incredibly rare sightings of great whites mating suggest that they roll around together on the surface!
While all sharks reproduce sexually, some species have shown evidence of reproducing asexually. Parthenogenesis – which translates as ‘virgin birth’ from Greek – is where females can produce offspring without a male contribution. It’s been recorded in a few species, including bonnethead, blacktip and zebra sharks. Sounds weird, but a great survival strategy – if all the males die out, it’s not a big problem!
Both! With more than 1,000 species of sharks and rays, there is a huge diversity of reproductive strategies. But there are three main types:
Oviparous species produce eggs. The eggs are generally protected in a tough, leathery case, but the cases look different, depending on the species. Some have curly tendrils, whereas others have ‘horns’ or sticky mucus filaments. All these appendages serve as ways to attach the egg case to seaweed, kelp or other forms of substrate and stop it from floating off into the open ocean, where it is more vulnerable to predators. This is a more ‘hands-off’ form of parenting, although the female may spend a long time laying and tending to her eggs to make sure they’re firmly fixed in place. Once that’s done, however, she’ll leave them to the mercy of the elements. The egg case has everything the shark pup needs to survive: food, nutrients and protection. Once the pup is big enough, it will emerge as a mini-me of the adult and begin its own adventure into adulthood.
Viviparous species give birth to live young. This is the most resource-intensive method, as the shark pup develops inside the female, getting nutrients from the placenta – just like mammals. Hammerhead and bull sharks are among the species that are viviparous. Once the pups are born, however, they are left to fend for themselves. They do not stay with the mother for their young life, unlike many mammals and birds.
Ovoviviparous species are a mixture of the two. The female produces eggs, but rather than lay them, she carries them inside her body until they are ready to hatch. The egg has a thinner casing than that of oviparous species, as they don’t need a tough casing to protect them from the outside world. The young actually hatch inside the mother and are then effectively born live. Sharks that are known to do this include the sand tiger and basking sharks.
The frequency of mating varies greatly between species, from as much as once a year (as in blue sharks) to once every other or even several years, with a ‘resting phase’ in between. Some mate year-round (e.g. the spadenose shark), whereas others prefer certain times of the year, known as the ‘mating season’. That being said, there is still much we don’t know about the mating habits of many species. For example, the basking shark – even though it is the second largest species in the world – has never been scientifically recorded mating and we have no idea where its mating and breeding grounds are.
Again, for many species we still don’t know – but we do know there is a huge diversity. We also know that most sharks are long-lived and reach sexual maturity – the age at which they can produce offspring – at a much greater age than other species of fish. The copper shark, for instance, becomes sexually mature between 13 and 19 years (for males) and at 19 or 20 years (for females). One of the longest lived species of shark, the Greenland shark, remains shrouded in mystery, but scientists think a female does not produce young until it’s 150 years old!
The time it takes for the young to develop, from conception to birth (also known as the gestation period) varies greatly between species – from a matter of months to years. The spiny dogfish, for example, has a gestation period of 24 months; that’s two years!
As animals go, sharks and rays are extremely successful in evolutionary terms. They’ve been around, in some form, for millennia – much, much longer than the human species! But their strategies of being long-lived and slow to reproduce and investing in fewer but higher-quality offspring have also made them extremely vulnerable to exploitation. As many species take 10+ years to reach sexual maturity, many individuals are caught before they are able to produce young. It takes the population much longer to recover from fishing pressure.
Williamson, M.J., Dudgeon, C. & Slade, R. Tonic immobility in the zebra shark, Stegostoma fasciatum, and its use for capture methodology, Springer Link.
Lisa J. Natanson, 2017, Gestation period and pupping seasonality of female spiny dogfish (Squalus acanthias) off southern New England, EBSCO.
R. G. Edwards, 2007, The significance of parthenogenetic virgin mothers in bonnethead sharks and mice
D. D. Chapman, et al., 2008, Parthenogenesis in a large-bodied requiem shark, the blacktip Carcharhinus limbatus, Wiley Online Library.
D. P. Robinson, et al., Annually recurring parthenogenesis in a zebra shark Stegostoma fasciatum, Wiley Online Library.
Yuki Fujinami, et al., 2016, Reproductive biology of the blue shark, Prionace glauca, in the western North Pacific Ocean, ISC.
Swatipriyanka Sen, et al., 2018, Reproductive strategy of spadenose shark, Scoliodon laticaudus Muller and Henle, 1839 along north-eastern Arabian Sea, Wiley Online Library.
WalterLandini, et al., 2017, A secondary nursery area for the copper shark Carcharhinus brachyurus from the late Miocene of Peru, Science Direct.
Julius Nielsen, et al., 2016, Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (Somniosus microcephalus), Science.
Lisa J. Natanson, et al., 2017 Gestation period and pupping seasonality of female spiny dogfish (Squalus acanthias) off southern New England., EBSCO.