From snouts studded with complex electro-receptors to skin made of tiny tooth-like structures, called dermal denticles, sharks are perfectly adapted to their environment. How a shark reproduces is one of the most interesting aspects of its biology. The precise mating and reproduction rituals of most shark species are still unknown, thanks to the difficulty of researching subjects dispersed across the vast expanse of the ocean. However, through observation of individuals both in captivity and in the wild, we do know that sharks produce young in a variety of ways; by laying eggs, by producing eggs that hatch in the womb, and by giving birth to live babies. There is one specific method of shark reproduction, however, that is truly astounding, known scientifically as parthenogenesis.
The word parthenogenesis comes from ancient Greek, and translates literally to “virgin birth.” It describes a phenomenon seen in several plant species, some bony fish species and very few vertebrates, whereby a female reproduces without first procreating with a male. In animals, parthenogenesis is defined as the development of an embryo from an unfertilized egg cell. In the absence of a father, the young that are produced derive their DNA solely from their mother. There are several different types of parthenogenesis; the kind seen in sharks results in offspring that have only half of their mother’s genetic material. Consequently, they are not complete clones of their parent, and are unique not only from their mother, but from any siblings as well. Parthenogenesis was first confirmed in sharks in May 2007, when advances in DNA profiling techniques were used to establish the genetics of a baby bonnethead shark born in 2001 at the Henry Doorly Zoo in Omaha, Nebraska. A female bonnethead gave birth to the pup despite having had no contact with any males of her species for over three years. At the time, parthenogenesis was put forward as a possible hypothesis to explain the birth, along with suggestions that the bonnethead had stored sperm from a long-ago encounter with a male or that she may have been a hermaphrodite.
The parthenogenesis theory was not confirmed until six years later, when a team of experts, including shark scientists Dr. Demian Chapman and Dr. Mahmood Shivji, conducted DNA testing on the pup and concluded that the birth had indeed been a virgin one. The test was identical to a human paternity test, and found that only the mother’s DNA was present in the baby shark. Sadly, the very first confirmed instance of parthenogenesis in sharks ended in tragedy; the pup was killed days after its birth by a stingray also living in the tank. That the bonnethead birth was not an isolated phenomenon was proved a year later, when a whitespotted bamboo shark in an aquarium in Detroit produced a clutch of eggs parthenogenetically, and again in 2008 with the virgin pregnancy of an oceanic blacktip in Virginia. Perhaps the most successful example of parthenogenesis in sharks is that of a zebra shark in a Dubai aquarium, who has laid eggs for several consecutive years, all of which hatched into viable offspring despite the female never having had contact with a male.
Although parthenogenetic reproduction amongst sharks in the wild has never been proven, experts believe that it may be a plausible reaction for female sharks in areas where drastic population collapse has made it impossible for them to find a mate. Parthenogenesis is not, however, a solution to the depletion of global shark stocks as a result of mass overfishing. The phenomenon has three significant drawbacks. First, whereas both bonnethead and oceanic blacktip sharks produce between four and 12 babies at a time in the wild, in both cases of parthenogenetic reproduction only one pup was born. In the wild, sharks give birth to multiple young to increase the chances that one will reach sexual maturity; the likelihood of a lone pup surviving would be small. Second, the mechanics of parthenogenesis mean that instead of a baby shark receiving half of its chromosomes from its mother and half from its father, the maternal half of its chromosomes pairs with a copy of itself. This results in reduced genetic diversity that could ultimately lead to the collapse of a population’s gene pool. Finally, because sharks have an XY chromosome sex-determination system, all offspring produced by parthenogenesis are female, thereby lessening the chances that normal reproduction methods could resume in the future.
The extent to which parthenogenesis occurs in sharks in the wild is currently unknown, as is the value of the phenomenon in replenishing dwindling shark populations. What is certain, however, is that the ability to reproduce without fertilization is an astonishing feat; that sharks can give birth in this way reminds us of how fascinating they are as a species. Even more importantly, that we learned about parthenogenesis in sharks just seven years ago serves to underline how much more we still have to learn.
