refers to the reproductive strategy where sharks lay eggs that develop and hatch outside the mother’s body. Approximately 30-40% of shark species are oviparous, depositing their fertilized eggs into the marine environment. These eggs are encased in a tough, leathery pouch, often called a “mermaid’s purse” or “devil’s purse,” which protects the developing embryo from predators and environmental stressors.
Shark egg cases vary in shape and size, from rectangular with tendrils to vase-shaped. These tendrils anchor the egg case to structures like seaweed or rocky seafloors, preventing them from being swept away. Inside, the embryo develops, nourished by a yolk sac, similar to a chicken egg. This process can be lengthy, ranging from several months to over a year; for example, catshark eggs typically take 6 to 9 months to hatch, and horn shark eggs take 9 to 12 months. Examples include horn sharks, catsharks, and swellsharks.
Ovoviviparous Sharks: Internal Egg Development
Ovoviviparity is a reproductive method where eggs hatch inside the mother’s body, without a direct placental connection for nourishment. Embryos develop within an egg membrane inside the female’s uterus, primarily sustained by their yolk. Once absorbed, some species receive additional nutrition from fluids secreted by the uterine lining. The mother retains these eggs internally until the young are fully developed and born live.
A unique aspect of ovoviviparous development in some species is “oophagy,” where developing embryos feed on unfertilized eggs produced by the mother. This practice provides sustenance, leading to larger, robust pups at birth. In a more extreme form, “adelphophagy” or intrauterine cannibalism, the first and strongest embryo to hatch consumes its weaker siblings within the uterus. This occurs in species like the sand tiger shark, where often only one pup from each uterus survives to birth, having consumed other embryos and unfertilized eggs. Ovoviviparous sharks include great white sharks, mako sharks, and sand tiger sharks.
Viviparous Sharks: Direct Live Birth
Viviparity is a reproductive strategy where embryos develop inside the mother’s body and receive direct nourishment through a placental connection or similar uterine structures. This method is comparable to mammalian reproduction, with the mother supplying nutrients and oxygen to the developing young. The embryo’s yolk sac initially provides nourishment, but later forms a pseudoplacenta that attaches to the uterine wall, facilitating nutrient transfer.
In some viviparous species, the uterine lining secretes a lipid-rich fluid, “uterine milk” or “histotroph,” which nourishes embryos. This fluid provides fats and proteins for pup growth, allowing them to develop into independent individuals before birth. Pups are born ready to fend for themselves, often equipped with a full set of teeth. Examples include hammerhead, blue, bull, and lemon sharks. This method often results in fewer, but larger and more developed, offspring compared to egg-laying strategies.
The Evolutionary Success of Diverse Shark Reproduction
The diverse reproductive strategies of sharks—oviparity, ovoviviparity, and viviparity—have played a significant role in their long-term survival and adaptability across marine habitats. Each method represents a distinct evolutionary investment in offspring survival, allowing different shark species to thrive in specific ecological niches. This reproductive diversity enables sharks to exploit different environments and resources.
This flexibility in reproduction contributes to the widespread distribution and resilience of sharks throughout geological time. These varied approaches have allowed sharks to persist and diversify in the ocean’s complex ecosystems.