SHARK REPRODUCTION

How do sharks reproduce? Unlike bony fish, who shed eggs and sperm into the water column, sharks have developed internal fertilization as a mode of reproduction. The male’s sexual organ, called a “clasper” is located on the pelvic fin. Females have oviducts, a tube leading to the womb, this opening is referred to as a “cloaca”. The male will insert his clasper into the female’s cloaca, releasing sperm and fertilizing her eggs. This usually happens whilst the sharks are swimming parallel to one another, the male will hold on to the female with his teeth, often inflicting bite marks along the female’s body.

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Whitetip Reef Shark Mating

Mating is difficult to observe in the wild, and most sharks will not mate in captivity making it very difficult to study mating habits of sharks. Sharks are late maturing, meaning that they will not become sexually mature until later on in their lives, whale sharks do not reach sexual maturity till the grand old age of 30!!! This means they are very vulnerable to population decline. Once they are mature, sharks will usually mate in the spring and summer. The period of gestation can be anything from 9 months to 2 years ( frilled shark gestation period may be as long as 3.5 years). Most species have an average gestation period of 9-12 months. The pups are born as mini replicas of their parents, and once they are born there is no parental care invested in them, they simply swim off into the vast ocean to fend for themselves.

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Whitetip reef SHaRk MATING

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nurse SHaRk MATING

Mating bite marks/scars

Like whale and humans, Sharks are k/selected species, which means they adhere to a life history strategy of slow growth, late maturation, long gestation and the production of small litters of high quality pups who are born ready to evade predators and competitors, enhancing their survival rate.

 

Nutrients are provided from several sources. Some species receive their nourishment through the sole use of yolk reserves from within the egg, this is refereed to as lecithotrophic. Lecithotrophic forms of reproduction include yolk sac viviparity and oviparity. For other species, the energy reserves present in the egg are supplemented by additional nutrients from the mother during gestation, this is referred to as matrotrophic. Matrotrophic methods of reproduction include placental viviparity and oophagy.

Oophagy  (egg eating) – Oophagy, or egg eating, is a means of providing nutrition by which the developing embryos are provided unfertilized eggs (potential siblings) whilst still in the womb. These embryos are referred to as ‘intra-uterine’ cannibals. Species of sharks that practice oophagy include Shortfin Mako (Isurus oxyrinchus) and Porbeagle (Lamna nasus). The Sandtiger shark (Carcharias taurus) is another species that practices intra-uterine cannibalism. The first embryo to hatch will feed on other embryos, not just the unfertilized eggs, which means only two pups will be born, one from each uterus. 

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Placental Viviparity –Viviparity literally means “giving live birth”. In placental viviparity there is a connection between the embryo and its mother, it is via this connection that nutrition is provided to the developing pup. In the early stages, the embryo receives nourishment from a yolk sac, once this is used up the sac attaches to the wall of the uterus and forms a placenta. The pup will continue to receive nourishment directly from the mother’s bloodstream and waste products will be transferred to the mother for elimination. The number of pups in the litter ranges from 2-20. Examples of species using this method of reproduction include: Bulll sharks (Carcharhinus leucas), Lemon sharks (Negaprion brevirostris), Blue shark (Prionace glauca) and the Hammerhead sharks Sphyrnidae.

Lemon Shark Live Birth

Aplacental Viviparity  –The embryos will develop within an egg, which will hatch inside the female’s body. No placenta is present in the uterus and the developing pup will then continue to be nourished via any unfertilized eggs and each other. Very few pups in a litter survive until birth as a result of sibling cannibalism. Examples of species using this method of reproduction include: Great white sharks (Carcharhinus carcaradon), Pelagic thresher (Alopias pelagicus), Nurse Sharks ( Ginglymostoma cirratum), Tiger Sharks ( Galeocerdo cuvier) and the Greenland shark (Somniosus microcephalus).

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Oviparity –Oviparous sharks lay eggs, which are protected by an egg case, these are referred to as a “mermaid’s purse”. The female shark deposits egg cases somewhere safe to protect them from being eaten by predators. The eggs are attached to structures on the sea floor by tendrils to prevent them from floating away. Oxygenation takes place through slits in the side of the egg, with the shark constantly moving its tail from side to side to increase water flow. This form of reproduction is considered a primitive form and is common in more bottom dwelling (benthic) species. Examples of species using this method of reproduction include: Zebra shark (Stegostoma fasciatum),  Swellshark (Cephaloscyllium ventriosum), Epaullete (Hemiscyllim ocellatum) and Horn Shark (Heterodontus francisci).

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nursehound 
CrediT Dr. Lauren Smith

Nursehound embryo
Credit Dr. Lauren Smith

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Small-Spotted Catshark Embryo
Credit Dr Lauren Smith

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Small-Spotted Catshark
CrediT andy murch

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Parthenogenesis, which means “virgin birth” in Greek, occurs when an embryo develops without any fertilization by sperm. Many invertebrates reproduce parthenogenetically, including bees and wasps but it was thought to be quite rare among vertebrates, other than in some species of reptiles, chickens and turkeys.  Females are able to produce offspring without the male genetic contribution when an egg progenitor cell, which is usually absorbed by the female’s body, acts as surrogate sperm and fertilizes” the female’s egg. This creates an embryo that is equipped with the correct number of chromosomes. 

 

Recently scientists have been documenting an increasing number of vertebrate species that have virgin births, even when they normally reproduce sexually. The majority of these cases were females living in captive environments and a number of these cases included sharks. 

 

On 14 the December 2001, a captive female bonnethead (Sphyrna tiburo) gave birth to a single female pup. The female in question had been in captivity for 3 years and was captured in Florida before she was sexually mature, at less than one-year-old. This ruled out the possibility of her engaging in any previous sexual activity and sperm storage prior to capture (long-term sperm storage and later fertilization is well documented in sharks). Another intriguing case of parthenogenesis was revealed in May 2007. A captive female blacktip shark (Carcharhinus limbatus) that did not survive a routine operation was found to be carrying a single well-developed female embryo, despite the mum being isolated from any other individuals of the same species for all 8 years of its captivity. In another famous case, Leonie, a captive zebra shark (Stegostoma fasciatum) gave birth to three pups, although she had not had contact with a male for years. The 20 something shark laid 41 eggs (zebra sharks are oviparous), three of them hatched into 3 healthy female pups named Cleo, CC and Gemini. 

 

The offspring produced by parthenogenesis are always female in any species that use the XY sex-determination system and males in those animals that use the ZW sex-determination system, sharks use the XY system. Scientists believe this incredible phenomenon occurs when sharks get pushed into an evolutionary corner, for example in a male-less captive environment. Although it’s exciting to see that life finds a way to make miracles happen, there is a down side to this phenomenon too. There is a loss of genetic diversity and the resulting pups will be less equipped to fight off infections and deal with other challenges. 

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ADDITIONAL RESOURCES
CLICK IMAGES TO DOWNLOAD
 

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Egg case
Matching

Egg case
craft

Shark Reproduction
with Dr. James Sulikowski

Finding Elasmobranch Egg cases 
with Dr. Lauren Smith

Mystery in a Mermaid's Purse 
with Cat Gordon (Shark Trust)