Sharks, those ancient and diverse inhabitants of our oceans, exhibit a remarkable array of reproductive strategies. While many people might envision all sharks giving birth to live young, a significant number of species actually lay eggs.
The Egg-Layers: Oviparous Sharks
Some sharks are egg-layers, a reproductive method known as oviparity. In these species, the female shark deposits fertilized eggs, each enclosed within a protective casing, into the marine environment. The egg cases are often referred to as “mermaid’s purses” due to their unique appearance. They are typically tough and leathery, made of collagen protein strands, and range in shape from rectangular pouches with tendrils to distinctive corkscrew designs. These tendrils or spiral flanges help anchor the egg cases to structures like seaweed or rocky crevices, preventing them from being washed away by currents or tides.
Inside this protective capsule, the developing shark embryo is nourished solely by a yolk sac until the pup is ready to hatch. The incubation period can vary widely depending on the species and water temperature, often lasting several months, or even over two years for some catsharks. Once fully developed, the miniature shark emerges from the egg case, independent, as there is no parental care after the egg is laid. Examples of oviparous sharks include horn sharks, catsharks, and swellsharks.
Live Birth and Other Strategies: Viviparous and Ovoviviparous Sharks
Beyond egg-laying, many shark species give birth to live young, employing two distinct strategies: ovoviviparity and viviparity. Ovoviviparous sharks produce eggs that hatch inside the mother’s body, where the embryos continue to develop. Initially, these embryos rely on a yolk sac for nourishment. However, some species, such as sand tiger sharks and mako sharks, exhibit more complex internal feeding mechanisms like oophagy, where embryos consume unfertilized eggs produced by the mother, or even adelphophagy, involving the consumption of siblings within the uterus. The young are born live, protected within the mother’s body throughout their development. Great white sharks are also examples of ovoviviparous species.
Viviparous sharks, in contrast, demonstrate live birth, a method akin to mammalian reproduction. In these species, the embryo develops inside the mother’s uterus and receives direct nourishment from her. This often occurs through a placental connection, similar to an umbilical cord, which transfers nutrients and oxygen from the mother’s bloodstream to the developing pups. Other viviparous sharks may nourish their young via “uterine milk,” a nutrient-rich fluid secreted by the uterine lining that the embryos absorb. Hammerhead sharks, bull sharks, and blue sharks are common examples of viviparous species. This live-bearing approach offers several advantages, including greater protection for the developing young from external predators and harsh environmental conditions, resulting in larger, more developed offspring at birth with enhanced survival rates.