Understanding Shark Fins

Sharks are an incredibly diverse animal. Their physical characteristics, habitats, and behaviors vary greatly among the species. These biological variances include how sharks reproduce. There are four different ways that sharks reproduce: Viviparous, oviparous, ovoviviparous, and asexually. The differences depend on the species and sometimes, environmental pressures. In the face of declining shark populations, it is essential to know about their reproduction to answer the question: Are sharks endangered?

Shark Academy: How Do Sharks Reproduce?

The Reproductive Anatomy Of Sharks

Shark sexual organs are very different from other animals. The males have a sex organ called a “clasper.” They are typically located on the pelvic fin, however chimaeras have claspers on their heads. Females have oviducts, essentially a fallopian tube that leads to the womb. Male sharks will insert their claspers into the oviduct and release semen fertilizing the female’s eggs. It is believe that this happens with sharks swimming parallel to one another or possibly in smaller or bottom dwelling species, they may mount one another. Once an egg is fertilized, the reproduction process begins.

Shark Mating Habits

Since shark mating is difficult to observe in the wild, very little is know about shark mating habits. This is further complicated by the fact most sharks will not mate in captivity. What is know is that most sharks take years, even decades, to reach the age of reproductive maturity. It is also believed that many sharks species migrate to open waters for mating season, though it is unclear where or when this happens with most species so it is difficult to track. Most sharks do have nursing grounds, places where the environment is conducive for healthy young where scientists can observe birthing. As for the mating ritual, scientists have only ever observed that many pregnant females have bite marks on their bodies and fins. This may either be a product of courtship displays or a means for the male shark to mount them.

Particular Breeders

Scientists believe that all sharks are r/K-selected reproducers, meaning that they have small litters of high quality pups rather than large litters of low quality pups. Sharks tend to have 2-100 pups in a littler depending on species. This could also mean that during courtship, sharks are very particular in mate selection, looking for the best genetic match for reproduction.

Shark Fetus Development

Once the eggs are fertilized, there are three ways the shark pups will develop:

Viviparity

Viviparity is when the egg is developed inside of the womb, eventually leading to a live birth, similar to how humans give birth. Viviparous sharks have placental viviparity. The shark pups will hatch inside of the womb and live off an placenta until they are ready to be born. Sharks born through viviparity will have an umbilical cord located between the pectoral fins that delivers them nutrients and oxygen from the mother’s bloodstream. Viviparous sharks give birth to small litters, typically 2-20 depending on species. The gestation period varies greatly, they tend to carry their pups anywhere from 7 months to three years. Some species of shark that are viviparous include:

Oviparity

About 40% of all sharks are oviparous. Oviparity is when a shark lays eggs in a deposit in the water. Sharks are “true oviparous” animals meaning the egg is fertilized in the womb then laid. The eggs contain a zygote and yolk sac. They are typical encased a leathery pouch, referred to as a “mermaid’s purse.” The eggs are left unprotected and are often times eaten by predators before they can hatch. These eggs can spend weeks in the womb before being laid, and may taken months to hatch. Depending on the species sharks will lay anywhere from 10 to 200 eggs. Oviparous shark species include:

Ovoviviparity

Oviviparity is when an egg hatches inside of the womb and the shark has a live birth. Pups living inside the womb will live off a yolk sac from the egg until they are ready to hatch. The mother’s body does not provide a placenta, but does provide oxygen. In some species of shark, the first shark to hatch from their egg will then consume the other shark fetuses and their yolk sacs. This form of fetal cannibalism helps ensure only the strongest pups survive. Sharks that are ovoviviparous give birth to very small litters with only 1-8 pups. Ovoviviparous shark species include:

Asexual Reproduction

Viviparity, oviparity, and ovoviviparity are not the only ways sharks can reproduce. There is one other surprising way sharks can reproduce, asexually. Asexual reproduction is when a female shark gives birth without any contact with a male. Although very rare, there have been two well-documented cases of asexual reproduction in sharks. Female sharks may asexually reproduce when they are in captivity or when there are no available male sharks in the wild. Asexual reproduction is not an ideal way of reproducing because it limits the genetic diversity of the species. However, asexual reproduction may be useful in propagating the species when under environmental stress.

Are sharks endangered? Unfortunately, they are. With 100 million sharks being killed every year, even with the range of reproduction types and the unusual possibility of asexual reproduction, most sharks have very low birth numbers and cannot reproduce quick enough to replenish their species at the rate we are killing them. This is especially highlighted by the fact most sharks take a long time to mature to the age of reproduction. Learning more about reproductive habits can help protect sharks. However since it is so difficult to observe, the best way to help preserve the species is to reduce the number of sharks killed every year.