Sharks are found all over the world in tropical and arctic fresh- and marine waters. Like all creatures, they reproduce to ensure the continuation of their species and play out their ecological role.
Even though sharks are fish, not all species lay eggs.
Sharks can give birth or lay eggs, depending on the species. Sharks produce eggs that develop and hatch inside, called viviparous reproduction, resulting in live births. Here the baby receives nutrition from the egg yolk, a placenta-like connection, or by feeding on unfertilized eggs or siblings in the womb. Other species lay eggs outside of the body referred to as oviparous reproduction. In rare instances, captive female sharks have been found to reproduce without a male.
Read on to learn more about the development and birthing process of baby sharks.
The Shark Family
Over 500 species of sharks belong to the Chondrichthyes class in the subclass family of Elasmobranchii. They are vertebrate cartilaginous fish.
They are not mammals and have characteristics much like fish such as:
- Ectothermy (cold-blooded)
- Lateral line sense organs (sensory capabilities in response to water movements and pressure)
Mating typically occurs in the spring or summer.
Male sharks have two claspers near their anal fins, and female sharks have two uteruses. The male will mount the female by swimming beside or underneath it, holding onto the female with his teeth.
Often female sharks have scars from the mating process. The male inserts a clasper into the female’s cloaca, transferring semen to the oviduct to fertilize eggs.
If it is a viviparous shark, the fertilized egg will incubate inside the mother, resulting in an eventual live birth.
For oviparous sharks, the fertilized egg is laid and eventually hatches in the water.
When sharks are born, they are left to fend for survival on their own. Sea snails, seals, other sharks, and marine creatures will eat eggs and young pups (baby sharks). However, sharks will search for safe, protective places to lay eggs or give birth.
3 Types Of Reproduction
The majority of sharks give birth to live young (viviparous) offspring, and those that do not are oviparous (lay eggs).
The eggs inside a viviparous mother are generally softer- and thinner-shelled than those laid outside of the body.
Viviparous species’ offspring are larger than oviparous.
Sharks species vary in how many eggs they produce resulting in litters ranging from single or double births up to 100 pups.
It is possible that some female sharks can reproduce without semen from a male.
More on that below.
Viviparous sharks keep developing eggs in the final section of their oviduct (womb, uterus) for an average gestation of 9 to 12 months.
Some species are longer than this time frame, up to 3 ½ years, such as found in the frilled shark (Chlamydoselachus anguineus).
The fetus will break through the egg’s protective case once the yolk has been consumed. Some species of pups (babies) obtain additional nutrition and grow inside longer before being birthed.
Yolk Nutrition: Aplacental And Placental
Most commonly, the embryo receives nutrients from the yolk in the egg until birth, called lecithotrophic nutrition. No placenta is present and is called aplacental viviparity.
In placental viviparity, some species of sharks have a yolk-placenta, called matrotrophic nutrition.
In these cases, the egg covering (case) and yolk interlock with the uterus creating a large contact surface that transports additional nutrients to the fetus. This serves a function much like an umbilical cord.
This nutritional fluid is referred to as “uterus milk” but is not a fluid as a result of mammary glands, which are found only in mammals.
Some species, such as several of the mackerel shark species (Lamnidae), the porbeagle shark (Lamna nasus), and the sand tiger shark (Carcharias taurus) have developing fetuses that hatch inside and obtain more nutrition before birth.
These intrauterine pups will feed on each other (called embryophagy) or on unfertilized eggs (called oophagy) inside the mother. This leaves only a few young, yet the strongest survivors, to be born.
This massive amount of nutrition through cannibalism allows the fetus to grow healthy and become strong, skilled predators from birth.
This video shows a mother shark giving live birth:
2. Oviparous: Egg-Laying Sharks
Some species of oviparous, egg-laying sharks are the small-spotted catshark (Scyliorhinus canicula), zebra shark (Stegostoma fasciatum), and nursehound (Scyliorhinus stellaris).
Egg-laying shark species tend to be benthic (bottom-dwelling) species.
External shark egg cases are different from the eggshells of chickens and other birds. The egg is produced in water instead of a land environment, and can appear opaque, showing the developing fetus inside.
Some species of sharks keep the eggs inside for further development before laying them.
The eggs have tendrils or fibers extending from the egg case to help them attach to the ocean floor, seaweed, or rocky surfaces. Bullhead sharks (Carcharhinus leucas) produce spiral, corkscrew-shaped eggs to help them attach to surfaces.
The unique design of the eggs provides functionality in preventing the egg from being washed away or brought up to the surface or shore.
These eggs are referred to as mermaid’s or devil’s purses. A yolk sac inside the egg provides lecithotrophic nutrition to the fetus.
The egg cases typically house one embryo, but in some species, there may be more inside one case. The egg cases have slits on them that allow water to flow through, providing oxygenation.
Once the yolk has been depleted, the shark will break through the shell.
This video shows a baby shark inside an egg case:
3. Parthenogenesis: Reproduction Without Mating
Parthenogenesis occurs when an embryo forms and develops without fertilization from the semen of a male shark.
While not common, female sharks living in capacity may be able to do this. An egg progenitor cell can act as surrogate semen to fertilize an egg.
On December 14, 2001, researchers discovered that a female bonnethead shark (Sphyrna tiburo) gave live birth to a single pup.
She had been captured before maturity and held for three years without the opportunity to mate or store semen.
Since then other instances of parthenogenesis have been discovered in both viviparous and oviparous sharks.
Depending on the species, sharks give live birth or lay eggs.
Live birth, viviparous reproduction, results from an egg that was fertilized and stored in the mother’s uterus. The embryo and fetus are fed via the egg’s yolk, a placenta-like connection, or cannibalism of siblings or unfertilized eggs.
Oviparous reproduction is when a female shark lays a fertilized egg in the water. The embryo is fed and develops as a result of the nutrition from the egg’s yolk.
Some female sharks kept in captivity, and without interaction with a male shark’s semen, have been able to fertilize their own eggs in a process called parthenogenesis.
Learn More About Sharks: