Sharks exhibit diverse reproductive strategies. While some species lay eggs, many others give birth to live young. This diversity highlights their adaptability, allowing them to thrive in various ocean environments. Understanding these methods provides insight into their complex life cycles.
Egg-Laying Sharks
Approximately 40% of shark species are oviparous, meaning they lay eggs externally. These eggs are encased in a protective structure often called a “mermaid’s purse.” Initially soft, these cases quickly harden and darken. They often feature tendrils or sticky filaments that allow them to attach securely to underwater structures like seaweed, coral, or rocks, preventing them from drifting.
The developing embryo inside the egg case receives nutrients from a yolk sac. These eggs can take several months to hatch, during which time the young shark grows within its protective shell. Species such as horn sharks, zebra sharks, bamboo sharks, and many catsharks exhibit this behavior. Horn sharks, for instance, lay distinct spiral-shaped egg cases, sometimes wedging them into rock crevices for added protection.
Live-Bearing Sharks
Many sharks give birth to live young, a reproductive method known as viviparity. About 60% of shark species are live-bearing, with some exhibiting placental viviparity. In this method, the embryo develops inside the mother’s uterus, receiving direct nourishment.
A specialized yolk stalk connects the developing embryo to the mother’s uterus, forming a placenta-like structure. Through this connection, nutrients are transferred directly from the mother’s tissues to the growing pup. Some viviparous species also secrete “uterine milk,” a nutritive fluid that provides additional sustenance. Examples include hammerhead sharks, blue sharks, and lemon sharks. After birth, these pups are independent, with no further parental care.
Internal Hatching Sharks
A third reproductive strategy, known as ovoviviparity, involves eggs hatching inside the mother’s body, with live young emerging afterwards. This is often considered the most common type of development among sharks. The female retains the eggs internally, and the embryos develop within a thin membrane or egg capsule.
In ovoviviparous species, there is no direct placental connection or nourishment from the mother to the developing embryos. Instead, the young rely on the yolk sac of their individual egg for nutrients. Once the yolk is absorbed and the embryo is sufficiently developed, it hatches internally, and the mother then gives birth to live, fully formed pups. Some ovoviviparous sharks exhibit unique feeding behaviors while in the womb, such as oophagy (where embryos consume unfertilized eggs) or embryophagy (intrauterine cannibalism). Whale sharks, tiger sharks, and sand tiger sharks are examples of species that reproduce in this way.