Sharks, belonging to the group of cartilaginous fish called elasmobranchs, exhibit a reproductive biology vastly different from most other aquatic life. Unlike bony fish that release eggs and sperm directly into the water, sharks rely entirely on internal fertilization. Shark birth is not a single, uniform method but a spectrum of diverse strategies, ranging from laying eggs to nourishing developing embryos through a direct placental connection. This variety demonstrates an evolutionary investment in producing fewer, more developed offspring with a greater chance of survival.
How Fertilization Occurs
Reproduction for all shark species begins with internal fertilization. The male possesses specialized, paired organs called claspers, which are modified extensions of the pelvic fins. These firm, cartilaginous structures deliver sperm into the female’s reproductive tract, known as the cloaca. During mating, a male typically inserts only one clasper. The clasper contains a groove that channels the seminal fluid, often aided by seawater flushed from a sac at the base of the organ. This internal process necessitates complex mating rituals, which can involve the male grasping the female with his teeth, often resulting in noticeable bite marks. Once fertilization occurs, the developing embryo is protected within the female, setting the stage for one of three distinct birth strategies.
Sharks That Lay Eggs
The first reproductive strategy, oviparity, involves the female depositing eggs that develop entirely outside her body. These eggs are protected by a tough, leathery encasement often referred to as a “mermaid’s purse.” The egg case is secreted by a gland as the fertilized egg moves through the oviduct, providing a secure capsule for the growing embryo. Species such as the Horn Shark and various Catsharks are oviparous, anchoring their egg cases to the seafloor or submerged structures using sticky tendrils or corkscrew shapes. The embryo receives its sole source of nutrition from the large yolk sac contained within the purse. This method provides no further maternal care once the egg is laid, and the young shark must chew its way out when fully developed.
Sharks That Hatch Internally
Many sharks employ aplacental viviparity, or ovoviviparity, where the eggs are retained and hatch inside the mother’s uterus. The developing young are initially nourished by their yolk sac, but no direct placental connection forms between the mother and the embryos. Since the yolk reserves are often depleted before the long gestation period ends, some species have evolved unique methods to sustain their growth inside the mother.
One common tactic is oophagy, or “egg-eating,” where the developing embryos consume a continuous supply of unfertilized eggs produced by the mother’s ovaries. This practice allows the pups of species like the Shortfin Mako and Porbeagle sharks to grow significantly larger before birth. An even more extreme survival tactic is adelphophagy, or intrauterine cannibalism, observed in sharks such as the Sand Tiger Shark. The first embryo to hatch in each of the female’s two uteri consumes the remaining developing embryos and unfertilized eggs. This results in a litter size of only one highly developed, large pup from each uterus, maximizing the survival chances of the few strongest individuals.
Sharks With True Placental Birth
The most mammal-like form of reproduction in sharks is viviparity, which involves a direct physiological connection between the mother and the developing young. This method utilizes a yolk sac placenta, which forms when the embryo’s yolk sac connects to the uterine wall. This structure allows for the transfer of nutrients and oxygen from the mother’s bloodstream directly to the embryo, while also removing waste products. The connection is analogous to an umbilical cord in mammals, and pups born this way, such as those from Hammerhead and Bull Sharks, will even have a temporary scar that resembles a belly button. Requiem sharks, including the Lemon Shark, are among the groups that employ this reproductive strategy. Viviparity ensures that the pups are born at a large size and an advanced developmental stage, immediately ready for an independent life in the ocean.