Sharks, ancient inhabitants of the world’s oceans, display diverse reproductive strategies. Unlike many fish that release eggs and sperm into the water, sharks have evolved complex ways of bringing their young into the world. This variety highlights their adaptability, ensuring species continuation across different marine environments.
Understanding Viviparity in Sharks
Viviparity, or live birth, involves the development of embryos inside the mother’s body, with young emerging fully formed. This category encompasses two primary forms: ovoviviparity and true viviparity. Both methods result in live young, but they differ significantly in how embryos receive nourishment.
Ovoviviparity, also known as aplacental viviparity, is the most common form of live birth among sharks. Fertilized eggs hatch internally within the mother’s oviduct, and embryos primarily draw nourishment from a yolk sac. Some species provide additional nutrition through oophagy, where embryos consume unfertilized eggs, or histotrophy, where they absorb nutritive fluids from the uterine lining. Young are brooded to a large size before birth, emerging as miniature adults.
True viviparity, or placental viviparity, is a reproductive strategy akin to that seen in mammals. After the initial yolk sac is depleted, a direct connection forms between the developing embryo and the mother’s uterine wall. This specialized structure, a placenta, allows for the direct transfer of nutrients and oxygen from the mother to the embryo, while removing waste. This direct maternal provisioning supports substantial growth within the womb.
Sharks That Give Live Birth: Specific Examples
Many shark species exhibit live birth. These can be categorized based on their method of internal development and nourishment.
Among ovoviviparous sharks, the Great White Shark (Carcharodon carcharias) nourishes its embryos initially with a yolk sac, followed by the ingestion of unfertilized eggs in a process called oophagy. Mako Sharks (Isurus oxyrinchus) also engage in oophagy, where developing embryos consume unfertilized eggs. The Sand Tiger Shark (Carcharias taurus) is known for adelphophagy, an intrauterine cannibalism where the strongest embryo consumes its developing siblings and unfertilized eggs within the womb, resulting in only one or two large, robust pups.
Nurse Sharks (Ginglymostoma cirratum) are ovoviviparous, with pups hatching inside the mother and feeding on a yolk sac. Lemon Sharks (Negaprion brevirostris) are primarily ovoviviparous, with embryos developing inside eggs within the mother.
True viviparous sharks, which form a placental connection, include species like the Bull Shark (Carcharhinus leucas). These sharks develop a yolk-sac placenta that provides direct nourishment to the pups. Various Hammerhead Shark species (Sphyrna spp.) also demonstrate placental viviparity, where the yolk sac forms a placenta-like structure to support the growing embryos. The Blue Shark (Prionace glauca) is another truly viviparous species, giving birth to large litters of well-developed young through a placental connection.
Other Ways Sharks Reproduce
While live birth is common, some shark species employ oviparity. Oviparous sharks lay eggs, which develop externally in the marine environment. These eggs are often encased in a tough, leathery protective shell, commonly referred to as a “mermaid’s purse.”
The egg case provides a secure environment for the developing embryo, protecting it from predators and hazards. The embryo within is nourished entirely by a yolk sac, similar to a chicken egg. Species such as the Horn Shark, Catshark, and Port Jackson Shark are oviparous examples; the Port Jackson shark even carries its corkscrew-shaped egg cases in its mouth to find a safe depositing spot. These eggs can take several months to hatch, with young emerging as miniature adults.
Why Viviparity is an Advantage for Sharks
Viviparity offers several evolutionary advantages for shark species, contributing to higher offspring survival rates. By retaining embryos within the mother’s body, developing young receive continuous protection from external predators and harsh environmental conditions. This internal development shields vulnerable embryos from ocean dangers.
The direct maternal provisioning, whether through a yolk sac or a placental connection, ensures a consistent nutrient supply, allowing pups to develop more fully before birth. Pups born live are typically larger and more developed, making them better equipped to fend for themselves immediately after birth. This increased size and maturity enhances their chances of survival and and contributes to the species’ overall success.