Shark reproduction is remarkably diverse, varying significantly across the hundreds of species inhabiting the world’s oceans. This variety allows different shark species to thrive in diverse environments, reflecting millions of years of evolution.
The Egg-Laying Sharks
Approximately 40% of all shark species reproduce by laying eggs, a method known as oviparity. These eggs are encased in a tough, protective covering often referred to as a “mermaid’s purse.” The appearance of these egg cases varies, with some being spiral-shaped, like those of the horn shark, which wedge firmly into rocks. Others, such as those from catsharks and some bamboo sharks, are more rectangular and possess tendrils that help anchor them to underwater structures like seaweed, kelp, or the seabed, preventing them from being washed away.
The developing embryo inside the egg case receives all its nourishment from a yolk sac. Egg-laying species, including carpet sharks and zebra sharks, typically deposit their eggs in safe, protected locations. The incubation period can range from a few months to over a year, after which a miniature version of the adult shark hatches.
Sharks That Give Live Birth
Many shark species give birth to live young, a reproductive strategy called viviparity, which accounts for the majority of shark species. This method means the young develop inside the mother’s body and are born fully formed. There are different ways viviparous sharks nourish their developing young.
One form is placental viviparity, where a yolk sac initially provides nourishment, then attaches to the uterine wall, forming a connection similar to a mammalian placenta. Through this connection, the mother directly provides nutrients and oxygen to the pups. Examples include hammerhead sharks, blue sharks, and bull sharks.
Another form is aplacental viviparity, where embryos develop without a placental connection. In some cases, such as with great white sharks, shortfin mako sharks, and porbeagle sharks, embryos consume unfertilized eggs produced by the mother, a process known as oophagy. Sand tiger sharks exhibit a version where the first embryo to hatch internally may consume unfertilized eggs and even its unhatched siblings, a behavior called embryophagy or intrauterine cannibalism.
Sharks Where Eggs Hatch Inside
A significant number of shark species employ a reproductive strategy known as ovoviviparity, where eggs develop and hatch inside the mother’s body, and the young are subsequently born live. While internally, the embryos are still enclosed within an egg membrane, the mother does not provide direct nutritional support after the egg is formed; instead, the developing sharks rely entirely on the yolk sac within their individual egg for sustenance.
This method acts as a middle ground between external egg-laying and true live birth with a placental connection. Internal development offers protection from external predators, and live birth ensures the young are more developed and better equipped for survival. Common examples of ovoviviparous sharks include nurse sharks, thresher sharks, and whale sharks.
The Benefits of Reproductive Diversity
The wide array of reproductive strategies observed in sharks reflects various evolutionary adaptations that offer distinct advantages for survival in different marine environments. Each method presents trade-offs related to parental investment, offspring protection, and development.
Oviparity allows a female shark to deposit eggs and then leave, freeing her from further direct parental care. The tough egg cases provide protection against predators and harsh conditions, enabling the embryo to grow in a stable environment. This strategy is often seen in bottom-dwelling species where eggs can be securely anchored.
Live birth, whether placental or aplacental viviparity, generally results in fewer but more developed offspring. Internal development provides protection from predators during vulnerable embryonic stages, and pups are born larger and more capable of fending for themselves. This increased investment often translates to higher individual survival rates.