Sharks exhibit a remarkable array of reproductive strategies, challenging the common assumption that all sharks give live birth. While some species do deliver live young, this is not a universal characteristic. Their methods are complex, encompassing a spectrum from laying eggs to various forms of internal development. This diversity highlights the incredible adaptations sharks have evolved to ensure offspring survival.
Sharks That Give Live Birth
Some sharks reproduce through viviparity, a method where embryos develop inside the mother and are born alive. This includes placental viviparity, where a yolk sac forms a placenta-like connection to the uterine wall, directly transferring nutrients from the mother’s bloodstream to the developing pup. Hammerhead sharks, bull sharks, lemon sharks, and blue sharks are examples of species that utilize this advanced form of reproduction. Pups born via this method often have an umbilical scar that fades after birth, similar to a belly button.
Sharks That Lay Eggs
Approximately 40% of shark species are oviparous, meaning they lay eggs that develop and hatch outside the mother’s body. These eggs are encased in a tough, leathery protective capsule commonly known as a “mermaid’s purse.” The egg cases vary in shape and size, often featuring tendrils or horns that help anchor them to structures on the seabed, such as seaweed or rocks, preventing them from being swept away by currents.
Inside the mermaid’s purse, the embryo develops, nourished entirely by the yolk within the egg. These eggs are typically large, ranging from 5 to 25 cm long, providing ample nutrients for the developing shark. Once fully developed, the miniature shark hatches from the egg case, which is often found washed up on beaches. Examples of oviparous sharks include horn sharks, catsharks, bamboo sharks, and some carpet sharks.
Sharks With Internal Egg Hatching
Ovoviviparity is a reproductive strategy where eggs hatch inside the mother’s uterus, and the young continue to develop internally until they are born live. While the birth appears live, there is no direct placental connection for nourishment from the mother’s bloodstream. Instead, the developing pups rely primarily on the yolk reserves from their own egg.
In some ovoviviparous species, embryos acquire additional nutrition by consuming unfertilized eggs produced by the mother, a practice known as oophagy, or even by consuming their weaker siblings, a behavior called adelphophagy. Some species, including great white sharks, also provide nourishment via “uterine milk” or histotroph, a lipid-rich secretion. Sand tiger sharks are a notable example of adelphophagy, where the strongest embryo consumes other embryos and unfertilized eggs, resulting in only one or two large, robust pups per litter. Mako sharks and nurse sharks also exhibit ovoviviparity.