The reproductive biology of sharks, rays, and skates, collectively known as cartilaginous fish or Chondrichthyes, introduces a far greater complexity. Sharks employ multiple reproductive strategies that range from true egg-laying to placental live birth, making their life cycles some of the most varied in the animal kingdom. These ancient predators have evolved diverse methods for nurturing their young, all of which prioritize internal fertilization and the production of a small number of well-developed offspring. Understanding the answer to “how many sharks lay eggs” requires first defining these distinct reproductive approaches.
Three Categories of Shark Reproduction
Unlike most bony fish, all sharks reproduce through internal fertilization, where the male uses specialized pelvic fins called claspers to deliver sperm into the female’s reproductive tract. This fundamental difference sets the stage for three primary modes of embryonic development observed across shark species. These strategies are broadly categorized by where the young develop and how they obtain nourishment before being born or hatched.
The first strategy is oviparity, which involves the female depositing eggs that develop and hatch outside her body in the marine environment. The remaining two strategies, ovoviviparity and viviparity, both result in the birth of live young, but they differ significantly in the maternal investment required. This spectrum of development highlights an evolutionary trend toward increasing protection and nourishment provided to the developing embryo.
Oviparity: The True Egg-Layers
Oviparity is the reproductive strategy of the true egg-laying sharks, representing a distinct minority among the overall shark population. Following internal fertilization, the female secretes a protective, collagenous shell, or egg case, around the developing embryo, which is nourished entirely by a large yolk sac. This specialized casing is commonly called a “mermaid’s purse” when found washed ashore on beaches.
The egg case is characteristically tough and leathery, providing substantial protection against predators and the physical environment. Many species, such as Catsharks and Zebra Sharks, produce cases with long, coiled tendrils at the corners, which serve to anchor the egg firmly to seaweed, sponges, or rocky substrate. Horn Sharks, including the Port Jackson Shark, produce a distinctive spiral-shaped egg case that helps wedge it securely between crevices on the seafloor.
The embryonic development within this external capsule is a prolonged process, often lasting between six to nine months, depending on the species and water temperature. The tough capsule features slits or pores that allow oxygenated water to flow in, while the embryo absorbs nutrients from the substantial yolk sac. Once development is complete, the miniature shark emerges from the case, fully formed and ready to begin its independent life without any parental care.
Ovoviviparity and Viviparity: The Live Birth Majority
The majority of shark species give birth to live young, utilizing either ovoviviparity or viviparity, both of which involve the embryo developing internally within the female’s uterus. Ovoviviparity, often considered the most common strategy among sharks, involves the eggs hatching inside the mother’s reproductive tract. The young are primarily nourished by their own yolk sac, which is retained until they are ready to be born.
In many ovoviviparous species, such as Great White Sharks and Mako Sharks, the nutrient supply is supplemented after the yolk is depleted through a process called oophagy, where the pups consume unfertilized eggs produced by the mother. Sand Tiger Sharks exhibit a more extreme form, intrauterine cannibalism, where the largest pup in the uterus will consume its developing siblings. The pups are born alive and fully developed once their internal gestation is complete.
Viviparity, the strategy most analogous to mammalian reproduction, involves a direct physiological connection between the mother and the developing embryo. This method is seen in species like Bull Sharks and Hammerhead Sharks, where the yolk sac transforms into a structure known as a yolk sac placenta. This placenta attaches to the uterine wall and facilitates the transfer of oxygen and nutrients directly from the mother’s bloodstream to the embryo. Viviparous sharks invest heavily in their young throughout the gestation period, resulting in larger, more robust pups at birth.
Quantifying Oviparous Species
The total number of currently recognized shark species is estimated to be around 500 to 537 worldwide, and the question of how many lay eggs is answered by looking at the proportion of oviparous species within this total. When focusing strictly on sharks, only about 30 to 40 percent of all known species reproduce through oviparity. This percentage is significantly lower than the proportion of oviparous species within the broader group of cartilaginous fish, which includes all skates that are exclusively egg-layers.
This means that out of the approximately 500 known shark species, the number that lay eggs is somewhere in the range of 150 to 200 species. The largest family contributing to this group is the Catsharks, which contains numerous species that deposit the distinctively shaped mermaid’s purses. The remaining majority of sharks utilize the live-bearing strategies of ovoviviparity and viviparity.