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 usually hold on to the female with his teeth, often inflicting bite marks along the female’s body.
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 very late 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. 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.
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.
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.
Aplacental Viviparity (Ovoviviparous) –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) and the Greenland shark (Somniosus microcephalus).
Oviparity –Oviparous sharks lay eggs, which are protected by an egg case, these are referred to as “mermaid’s purses”. 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), Epaulete (Hemiscyllim ocellatum) and Hornshark (Heterodontus francisci).