SHARK REPRODUCTION
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 onto the female with his teeth, often inflicting bite marks along the female’s body.
MATING HABITS
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 in their lives. Whale sharks reach sexual maturity at the 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 range 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. Once they are born there is no parental care - they simply swim off into the vast ocean to fend for themselves.
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Photo of whitetip reef sharks mating by Ron Watkins
Like whales and humans, Sharks are a k-selected species, which means they adhere to a life strategy of slow growth, late maturation, long gestation, and the production of small litters of pups who are born ready to evade predators and competitors, enhancing their survival rate.
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Internal Nourishment
As shark embryos grow internally (prior to birth), 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, refers to the developing shark embryos method of receiving nutrients while in the womb through the consumption of unfertilized eggs (potential siblings). These embryos are referred to as ‘intra-uterine’ cannibals. There are several species of sharks that practice oophagy, such as the shortfin mako (Isurus oxyrinchus), porbeagle (Lamna nasus), and sandtiger shark (Carcharias taurus). The first embryo to hatch within the womb will feed on other embryos, not just the unfertilized eggs. This means only two pups will be born, one from each uterus.
PLACENTAL VIVIPARITY
Viviparity literally means “giving live birth”. In placental viviparity there is a connection between the embryo and its mother that provides nutrition as the pup develops. 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 then receives 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. Some species using this method of reproduction include: bull sharks (Carcharhinus leucas), lemon sharks (Negaprion brevirostris), blue sharks (Prionace glauca), and Hammerhead sharks (Sphyrnidae).
APLACENTAL VIVIPARITY
In this reproductive method, the shark embryos develop within an egg, which will hatch inside the mother’s body. No placenta is present in the uterus and the developing pup will then continue to be nourished by consuming 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 Greenland sharks (Somniosus microcephalus).
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Shark ultrasound by Sulikowski Lab
Tiger shark litter size can range from 10-82 pups - you can see multiples in this ultrasound. At birth, tiger shark pups are 80-90 cm (2.6-2.95 ft) in length. As you can see in the photo (by Jillian Morris) juvenile tiger sharks have a bold and striking pattern, which becomes less prominent as they grow.
OVIPARITY
Oviparous sharks lay eggs, which are protected by an egg case, referred to as a “mermaid’s purse”. Eggs are deposited somewhere safe to protect them from being eaten by predators. The eggs attach onto 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 common in 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 embryo - video by Dr. Lauren Smith
Small-spotted Catshark (Scyliorhinus canicula)
​Photo by Andy Murch
Small-spotted Catshark embryo
​Photo by Dr. Lauren Smith
​Horn shark hatching from egg case
Photo by Marty Snyderman
PARTHENOGENESIS
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 rare among vertebrates (other than specific reptiles, chickens, and turkeys). This reproductive method enables females to produce offspring without the male genetic contribution. This means an egg progenitor cell, which is usually absorbed by the female’s body, acts as a surrogate sperm and fertilizes the female’s egg. This creates an embryo that is equipped with the correct number of chromosomes.
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Zebra shark photo by Andy Murch
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.
December 14th, 2001 - a captive female bonnethead (Sphyrna tiburo) gave birth to a single female pup. The female in question had been captured in Florida at less than one year old (before she was sexually mature) and lived in captivity for 3 years before the parthenogenesis birth. 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).
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. The female had been isolated from any other individuals of the same species for 8 years.
Between February and April 2016 - Leonie, a captive female zebra shark (Stegostoma fasciatum) gave birth to three pups. Over the course of three years (without any male interaction during that time), she laid 41 eggs (zebra sharks are oviparous). Three of the eggs hatched, resulting in three healthy female pups named Cleo, CC, and Gemini. [Source]
The offspring produced by parthenogenesis are always female in any species that use the XY sex-determination system (like sharks). In any species that use the ZW sex-determination system, the offspring are always males. Scientists believe this incredible phenomenon occurs when sharks get pushed into an evolutionary corner. For example: a captive female shark in a male-less environment. Although it may seem exciting, there is a down-side to this phenomenon. There is a loss of genetic diversity and the resulting pups are less equipped to fight off infections and deal with other challenges.
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